nu_parser/

parser.rs

#![allow(clippy::byte_char_slices)]

use crate::{
    Token, TokenContents,
    lex::{LexState, is_assignment_operator, lex, lex_n_tokens, lex_signature},
    lite_parser::{LiteCommand, LitePipeline, LiteRedirection, LiteRedirectionTarget, lite_parse},
    parse_keywords::*,
    parse_patterns::parse_pattern,
    parse_shape_specs::{parse_completer, parse_shape_name, parse_type},
    type_check::{self, check_range_types, math_result_type, type_compatible},
};
use itertools::Itertools;
use log::trace;
use nu_engine::DIR_VAR_PARSER_INFO;
use nu_protocol::{
    BlockId, DeclId, DidYouMean, ENV_VARIABLE_ID, FilesizeUnit, Flag, IN_VARIABLE_ID, ParseError,
    PositionalArg, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value, VarId,
    ast::*,
    casing::Casing,
    did_you_mean,
    engine::{CommandType, StateWorkingSet},
    eval_const::eval_constant,
};
use std::{
    collections::{HashMap, HashSet},
    str,
    sync::Arc,
};

pub fn garbage(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    Expression::garbage(working_set, span)
}

pub fn garbage_pipeline(working_set: &mut StateWorkingSet, spans: &[Span]) -> Pipeline {
    Pipeline::from_vec(vec![garbage(working_set, Span::concat(spans))])
}

fn is_identifier_byte(b: u8) -> bool {
    b != b'.'
        && b != b'['
        && b != b'('
        && b != b'{'
        && b != b'+'
        && b != b'-'
        && b != b'*'
        && b != b'^'
        && b != b'%'
        && b != b'/'
        && b != b'='
        && b != b'!'
        && b != b'<'
        && b != b'>'
        && b != b'&'
        && b != b'|'
}

pub fn is_math_expression_like(working_set: &mut StateWorkingSet, span: Span) -> bool {
    let bytes = working_set.get_span_contents(span);
    if bytes.is_empty() {
        return false;
    }

    if bytes == b"true"
        || bytes == b"false"
        || bytes == b"null"
        || bytes == b"not"
        || bytes == b"if"
        || bytes == b"match"
    {
        return true;
    }

    let b = bytes[0];

    // check for raw string
    if bytes.starts_with(b"r#") {
        return true;
    }

    if b == b'(' || b == b'{' || b == b'[' || b == b'$' || b == b'"' || b == b'\'' || b == b'-' {
        return true;
    }

    let starting_error_count = working_set.parse_errors.len();

    // Number
    parse_number(working_set, span);
    if working_set.parse_errors.len() == starting_error_count {
        return true;
    }
    working_set.parse_errors.truncate(starting_error_count);

    // Filesize
    parse_filesize(working_set, span);
    if working_set.parse_errors.len() == starting_error_count {
        return true;
    }
    working_set.parse_errors.truncate(starting_error_count);

    parse_duration(working_set, span);
    if working_set.parse_errors.len() == starting_error_count {
        return true;
    }
    working_set.parse_errors.truncate(starting_error_count);

    parse_datetime(working_set, span);
    if working_set.parse_errors.len() == starting_error_count {
        return true;
    }
    working_set.parse_errors.truncate(starting_error_count);

    parse_binary(working_set, span);
    // We need an additional negate match to check if the last error was unexpected
    // or more specifically, if it was `ParseError::InvalidBinaryString`.
    // If so, we suppress the error and stop parsing to the next (which is `parse_range()`).
    if working_set.parse_errors.len() == starting_error_count {
        return true;
    } else if !matches!(
        working_set.parse_errors.last(),
        Some(ParseError::Expected(_, _))
    ) {
        working_set.parse_errors.truncate(starting_error_count);
        return true;
    }
    working_set.parse_errors.truncate(starting_error_count);

    let is_range = parse_range(working_set, span).is_some();
    working_set.parse_errors.truncate(starting_error_count);
    is_range
}

fn is_env_variable_name(bytes: &[u8]) -> bool {
    if bytes.is_empty() {
        return false;
    }

    let first = bytes[0];
    if !first.is_ascii_alphabetic() && first != b'_' {
        return false;
    }

    bytes
        .iter()
        .skip(1)
        .all(|&b| b.is_ascii_alphanumeric() || b == b'_')
}

fn is_identifier(bytes: &[u8]) -> bool {
    bytes.iter().all(|x| is_identifier_byte(*x))
}

pub fn is_variable(bytes: &[u8]) -> bool {
    if bytes.len() > 1 && bytes[0] == b'$' {
        is_identifier(&bytes[1..])
    } else {
        is_identifier(bytes)
    }
}

pub fn trim_quotes(bytes: &[u8]) -> &[u8] {
    if (bytes.starts_with(b"\"") && bytes.ends_with(b"\"") && bytes.len() > 1)
        || (bytes.starts_with(b"\'") && bytes.ends_with(b"\'") && bytes.len() > 1)
        || (bytes.starts_with(b"`") && bytes.ends_with(b"`") && bytes.len() > 1)
    {
        &bytes[1..(bytes.len() - 1)]
    } else {
        bytes
    }
}

pub fn trim_quotes_str(s: &str) -> &str {
    if (s.starts_with('"') && s.ends_with('"') && s.len() > 1)
        || (s.starts_with('\'') && s.ends_with('\'') && s.len() > 1)
        || (s.starts_with('`') && s.ends_with('`') && s.len() > 1)
    {
        &s[1..(s.len() - 1)]
    } else {
        s
    }
}

/// Return type of `check_call`
#[derive(Debug, PartialEq, Eq)]
pub enum CallKind {
    Help,
    Valid,
    Invalid,
}

pub(crate) fn check_call(
    working_set: &mut StateWorkingSet,
    command: Span,
    sig: &Signature,
    call: &Call,
) -> CallKind {
    // Allow the call to pass if they pass in the help flag
    if call.named_iter().any(|(n, _, _)| n.item == "help") {
        return CallKind::Help;
    }

    if call.positional_len() < sig.required_positional.len() {
        let end_offset = call
            .positional_iter()
            .last()
            .map(|last| last.span.end)
            .unwrap_or(command.end);
        // Comparing the types of all signature positional arguments against the parsed
        // expressions found in the call. If one type is not found then it could be assumed
        // that that positional argument is missing from the parsed call
        for argument in &sig.required_positional {
            let found = call.positional_iter().fold(false, |ac, expr| {
                if argument.shape.to_type() == expr.ty || argument.shape == SyntaxShape::Any {
                    true
                } else {
                    ac
                }
            });
            if !found {
                working_set.error(ParseError::MissingPositional(
                    argument.name.clone(),
                    Span::new(end_offset, end_offset),
                    sig.call_signature(),
                ));
                return CallKind::Invalid;
            }
        }

        let missing = &sig.required_positional[call.positional_len()];
        working_set.error(ParseError::MissingPositional(
            missing.name.clone(),
            Span::new(end_offset, end_offset),
            sig.call_signature(),
        ));
        return CallKind::Invalid;
    } else {
        for req_flag in sig.named.iter().filter(|x| x.required) {
            if call.named_iter().all(|(n, _, _)| n.item != req_flag.long) {
                working_set.error(ParseError::MissingRequiredFlag(
                    req_flag.long.clone(),
                    command,
                ));
                return CallKind::Invalid;
            }
        }
    }
    CallKind::Valid
}

/// Parses an unknown argument for the given signature. This handles the parsing as appropriate to
/// the rest type of the command.
fn parse_unknown_arg(
    working_set: &mut StateWorkingSet,
    span: Span,
    signature: &Signature,
) -> Expression {
    let shape = signature
        .rest_positional
        .as_ref()
        .map(|arg| arg.shape.clone())
        .unwrap_or(SyntaxShape::Any);

    parse_value(working_set, span, &shape)
}

/// Parses a string in the arg or head position of an external call.
///
/// If the string begins with `r#`, it is parsed as a raw string. If it doesn't contain any quotes
/// or parentheses, it is parsed as a glob pattern so that tilde and glob expansion can be handled
/// by `run-external`. Otherwise, we use a custom state machine to put together an interpolated
/// string, where each balanced pair of quotes is parsed as a separate part of the string, and then
/// concatenated together.
///
/// For example, `-foo="bar\nbaz"` becomes `$"-foo=bar\nbaz"`
fn parse_external_string(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let contents = working_set.get_span_contents(span);

    if contents.starts_with(b"r#") {
        parse_raw_string(working_set, span)
    } else if contents
        .iter()
        .any(|b| matches!(b, b'"' | b'\'' | b'(' | b')' | b'`'))
    {
        enum State {
            Bare {
                from: usize,
            },
            BackTickQuote {
                from: usize,
            },
            Quote {
                from: usize,
                quote_char: u8,
                escaped: bool,
            },
            Parenthesized {
                from: usize,
                depth: usize,
            },
        }
        // Find the spans of parts of the string that can be parsed as their own strings for
        // concatenation.
        //
        // By passing each of these parts to `parse_string()`, we can eliminate the quotes and also
        // handle string interpolation.
        let make_span = |from: usize, index: usize| Span {
            start: span.start + from,
            end: span.start + index,
        };
        let mut spans = vec![];
        let mut state = State::Bare { from: 0 };
        let mut index = 0;
        while index < contents.len() {
            let ch = contents[index];
            match &mut state {
                State::Bare { from } => match ch {
                    b'"' | b'\'' => {
                        // Push bare string
                        if index != *from {
                            spans.push(make_span(*from, index));
                        }
                        // then transition to other state
                        state = State::Quote {
                            from: index,
                            quote_char: ch,
                            escaped: false,
                        };
                    }
                    b'$' => {
                        if let Some(&quote_char @ (b'"' | b'\'')) = contents.get(index + 1) {
                            // Start a dollar quote (interpolated string)
                            if index != *from {
                                spans.push(make_span(*from, index));
                            }
                            state = State::Quote {
                                from: index,
                                quote_char,
                                escaped: false,
                            };
                            // Skip over two chars (the dollar sign and the quote)
                            index += 2;
                            continue;
                        }
                    }
                    b'`' => {
                        if index != *from {
                            spans.push(make_span(*from, index))
                        }
                        state = State::BackTickQuote { from: index }
                    }
                    b'(' => {
                        if index != *from {
                            spans.push(make_span(*from, index))
                        }
                        state = State::Parenthesized {
                            from: index,
                            depth: 1,
                        }
                    }
                    // Continue to consume
                    _ => (),
                },
                State::Quote {
                    from,
                    quote_char,
                    escaped,
                } => match ch {
                    ch if ch == *quote_char && !*escaped => {
                        // quoted string ended, just make a new span for it.
                        spans.push(make_span(*from, index + 1));
                        // go back to Bare state.
                        state = State::Bare { from: index + 1 };
                    }
                    b'\\' if !*escaped && *quote_char == b'"' => {
                        // The next token is escaped so it doesn't count (only for double quote)
                        *escaped = true;
                    }
                    _ => {
                        *escaped = false;
                    }
                },
                State::BackTickQuote { from } => {
                    if ch == b'`' {
                        spans.push(make_span(*from, index + 1));
                        state = State::Bare { from: index + 1 };
                    }
                }
                State::Parenthesized { from, depth } => {
                    if ch == b')' {
                        if *depth == 1 {
                            spans.push(make_span(*from, index + 1));
                            state = State::Bare { from: index + 1 };
                        } else {
                            *depth -= 1;
                        }
                    } else if ch == b'(' {
                        *depth += 1;
                    }
                }
            }
            index += 1;
        }

        // Add the final span
        match state {
            State::Bare { from }
            | State::Quote { from, .. }
            | State::Parenthesized { from, .. }
            | State::BackTickQuote { from, .. } => {
                if from < contents.len() {
                    spans.push(make_span(from, contents.len()));
                }
            }
        }

        // Log the spans that will be parsed
        if log::log_enabled!(log::Level::Trace) {
            let contents = spans
                .iter()
                .map(|span| String::from_utf8_lossy(working_set.get_span_contents(*span)))
                .collect::<Vec<_>>();

            trace!("parsing: external string, parts: {contents:?}")
        }

        // Check if the whole thing is quoted. If not, it should be a glob
        let quoted =
            (contents.len() >= 3 && contents.starts_with(b"$\"") && contents.ends_with(b"\""))
                || is_quoted(contents);

        // Parse each as its own string
        let exprs: Vec<Expression> = spans
            .into_iter()
            .map(|span| parse_string(working_set, span))
            .collect();

        if exprs
            .iter()
            .all(|expr| matches!(expr.expr, Expr::String(..)))
        {
            // If the exprs are all strings anyway, just collapse into a single string.
            let string = exprs
                .into_iter()
                .map(|expr| {
                    let Expr::String(contents) = expr.expr else {
                        unreachable!("already checked that this was a String")
                    };
                    contents
                })
                .collect::<String>();
            if quoted {
                Expression::new(working_set, Expr::String(string), span, Type::String)
            } else {
                Expression::new(
                    working_set,
                    Expr::GlobPattern(string, false),
                    span,
                    Type::Glob,
                )
            }
        } else {
            // Flatten any string interpolations contained with the exprs.
            let exprs = exprs
                .into_iter()
                .flat_map(|expr| match expr.expr {
                    Expr::StringInterpolation(subexprs) => subexprs,
                    _ => vec![expr],
                })
                .collect();
            // Make an interpolation out of the expressions. Use `GlobInterpolation` if it's a bare
            // word, so that the unquoted state can get passed through to `run-external`.
            if quoted {
                Expression::new(
                    working_set,
                    Expr::StringInterpolation(exprs),
                    span,
                    Type::String,
                )
            } else {
                Expression::new(
                    working_set,
                    Expr::GlobInterpolation(exprs, false),
                    span,
                    Type::Glob,
                )
            }
        }
    } else {
        parse_glob_pattern(working_set, span)
    }
}

fn parse_external_arg(working_set: &mut StateWorkingSet, span: Span) -> ExternalArgument {
    let contents = working_set.get_span_contents(span);

    if contents.len() > 3
        && contents.starts_with(b"...")
        && (contents[3] == b'$' || contents[3] == b'[' || contents[3] == b'(')
    {
        ExternalArgument::Spread(parse_value(
            working_set,
            Span::new(span.start + 3, span.end),
            &SyntaxShape::List(Box::new(SyntaxShape::Any)),
        ))
    } else {
        ExternalArgument::Regular(parse_regular_external_arg(working_set, span))
    }
}

fn parse_regular_external_arg(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let contents = working_set.get_span_contents(span);

    if contents.starts_with(b"$") {
        parse_dollar_expr(working_set, span)
    } else if contents.starts_with(b"(") {
        parse_paren_expr(working_set, span, &SyntaxShape::Any)
    } else if contents.starts_with(b"[") {
        parse_list_expression(working_set, span, &SyntaxShape::Any)
    } else {
        parse_external_string(working_set, span)
    }
}

pub fn parse_external_call(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
    call_span: Span,
) -> Expression {
    trace!("parse external");

    let head_span = spans[0];

    let head_contents = working_set.get_span_contents(head_span).to_vec();

    let head = if head_contents.starts_with(b"$") || head_contents.starts_with(b"(") {
        // the expression is inside external_call, so it's a subexpression
        let arg = parse_expression(working_set, &[head_span]);
        Box::new(arg)
    } else {
        Box::new(parse_external_string(working_set, head_span))
    };

    let args = spans[1..]
        .iter()
        .map(|&span| parse_external_arg(working_set, span))
        .collect();

    Expression::new(
        working_set,
        Expr::ExternalCall(head, args),
        call_span,
        Type::Any,
    )
}

fn ensure_flag_arg_type(
    working_set: &mut StateWorkingSet,
    arg_name: String,
    arg: Expression,
    arg_shape: &SyntaxShape,
    long_name_span: Span,
) -> (Spanned<String>, Expression) {
    if !type_compatible(&arg.ty, &arg_shape.to_type()) {
        working_set.error(ParseError::TypeMismatch(
            arg_shape.to_type(),
            arg.ty,
            arg.span,
        ));
        (
            Spanned {
                item: arg_name,
                span: long_name_span,
            },
            Expression::garbage(working_set, arg.span),
        )
    } else {
        (
            Spanned {
                item: arg_name,
                span: long_name_span,
            },
            arg,
        )
    }
}

fn parse_long_flag(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
    spans_idx: &mut usize,
    sig: &Signature,
) -> (Option<Spanned<String>>, Option<Expression>) {
    let arg_span = spans[*spans_idx];
    let arg_contents = working_set.get_span_contents(arg_span);

    if arg_contents.starts_with(b"--") {
        // FIXME: only use the first flag you find?
        let split: Vec<_> = arg_contents.split(|x| *x == b'=').collect();
        let long_name = String::from_utf8(split[0].into());
        if let Ok(long_name) = long_name {
            let long_name = long_name[2..].to_string();
            if let Some(flag) = sig.get_long_flag(&long_name) {
                if let Some(arg_shape) = &flag.arg {
                    if split.len() > 1 {
                        // and we also have the argument
                        let long_name_len = long_name.len();
                        let mut span = arg_span;
                        span.start += long_name_len + 3; //offset by long flag and '='

                        let arg = parse_value(working_set, span, arg_shape);
                        let (arg_name, val_expression) = ensure_flag_arg_type(
                            working_set,
                            long_name,
                            arg,
                            arg_shape,
                            Span::new(arg_span.start, arg_span.start + long_name_len + 2),
                        );
                        (Some(arg_name), Some(val_expression))
                    } else if let Some(arg) = spans.get(*spans_idx + 1) {
                        let arg = parse_value(working_set, *arg, arg_shape);

                        *spans_idx += 1;
                        let (arg_name, val_expression) =
                            ensure_flag_arg_type(working_set, long_name, arg, arg_shape, arg_span);
                        (Some(arg_name), Some(val_expression))
                    } else {
                        working_set.error(ParseError::MissingFlagParam(
                            arg_shape.to_string(),
                            arg_span,
                        ));
                        // NOTE: still need to cover this incomplete flag in the final expression
                        // see https://github.com/nushell/nushell/issues/16375
                        (
                            Some(Spanned {
                                item: long_name,
                                span: arg_span,
                            }),
                            None,
                        )
                    }
                } else {
                    // A flag with no argument
                    // It can also takes a boolean value like --x=true
                    if split.len() > 1 {
                        // and we also have the argument
                        let long_name_len = long_name.len();
                        let mut span = arg_span;
                        span.start += long_name_len + 3; //offset by long flag and '='

                        let arg = parse_value(working_set, span, &SyntaxShape::Boolean);

                        let (arg_name, val_expression) = ensure_flag_arg_type(
                            working_set,
                            long_name,
                            arg,
                            &SyntaxShape::Boolean,
                            Span::new(arg_span.start, arg_span.start + long_name_len + 2),
                        );
                        (Some(arg_name), Some(val_expression))
                    } else {
                        (
                            Some(Spanned {
                                item: long_name,
                                span: arg_span,
                            }),
                            None,
                        )
                    }
                }
            } else {
                let suggestion = did_you_mean(sig.get_names(), &long_name)
                    .map(|name| format!("Did you mean: `--{name}`?"))
                    .unwrap_or("Use `--help` to see available flags".to_owned());
                working_set.error(ParseError::UnknownFlag(
                    sig.name.clone(),
                    long_name.clone(),
                    arg_span,
                    suggestion,
                ));
                (
                    Some(Spanned {
                        item: long_name.clone(),
                        span: arg_span,
                    }),
                    None,
                )
            }
        } else {
            working_set.error(ParseError::NonUtf8(arg_span));
            (
                Some(Spanned {
                    item: "--".into(),
                    span: arg_span,
                }),
                None,
            )
        }
    } else {
        (None, None)
    }
}

fn parse_short_flags(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
    spans_idx: &mut usize,
    positional_idx: usize,
    sig: &Signature,
) -> Option<Vec<Flag>> {
    let arg_span = spans[*spans_idx];

    let arg_contents = working_set.get_span_contents(arg_span);

    if let Ok(arg_contents_uft8_ref) = str::from_utf8(arg_contents) {
        if arg_contents_uft8_ref.starts_with('-') && arg_contents_uft8_ref.len() > 1 {
            let short_flags = &arg_contents_uft8_ref[1..];
            let num_chars = short_flags.chars().count();
            let mut found_short_flags = vec![];
            let mut unmatched_short_flags = vec![];
            for (offset, short_flag) in short_flags.char_indices() {
                let short_flag_span = Span::new(
                    arg_span.start + 1 + offset,
                    arg_span.start + 1 + offset + short_flag.len_utf8(),
                );
                if let Some(flag) = sig.get_short_flag(short_flag) {
                    // Allow args in short flag batches as long as it is the last flag.
                    if flag.arg.is_some() && offset < num_chars - 1 {
                        working_set
                            .error(ParseError::OnlyLastFlagInBatchCanTakeArg(short_flag_span));
                        break;
                    }
                    found_short_flags.push(flag);
                } else {
                    unmatched_short_flags.push(short_flag_span);
                }
            }

            if found_short_flags.is_empty()
                // check to see if we have a negative number
                && matches!(
                    sig.get_positional(positional_idx),
                    Some(PositionalArg {
                        shape: SyntaxShape::Int | SyntaxShape::Number | SyntaxShape::Float,
                        ..
                    })
                )
                && String::from_utf8_lossy(working_set.get_span_contents(arg_span))
                    .parse::<f64>()
                    .is_ok()
            {
                return None;
            } else if let Some(first) = unmatched_short_flags.first() {
                let contents = working_set.get_span_contents(*first);
                working_set.error(ParseError::UnknownFlag(
                    sig.name.clone(),
                    format!("-{}", String::from_utf8_lossy(contents)),
                    *first,
                    "Use `--help` to see available flags".to_owned(),
                ));
            }

            Some(found_short_flags)
        } else {
            None
        }
    } else {
        working_set.error(ParseError::NonUtf8(arg_span));
        None
    }
}

fn first_kw_idx(
    working_set: &StateWorkingSet,
    signature: &Signature,
    spans: &[Span],
    spans_idx: usize,
    positional_idx: usize,
) -> (Option<usize>, usize) {
    for idx in (positional_idx + 1)..signature.num_positionals() {
        if let Some(PositionalArg {
            shape: SyntaxShape::Keyword(kw, ..),
            ..
        }) = signature.get_positional(idx)
        {
            for (span_idx, &span) in spans.iter().enumerate().skip(spans_idx) {
                let contents = working_set.get_span_contents(span);

                if contents == kw {
                    return (Some(idx), span_idx);
                }
            }
        }
    }
    (None, spans.len())
}

fn calculate_end_span(
    working_set: &StateWorkingSet,
    signature: &Signature,
    spans: &[Span],
    spans_idx: usize,
    positional_idx: usize,
) -> usize {
    if signature.rest_positional.is_some() {
        spans.len()
    } else {
        let (kw_pos, kw_idx) =
            first_kw_idx(working_set, signature, spans, spans_idx, positional_idx);

        if let Some(kw_pos) = kw_pos {
            // We found a keyword. Keywords, once found, create a guidepost to
            // show us where the positionals will lay into the arguments. Because they're
            // keywords, they get to set this by being present

            let positionals_between = kw_pos - positional_idx - 1;
            if positionals_between >= (kw_idx - spans_idx) {
                kw_idx
            } else {
                kw_idx - positionals_between
            }
        } else {
            // Make space for the remaining require positionals, if we can
            // spans_idx < spans.len() is an invariant
            let remaining_spans = spans.len() - (spans_idx + 1);
            // positional_idx can be larger than required_positional.len() if we have optional args
            let remaining_positional = signature
                .required_positional
                .len()
                .saturating_sub(positional_idx + 1);
            // Saturates to 0 when we have too few args
            let extra_spans = remaining_spans.saturating_sub(remaining_positional);
            spans_idx + 1 + extra_spans
        }
    }
}

fn parse_oneof(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
    spans_idx: &mut usize,
    possible_shapes: &Vec<SyntaxShape>,
    multispan: bool,
) -> Expression {
    let starting_spans_idx = *spans_idx;
    let mut best_guess = None;
    let mut best_guess_errors = Vec::new();
    let mut max_first_error_offset = 0;
    let mut propagate_error = false;
    for shape in possible_shapes {
        let starting_error_count = working_set.parse_errors.len();
        *spans_idx = starting_spans_idx;
        let value = match multispan {
            true => parse_multispan_value(working_set, spans, spans_idx, shape),
            false => parse_value(working_set, spans[*spans_idx], shape),
        };

        let new_errors = working_set.parse_errors[starting_error_count..].to_vec();
        // no new errors found means success
        let Some(first_error_offset) = new_errors.iter().map(|e| e.span().start).min() else {
            return value;
        };

        if first_error_offset > max_first_error_offset {
            // while trying the possible shapes, ignore Expected type errors
            // unless they're inside a block, closure, or expression
            propagate_error = match working_set.parse_errors.last() {
                Some(ParseError::Expected(_, error_span))
                | Some(ParseError::ExpectedWithStringMsg(_, error_span)) => {
                    matches!(
                        shape,
                        SyntaxShape::Block | SyntaxShape::Closure(_) | SyntaxShape::Expression
                    ) && *error_span != spans[*spans_idx]
                }
                _ => true,
            };
            max_first_error_offset = first_error_offset;
            best_guess = Some(value);
            best_guess_errors = new_errors;
        }
        working_set.parse_errors.truncate(starting_error_count);
    }

    // if best_guess results in new errors further than current span, then accept it
    // or propagate_error is marked as true for it
    if max_first_error_offset > spans[starting_spans_idx].start || propagate_error {
        working_set.parse_errors.extend(best_guess_errors);
        best_guess.expect("best_guess should not be None here!")
    } else {
        working_set.error(ParseError::ExpectedWithStringMsg(
            format!("one of a list of accepted shapes: {possible_shapes:?}"),
            spans[starting_spans_idx],
        ));
        Expression::garbage(working_set, spans[starting_spans_idx])
    }
}

pub fn parse_multispan_value(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
    spans_idx: &mut usize,
    shape: &SyntaxShape,
) -> Expression {
    trace!("parse multispan value");
    match shape {
        SyntaxShape::VarWithOptType => {
            trace!("parsing: var with opt type");

            parse_var_with_opt_type(working_set, spans, spans_idx, false).0
        }
        SyntaxShape::RowCondition => {
            trace!("parsing: row condition");
            let arg = parse_row_condition(working_set, &spans[*spans_idx..]);
            *spans_idx = spans.len() - 1;

            arg
        }
        SyntaxShape::MathExpression => {
            trace!("parsing: math expression");

            let arg = parse_math_expression(working_set, &spans[*spans_idx..], None);
            *spans_idx = spans.len() - 1;

            arg
        }
        SyntaxShape::OneOf(possible_shapes) => {
            parse_oneof(working_set, spans, spans_idx, possible_shapes, true)
        }

        SyntaxShape::Expression => {
            trace!("parsing: expression");

            // is it subexpression?
            // Not sure, but let's make it not, so the behavior is the same as previous version of nushell.
            let arg = parse_expression(working_set, &spans[*spans_idx..]);
            *spans_idx = spans.len().saturating_sub(1);

            arg
        }
        SyntaxShape::Signature => {
            trace!("parsing: signature");

            let sig = parse_full_signature(working_set, &spans[*spans_idx..], false);
            *spans_idx = spans.len().saturating_sub(1);

            sig
        }
        SyntaxShape::ExternalSignature => {
            trace!("parsing: external signature");

            let sig = parse_full_signature(working_set, &spans[*spans_idx..], true);
            *spans_idx = spans.len().saturating_sub(1);

            sig
        }
        SyntaxShape::Keyword(keyword, arg) => {
            trace!(
                "parsing: keyword({}) {:?}",
                String::from_utf8_lossy(keyword),
                arg
            );
            let arg_span = spans[*spans_idx];

            let arg_contents = working_set.get_span_contents(arg_span);

            if arg_contents != keyword {
                // When keywords mismatch, this is a strong indicator of something going wrong.
                // We won't often override the current error, but as this is a strong indicator
                // go ahead and override the current error and tell the user about the missing
                // keyword/literal.
                working_set.error(ParseError::ExpectedKeyword(
                    String::from_utf8_lossy(keyword).into(),
                    arg_span,
                ))
            }

            *spans_idx += 1;
            if *spans_idx >= spans.len() {
                working_set.error(ParseError::KeywordMissingArgument(
                    arg.to_string(),
                    String::from_utf8_lossy(keyword).into(),
                    Span::new(spans[*spans_idx - 1].end, spans[*spans_idx - 1].end),
                ));
                let keyword = Keyword {
                    keyword: keyword.as_slice().into(),
                    span: spans[*spans_idx - 1],
                    expr: Expression::garbage(working_set, arg_span),
                };
                return Expression::new(
                    working_set,
                    Expr::Keyword(Box::new(keyword)),
                    arg_span,
                    Type::Any,
                );
            }

            let keyword = Keyword {
                keyword: keyword.as_slice().into(),
                span: spans[*spans_idx - 1],
                expr: parse_multispan_value(working_set, spans, spans_idx, arg),
            };

            Expression::new(
                working_set,
                Expr::Keyword(Box::new(keyword.clone())),
                keyword.span.merge(keyword.expr.span),
                keyword.expr.ty,
            )
        }
        _ => {
            // All other cases are single-span values
            let arg_span = spans[*spans_idx];

            parse_value(working_set, arg_span, shape)
        }
    }
}

pub struct ParsedInternalCall {
    pub call: Box<Call>,
    pub output: Type,
    pub call_kind: CallKind,
}

/// Sometimes the arguments of an internal command need to be parsed in dedicated functions, e.g. `parse_module`.
/// If so, `parse_internal_call` should be called with the appropriate parsing level to avoid repetition.
///
/// Defaults to `ArgumentParsingLevel::Full`
#[derive(Default)]
pub enum ArgumentParsingLevel {
    #[default]
    Full,
    /// Parse only the first `k` arguments
    FirstK { k: usize },
}

pub fn parse_internal_call(
    working_set: &mut StateWorkingSet,
    command_span: Span,
    spans: &[Span],
    decl_id: DeclId,
    arg_parsing_level: ArgumentParsingLevel,
) -> ParsedInternalCall {
    trace!("parsing: internal call (decl id: {})", decl_id.get());

    let mut call = Call::new(command_span);
    call.decl_id = decl_id;
    call.head = command_span;
    let _ = working_set.add_span(call.head);

    let decl = working_set.get_decl(decl_id);
    let signature = working_set.get_signature(decl);
    let output = signature.get_output_type();

    let deprecation = decl.deprecation_info();

    // storing the var ID for later due to borrowing issues
    let lib_dirs_var_id = match decl.name() {
        "use" | "overlay use" | "source-env" if decl.is_keyword() => {
            find_dirs_var(working_set, LIB_DIRS_VAR)
        }
        "nu-check" if decl.is_builtin() => find_dirs_var(working_set, LIB_DIRS_VAR),
        _ => None,
    };

    // The index into the positional parameter in the definition
    let mut positional_idx = 0;

    // The index into the spans of argument data given to parse
    // Starting at the first argument
    let mut spans_idx = 0;

    if let Some(alias) = decl.as_alias() {
        if let Expression {
            expr: Expr::Call(wrapped_call),
            ..
        } = &alias.wrapped_call
        {
            // Replace this command's call with the aliased call, but keep the alias name
            call = *wrapped_call.clone();
            call.head = command_span;
            // Skip positionals passed to aliased call
            positional_idx = call.positional_len();
        } else {
            working_set.error(ParseError::UnknownState(
                "Alias does not point to internal call.".to_string(),
                command_span,
            ));
            return ParsedInternalCall {
                call: Box::new(call),
                output: Type::Any,
                call_kind: CallKind::Invalid,
            };
        }
    }

    if let Some(var_id) = lib_dirs_var_id {
        call.set_parser_info(
            DIR_VAR_PARSER_INFO.to_owned(),
            Expression::new(working_set, Expr::Var(var_id), call.head, Type::Any),
        );
    }

    if signature.creates_scope {
        working_set.enter_scope();
    }

    while spans_idx < spans.len() {
        let arg_span = spans[spans_idx];

        let starting_error_count = working_set.parse_errors.len();
        // Check if we're on a long flag, if so, parse
        let (long_name, arg) = parse_long_flag(working_set, spans, &mut spans_idx, &signature);

        if let Some(long_name) = long_name {
            // We found a long flag, like --bar
            if working_set.parse_errors[starting_error_count..]
                .iter()
                .any(|x| matches!(x, ParseError::UnknownFlag(_, _, _, _)))
                && signature.allows_unknown_args
            {
                working_set.parse_errors.truncate(starting_error_count);
                let arg = parse_unknown_arg(working_set, arg_span, &signature);

                call.add_unknown(arg);
            } else {
                call.add_named((long_name, None, arg));
            }

            spans_idx += 1;
            continue;
        }

        let starting_error_count = working_set.parse_errors.len();

        // Check if we're on a short flag or group of short flags, if so, parse
        let short_flags = parse_short_flags(
            working_set,
            spans,
            &mut spans_idx,
            positional_idx,
            &signature,
        );

        if let Some(mut short_flags) = short_flags {
            if short_flags.is_empty() {
                // workaround for completions (PR #6067)
                short_flags.push(Flag {
                    long: "".to_string(),
                    short: Some('a'),
                    arg: None,
                    required: false,
                    desc: "".to_string(),
                    var_id: None,
                    default_value: None,
                    completion: None,
                })
            }

            if working_set.parse_errors[starting_error_count..]
                .iter()
                .any(|x| matches!(x, ParseError::UnknownFlag(_, _, _, _)))
                && signature.allows_unknown_args
            {
                working_set.parse_errors.truncate(starting_error_count);
                let arg = parse_unknown_arg(working_set, arg_span, &signature);

                call.add_unknown(arg);
            } else {
                for flag in short_flags {
                    let _ = working_set.add_span(spans[spans_idx]);

                    if let Some(arg_shape) = flag.arg {
                        if let Some(arg) = spans.get(spans_idx + 1) {
                            let arg = parse_value(working_set, *arg, &arg_shape);
                            let (arg_name, val_expression) = ensure_flag_arg_type(
                                working_set,
                                flag.long.clone(),
                                arg.clone(),
                                &arg_shape,
                                spans[spans_idx],
                            );

                            if flag.long.is_empty() {
                                if let Some(short) = flag.short {
                                    call.add_named((
                                        arg_name,
                                        Some(Spanned {
                                            item: short.to_string(),
                                            span: spans[spans_idx],
                                        }),
                                        Some(val_expression),
                                    ));
                                }
                            } else {
                                call.add_named((arg_name, None, Some(val_expression)));
                            }
                            spans_idx += 1;
                        } else {
                            working_set.error(ParseError::MissingFlagParam(
                                arg_shape.to_string(),
                                arg_span,
                            ));
                            // NOTE: still need to cover this incomplete flag in the final expression
                            // see https://github.com/nushell/nushell/issues/16375
                            call.add_named((
                                Spanned {
                                    item: String::new(),
                                    span: spans[spans_idx],
                                },
                                None,
                                None,
                            ));
                        }
                    } else if flag.long.is_empty() {
                        if let Some(short) = flag.short {
                            call.add_named((
                                Spanned {
                                    item: String::new(),
                                    span: spans[spans_idx],
                                },
                                Some(Spanned {
                                    item: short.to_string(),
                                    span: spans[spans_idx],
                                }),
                                None,
                            ));
                        }
                    } else {
                        call.add_named((
                            Spanned {
                                item: flag.long.clone(),
                                span: spans[spans_idx],
                            },
                            None,
                            None,
                        ));
                    }
                }
            }

            spans_idx += 1;
            continue;
        }

        {
            let contents = working_set.get_span_contents(spans[spans_idx]);

            if contents.len() > 3
                && contents.starts_with(b"...")
                && (contents[3] == b'$' || contents[3] == b'[' || contents[3] == b'(')
            {
                if signature.rest_positional.is_none() && !signature.allows_unknown_args {
                    working_set.error(ParseError::UnexpectedSpreadArg(
                        signature.call_signature(),
                        arg_span,
                    ));
                    call.add_positional(Expression::garbage(working_set, arg_span));
                } else if positional_idx < signature.required_positional.len() {
                    working_set.error(ParseError::MissingPositional(
                        signature.required_positional[positional_idx].name.clone(),
                        Span::new(spans[spans_idx].start, spans[spans_idx].start),
                        signature.call_signature(),
                    ));
                    call.add_positional(Expression::garbage(working_set, arg_span));
                } else {
                    let rest_shape = match &signature.rest_positional {
                        Some(arg) if matches!(arg.shape, SyntaxShape::ExternalArgument) => {
                            // External args aren't parsed inside lists in spread position.
                            SyntaxShape::Any
                        }
                        Some(arg) => arg.shape.clone(),
                        None => SyntaxShape::Any,
                    };
                    // Parse list of arguments to be spread
                    let args = parse_value(
                        working_set,
                        Span::new(arg_span.start + 3, arg_span.end),
                        &SyntaxShape::List(Box::new(rest_shape)),
                    );

                    call.add_spread(args);
                    // Let the parser know that it's parsing rest arguments now
                    positional_idx =
                        signature.required_positional.len() + signature.optional_positional.len();
                }

                spans_idx += 1;
                continue;
            }
        }

        // Parse a positional arg if there is one
        if let Some(positional) = signature.get_positional(positional_idx) {
            let end = calculate_end_span(working_set, &signature, spans, spans_idx, positional_idx);

            // Missing arguments before next keyword
            if end == spans_idx {
                let prev_span = if spans_idx == 0 {
                    command_span
                } else {
                    spans[spans_idx - 1]
                };
                let whitespace_span = Span::new(prev_span.end, spans[spans_idx].start);
                working_set.error(ParseError::MissingPositional(
                    positional.name.clone(),
                    whitespace_span,
                    signature.call_signature(),
                ));
                call.add_positional(Expression::garbage(working_set, whitespace_span));
                positional_idx += 1;
                continue;
            }
            debug_assert!(end <= spans.len());

            if spans[..end].is_empty() || spans_idx == end {
                working_set.error(ParseError::MissingPositional(
                    positional.name.clone(),
                    Span::new(spans[spans_idx].end, spans[spans_idx].end),
                    signature.call_signature(),
                ));
                positional_idx += 1;
                continue;
            }

            let compile_error_count = working_set.compile_errors.len();

            // HACK: avoid repeated parsing of argument values in special cases
            // see https://github.com/nushell/nushell/issues/16398
            let arg = match arg_parsing_level {
                ArgumentParsingLevel::FirstK { k } if k <= positional_idx => {
                    Expression::garbage(working_set, spans[spans_idx])
                }
                _ => parse_multispan_value(
                    working_set,
                    &spans[..end],
                    &mut spans_idx,
                    &positional.shape,
                ),
            };

            // HACK: try-catch's signature defines the catch block as a Closure, even though it's
            // used like a Block. Because closures are compiled eagerly, this ends up making the
            // following code technically invalid:
            // ```nu
            // loop { try { } catch {|e| break } }
            // ```
            // Thus, we discard the compilation error here
            if let SyntaxShape::OneOf(ref shapes) = positional.shape {
                for one_shape in shapes {
                    if let SyntaxShape::Keyword(keyword, ..) = one_shape
                        && keyword == b"catch"
                        && let [nu_protocol::CompileError::NotInALoop { .. }] =
                            &working_set.compile_errors[compile_error_count..]
                    {
                        working_set.compile_errors.truncate(compile_error_count);
                    }
                }
            }

            let arg = if !type_compatible(&positional.shape.to_type(), &arg.ty) {
                working_set.error(ParseError::TypeMismatch(
                    positional.shape.to_type(),
                    arg.ty,
                    arg.span,
                ));
                Expression::garbage(working_set, arg.span)
            } else {
                arg
            };

            call.add_positional(arg);
            positional_idx += 1;
        } else if signature.allows_unknown_args {
            let arg = parse_unknown_arg(working_set, arg_span, &signature);

            call.add_unknown(arg);
        } else {
            call.add_positional(Expression::garbage(working_set, arg_span));
            working_set.error(ParseError::ExtraPositional(
                signature.call_signature(),
                arg_span,
            ))
        }

        spans_idx += 1;
    }

    // TODO: Inline `check_call`,
    // move missing positional checking into the while loop above with two pointers.
    // Maybe more `CallKind::Invalid` if errors found during argument parsing.
    let call_kind = check_call(working_set, command_span, &signature, &call);

    deprecation
        .into_iter()
        .filter_map(|entry| entry.parse_warning(&signature.name, &call))
        .for_each(|warning| {
            // FIXME: if two flags are deprecated and both are used in one command,
            // the second flag's deprecation won't show until the first flag is removed
            // (but it won't be flagged as reported until it is actually reported)
            working_set.warning(warning);
        });

    if signature.creates_scope {
        working_set.exit_scope();
    }

    ParsedInternalCall {
        call: Box::new(call),
        output,
        call_kind,
    }
}

pub fn parse_call(working_set: &mut StateWorkingSet, spans: &[Span], head: Span) -> Expression {
    trace!("parsing: call");
    let call_span = Span::concat(spans);

    if spans.is_empty() {
        working_set.error(ParseError::UnknownState(
            "Encountered command with zero spans".into(),
            call_span,
        ));
        return garbage(working_set, head);
    }

    let call_sigil = match working_set.get_span_contents(spans[0]).first() {
        Some(b'^') => Some(b'^'),
        Some(b'%') => Some(b'%'),
        _ => None,
    };

    let mut adjusted_spans = Vec::new();
    let resolution_spans = match call_sigil {
        Some(b'^') | Some(b'%') => {
            adjusted_spans.reserve(spans.len());
            adjusted_spans.push(Span::new(spans[0].start + 1, spans[0].end));
            adjusted_spans.extend_from_slice(&spans[1..]);
            adjusted_spans.as_slice()
        }
        _ => spans,
    };

    // `^` always forces external command parsing and must bypass declaration
    // resolution, even when an internal command with the same name exists.
    if call_sigil == Some(b'^') {
        trace!("parsing: forced external call");
        return parse_external_call(working_set, resolution_spans, call_span);
    }

    let (cmd_start, pos, _name, maybe_decl_id) = if call_sigil == Some(b'%') {
        find_longest_decl_with_command_type(working_set, resolution_spans, CommandType::Builtin)
    } else {
        find_longest_decl(working_set, resolution_spans)
    };

    if let Some(decl_id) = maybe_decl_id {
        // Before the internal parsing we check if there is no let or alias declarations
        // that are missing their name, e.g.: let = 1 or alias = 2
        if resolution_spans.len() > 1 {
            let test_equal = working_set.get_span_contents(resolution_spans[1]);

            if test_equal == [b'='] {
                trace!("incomplete statement");

                working_set.error(ParseError::UnknownState(
                    "Incomplete statement".into(),
                    call_span,
                ));
                return garbage(working_set, call_span);
            }
        }

        let decl = working_set.get_decl(decl_id);

        let parsed_call = if let Some(alias) = decl.as_alias() {
            if let Expression {
                expr: Expr::ExternalCall(head, args),
                span: _,
                span_id: _,
                ty,
            } = &alias.clone().wrapped_call
            {
                trace!("parsing: alias of external call");

                let mut head = head.clone();
                head.span = Span::concat(&resolution_spans[cmd_start..pos]); // replacing the spans preserves syntax highlighting

                let mut final_args = args.clone().into_vec();
                for arg_span in &resolution_spans[pos..] {
                    let arg = parse_external_arg(working_set, *arg_span);
                    final_args.push(arg);
                }

                let expression = Expression::new(
                    working_set,
                    Expr::ExternalCall(head, final_args.into()),
                    Span::concat(spans),
                    ty.clone(),
                );

                return expression;
            } else {
                trace!("parsing: alias of internal call");
                parse_internal_call(
                    working_set,
                    Span::concat(&resolution_spans[cmd_start..pos]),
                    &resolution_spans[pos..],
                    decl_id,
                    ArgumentParsingLevel::Full,
                )
            }
        } else {
            trace!("parsing: internal call");
            parse_internal_call(
                working_set,
                Span::concat(&resolution_spans[cmd_start..pos]),
                &resolution_spans[pos..],
                decl_id,
                ArgumentParsingLevel::Full,
            )
        };

        Expression::new(
            working_set,
            Expr::Call(parsed_call.call),
            call_span,
            parsed_call.output,
        )
    } else {
        if call_sigil == Some(b'%') {
            working_set.error(ParseError::LabeledErrorWithHelp {
                error: "percent sigil requires a built-in command".into(),
                label: "unknown built-in command".into(),
                help:
                    "remove `%` to use normal resolution, or use `^` to run an external command explicitly".into(),
                span: resolution_spans[0],
            });

            // Preserve expression shape for features like completion while retaining the parse error.
            return parse_external_call(working_set, spans, call_span);
        }

        // We might be parsing left-unbounded range ("..10")
        let bytes = working_set.get_span_contents(spans[0]);
        trace!("parsing: range {bytes:?}");
        if let (Some(b'.'), Some(b'.')) = (bytes.first(), bytes.get(1)) {
            trace!("-- found leading range indicator");
            let starting_error_count = working_set.parse_errors.len();

            if let Some(range_expr) = parse_range(working_set, spans[0]) {
                trace!("-- successfully parsed range");
                return range_expr;
            }
            working_set.parse_errors.truncate(starting_error_count);
        }
        trace!("parsing: external call");

        // Otherwise, try external command
        parse_external_call(working_set, spans, call_span)
    }
}

fn find_decl_with_command_type(
    working_set: &StateWorkingSet<'_>,
    name: &[u8],
    command_type: CommandType,
) -> Option<DeclId> {
    // Search all known declarations so `%cmd` can still resolve a built-in even when
    // a custom command with the same name shadows it in normal visibility lookup.
    for idx in (0..working_set.num_decls()).rev() {
        let decl_id = DeclId::new(idx);
        let decl = working_set.get_decl(decl_id);
        if decl.command_type() == command_type && decl.name().as_bytes() == name {
            return Some(decl_id);
        }
    }

    None
}

// Build a command name from spaced spans, preserving the existing parser command-name behavior.
fn command_name_from_spans(
    working_set: &StateWorkingSet<'_>,
    spans: &[Span],
    prefix: &[u8],
) -> Vec<u8> {
    let mut name = Vec::with_capacity(prefix.len() + spans.len() * 2);
    name.extend(prefix);

    for span in spans {
        let name_part = working_set.get_span_contents(*span);
        if name.is_empty() {
            name.extend(name_part);
        } else {
            name.push(b' ');
            name.extend(name_part);
        }
    }

    name
}

// Variant of `find_longest_decl` that constrains matches to a specific command type.
fn find_longest_decl_with_command_type(
    working_set: &StateWorkingSet<'_>,
    spans: &[Span],
    command_type: CommandType,
) -> (
    usize,
    usize,
    Vec<u8>,
    Option<nu_protocol::Id<nu_protocol::marker::Decl>>,
) {
    let mut pos = spans.len();
    let cmd_start = 0;
    let mut name_spans = spans.to_vec();

    let mut name = command_name_from_spans(working_set, &name_spans, b"");

    let mut maybe_decl_id = find_decl_with_command_type(working_set, &name, command_type);

    while maybe_decl_id.is_none() {
        if name_spans.len() <= 1 {
            break;
        }

        name_spans.pop();
        pos -= 1;

        name = command_name_from_spans(working_set, &name_spans, b"");

        maybe_decl_id = find_decl_with_command_type(working_set, &name, command_type);
    }

    (cmd_start, pos, name, maybe_decl_id)
}

pub fn find_longest_decl(
    working_set: &mut StateWorkingSet<'_>,
    spans: &[Span],
) -> (
    usize,
    usize,
    Vec<u8>,
    Option<nu_protocol::Id<nu_protocol::marker::Decl>>,
) {
    find_longest_decl_with_prefix(working_set, spans, b"")
}

pub fn find_longest_decl_with_prefix(
    working_set: &mut StateWorkingSet<'_>,
    spans: &[Span],
    prefix: &[u8],
) -> (
    usize,
    usize,
    Vec<u8>,
    Option<nu_protocol::Id<nu_protocol::marker::Decl>>,
) {
    let mut pos = 0;
    let cmd_start = pos;
    let mut name_spans = vec![];

    for word_span in spans[cmd_start..].iter() {
        // Find the longest group of words that could form a command

        name_spans.push(*word_span);

        pos += 1;
    }

    let mut name = command_name_from_spans(working_set, &name_spans, prefix);

    let mut maybe_decl_id = working_set.find_decl(&name);

    while maybe_decl_id.is_none() {
        // Find the longest command match
        if name_spans.len() <= 1 {
            // Keep the first word even if it does not match -- could be external command
            break;
        }

        name_spans.pop();
        pos -= 1;

        name = command_name_from_spans(working_set, &name_spans, prefix);
        maybe_decl_id = working_set.find_decl(&name);
    }

    // If there is a declaration and there are remaining spans, check if it's an alias.
    // If it is, try to see if there are sub commands
    if let Some(decl_id) = maybe_decl_id
        && pos < spans.len()
    {
        let decl = working_set.get_decl(decl_id);
        if let Some(alias) = decl.as_alias() {
            // Extract the command name from the alias
            // The wrapped_call should be a Call expression for internal commands
            if let Expression {
                expr: Expr::Call(call),
                ..
            } = &alias.wrapped_call
            {
                let aliased_decl_id = call.decl_id;
                let aliased_name = working_set.get_decl(aliased_decl_id).name().to_string();

                // Try to find a longer match using the aliased command name with remaining spans
                let (_, new_pos, new_name, new_decl_id) = find_longest_decl_with_prefix(
                    working_set,
                    &spans[pos..],
                    aliased_name.as_bytes(),
                );

                // If we find a sub command, use it instead.
                if new_decl_id.is_some() && new_pos > 0 {
                    let total_pos = pos + new_pos;
                    return (cmd_start, total_pos, new_name, new_decl_id);
                }
            }
        }
    }

    (cmd_start, pos, name, maybe_decl_id)
}

pub fn parse_attribute(
    working_set: &mut StateWorkingSet,
    lite_command: &LiteCommand,
) -> (Attribute, Option<String>) {
    let _ = lite_command
        .parts
        .first()
        .filter(|s| working_set.get_span_contents(**s).starts_with(b"@"))
        .expect("Attributes always start with an `@`");

    assert!(
        lite_command.attribute_idx.is_empty(),
        "attributes can't have attributes"
    );

    let mut spans = lite_command.parts.clone();
    if let Some(first) = spans.first_mut() {
        first.start += 1;
    }
    let spans = spans.as_slice();
    let attr_span = Span::concat(spans);

    let (cmd_start, cmd_end, mut name, decl_id) =
        find_longest_decl_with_prefix(working_set, spans, b"attr");

    debug_assert!(name.starts_with(b"attr "));
    let _ = name.drain(..(b"attr ".len()));

    let name_span = Span::concat(&spans[cmd_start..cmd_end]);

    let Ok(name) = String::from_utf8(name) else {
        working_set.error(ParseError::NonUtf8(name_span));
        return (
            Attribute {
                expr: garbage(working_set, attr_span),
            },
            None,
        );
    };

    let Some(decl_id) = decl_id else {
        working_set.error(ParseError::UnknownCommand(name_span));
        return (
            Attribute {
                expr: garbage(working_set, attr_span),
            },
            None,
        );
    };

    let decl = working_set.get_decl(decl_id);

    let parsed_call = match decl.as_alias() {
        // TODO: Once `const def` is available, we should either disallow aliases as attributes OR
        // allow them but rather than using the aliases' name, use the name of the aliased command
        Some(alias) => match &alias.clone().wrapped_call {
            Expression {
                expr: Expr::ExternalCall(..),
                ..
            } => {
                let shell_error = ShellError::NotAConstCommand { span: name_span };
                working_set.error(shell_error.wrap(working_set, attr_span));
                return (
                    Attribute {
                        expr: garbage(working_set, Span::concat(spans)),
                    },
                    None,
                );
            }
            _ => {
                trace!("parsing: alias of internal call");
                parse_internal_call(
                    working_set,
                    name_span,
                    &spans[cmd_end..],
                    decl_id,
                    ArgumentParsingLevel::Full,
                )
            }
        },
        None => {
            trace!("parsing: internal call");
            parse_internal_call(
                working_set,
                name_span,
                &spans[cmd_end..],
                decl_id,
                ArgumentParsingLevel::Full,
            )
        }
    };

    (
        Attribute {
            expr: Expression::new(
                working_set,
                Expr::Call(parsed_call.call),
                Span::concat(spans),
                parsed_call.output,
            ),
        },
        Some(name),
    )
}

pub fn parse_binary(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    trace!("parsing: binary");
    let contents = working_set.get_span_contents(span);
    if contents.starts_with(b"0x[") {
        parse_binary_with_base(working_set, span, 16, 2, b"0x[", b"]")
    } else if contents.starts_with(b"0o[") {
        parse_binary_with_base(working_set, span, 8, 3, b"0o[", b"]")
    } else if contents.starts_with(b"0b[") {
        parse_binary_with_base(working_set, span, 2, 8, b"0b[", b"]")
    } else {
        working_set.error(ParseError::Expected("binary", span));
        garbage(working_set, span)
    }
}

fn parse_binary_with_base(
    working_set: &mut StateWorkingSet,
    span: Span,
    base: u32,
    min_digits_per_byte: usize,
    prefix: &[u8],
    suffix: &[u8],
) -> Expression {
    let token = working_set.get_span_contents(span);

    if let Some(token) = token.strip_prefix(prefix)
        && let Some(token) = token.strip_suffix(suffix)
    {
        let (lexed, err) = lex(
            token,
            span.start + prefix.len(),
            &[b',', b'\r', b'\n'],
            &[],
            true,
        );
        if let Some(err) = err {
            working_set.error(err);
        }

        let mut binary_value = vec![];
        for token in lexed {
            match token.contents {
                TokenContents::Item => {
                    let contents = working_set.get_span_contents(token.span);

                    binary_value.extend_from_slice(contents);
                }
                TokenContents::Pipe
                | TokenContents::PipePipe
                | TokenContents::ErrGreaterPipe
                | TokenContents::OutGreaterThan
                | TokenContents::OutErrGreaterPipe
                | TokenContents::OutGreaterGreaterThan
                | TokenContents::ErrGreaterThan
                | TokenContents::ErrGreaterGreaterThan
                | TokenContents::OutErrGreaterThan
                | TokenContents::OutErrGreaterGreaterThan
                | TokenContents::AssignmentOperator => {
                    working_set.error(ParseError::Expected("binary", span));
                    return garbage(working_set, span);
                }
                TokenContents::Comment | TokenContents::Semicolon | TokenContents::Eol => {}
            }
        }

        let required_padding =
            (min_digits_per_byte - binary_value.len() % min_digits_per_byte) % min_digits_per_byte;

        if required_padding != 0 {
            binary_value = {
                let mut tail = binary_value;
                let mut binary_value: Vec<u8> = vec![b'0'; required_padding];
                binary_value.append(&mut tail);
                binary_value
            };
        }

        let str = String::from_utf8_lossy(&binary_value).to_string();

        match decode_with_base(&str, base, min_digits_per_byte) {
            Ok(v) => return Expression::new(working_set, Expr::Binary(v), span, Type::Binary),
            Err(help) => {
                working_set.error(ParseError::InvalidBinaryString(span, help.to_string()));
                return garbage(working_set, span);
            }
        }
    }

    working_set.error(ParseError::Expected("binary", span));
    garbage(working_set, span)
}

fn decode_with_base(s: &str, base: u32, digits_per_byte: usize) -> Result<Vec<u8>, &str> {
    s.chars()
        .chunks(digits_per_byte)
        .into_iter()
        .map(|chunk| {
            let str: String = chunk.collect();
            u8::from_str_radix(&str, base).map_err(|_| match base {
                2 => "binary strings may contain only 0 or 1.",
                8 => "octal strings must have a length that is a multiple of three and contain values between 0o000 and 0o377.",
                16 => "hexadecimal strings may contain only the characters 0–9 and A–F.",
                _ => "internal error: radix other than 2, 8, or 16 is not allowed."
            })
        })
        .collect()
}

fn strip_underscores(token: &[u8]) -> String {
    String::from_utf8_lossy(token)
        .chars()
        .filter(|c| *c != '_')
        .collect()
}

pub fn parse_int(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let token = working_set.get_span_contents(span);

    fn extract_int(
        working_set: &mut StateWorkingSet,
        token: &str,
        span: Span,
        radix: u32,
    ) -> Expression {
        // Parse as a u64, then cast to i64, otherwise, for numbers like "0xffffffffffffffef",
        // you'll get `Error parsing hex string: number too large to fit in target type`.
        if let Ok(num) = u64::from_str_radix(token, radix).map(|val| val as i64) {
            Expression::new(working_set, Expr::Int(num), span, Type::Int)
        } else {
            working_set.error(ParseError::InvalidLiteral(
                format!("invalid digits for radix {radix}"),
                "int".into(),
                span,
            ));

            garbage(working_set, span)
        }
    }

    let token = strip_underscores(token);

    if token.is_empty() {
        working_set.error(ParseError::Expected("int", span));
        return garbage(working_set, span);
    }

    if let Some(num) = token.strip_prefix("0b") {
        extract_int(working_set, num, span, 2)
    } else if let Some(num) = token.strip_prefix("0o") {
        extract_int(working_set, num, span, 8)
    } else if let Some(num) = token.strip_prefix("0x") {
        extract_int(working_set, num, span, 16)
    } else if let Ok(num) = token.parse::<i64>() {
        Expression::new(working_set, Expr::Int(num), span, Type::Int)
    } else {
        working_set.error(ParseError::Expected("int", span));
        garbage(working_set, span)
    }
}

pub fn parse_float(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let token = working_set.get_span_contents(span);
    let token = strip_underscores(token);

    if let Ok(x) = token.parse::<f64>() {
        Expression::new(working_set, Expr::Float(x), span, Type::Float)
    } else {
        working_set.error(ParseError::Expected("float", span));

        garbage(working_set, span)
    }
}

pub fn parse_number(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let starting_error_count = working_set.parse_errors.len();

    let result = parse_int(working_set, span);
    if starting_error_count == working_set.parse_errors.len() {
        return result;
    } else if let Some(ParseError::Expected(_, _)) = working_set.parse_errors.last() {
        working_set.parse_errors.truncate(starting_error_count);
    }

    let result = parse_float(working_set, span);

    if starting_error_count == working_set.parse_errors.len() {
        return result;
    }
    working_set.parse_errors.truncate(starting_error_count);

    working_set.error(ParseError::Expected("number", span));
    garbage(working_set, span)
}

pub fn parse_range(working_set: &mut StateWorkingSet, span: Span) -> Option<Expression> {
    trace!("parsing: range");
    let starting_error_count = working_set.parse_errors.len();

    // Range follows the following syntax: [<from>][<next_operator><next>]<range_operator>[<to>]
    //   where <next_operator> is ".."
    //   and  <range_operator> is "..", "..=" or "..<"
    //   and one of the <from> or <to> bounds must be present (just '..' is not allowed since it
    //     looks like parent directory)
    //bugbug range cannot be [..] because that looks like parent directory

    let contents = working_set.get_span_contents(span);

    let token = if let Ok(s) = String::from_utf8(contents.into()) {
        s
    } else {
        working_set.error(ParseError::NonUtf8(span));
        return None;
    };

    if token.starts_with("...") {
        working_set.error(ParseError::Expected(
            "range operator ('..'), got spread ('...')",
            span,
        ));
        return None;
    }

    if !token.contains("..") {
        working_set.error(ParseError::Expected("at least one range bound set", span));
        return None;
    }

    let dotdot_pos: Vec<_> = token
        .match_indices("..")
        .filter_map(|(pos, _)| {
            // paren_depth = count of unclosed parens prior to pos
            let before = &token[..pos];
            let paren_depth = before
                .chars()
                .filter(|&c| c == '(')
                .count()
                .checked_sub(before.chars().filter(|&c| c == ')').count());
            paren_depth.and_then(|d| (d == 0).then_some(pos))
        })
        .collect();

    let (next_op_pos, range_op_pos) = match dotdot_pos.len() {
        1 => (None, dotdot_pos[0]),
        2 => (Some(dotdot_pos[0]), dotdot_pos[1]),
        _ => {
            working_set.error(ParseError::Expected(
                "one range operator ('..' or '..<') and optionally one next operator ('..')",
                span,
            ));
            return None;
        }
    };
    // Avoid calling sub-parsers on unmatched parens, to prevent quadratic time on things like ((((1..2))))
    // No need to call the expensive parse_value on "((((1"
    if dotdot_pos[0] > 0 {
        let (_tokens, err) = lex(
            &contents[..dotdot_pos[0]],
            span.start,
            &[],
            &[b'.', b'?', b'!'],
            true,
        );
        if let Some(_err) = err {
            working_set.error(ParseError::Expected("Valid expression before ..", span));
            return None;
        }
    }

    let (inclusion, range_op_str, range_op_span) = if let Some(pos) = token.find("..<") {
        if pos == range_op_pos {
            let op_str = "..<";
            let op_span = Span::new(
                span.start + range_op_pos,
                span.start + range_op_pos + op_str.len(),
            );
            (RangeInclusion::RightExclusive, "..<", op_span)
        } else {
            working_set.error(ParseError::Expected(
                "inclusive operator preceding second range bound",
                span,
            ));
            return None;
        }
    } else {
        let op_str = if token[range_op_pos..].starts_with("..=") {
            "..="
        } else {
            ".."
        };

        let op_span = Span::new(
            span.start + range_op_pos,
            span.start + range_op_pos + op_str.len(),
        );
        (RangeInclusion::Inclusive, op_str, op_span)
    };

    // Now, based on the operator positions, figure out where the bounds & next are located and
    // parse them
    // TODO: Actually parse the next number in the range
    let from = if token.starts_with("..") {
        // token starts with either next operator, or range operator -- we don't care which one
        None
    } else {
        let from_span = Span::new(span.start, span.start + dotdot_pos[0]);
        Some(parse_value(working_set, from_span, &SyntaxShape::Number))
    };

    let to = if token.ends_with(range_op_str) {
        None
    } else {
        let to_span = Span::new(range_op_span.end, span.end);
        Some(parse_value(working_set, to_span, &SyntaxShape::Number))
    };

    trace!("-- from: {from:?} to: {to:?}");

    if let (None, None) = (&from, &to) {
        working_set.error(ParseError::Expected("at least one range bound set", span));
        return None;
    }

    let (next, next_op_span) = if let Some(pos) = next_op_pos {
        let next_op_span = Span::new(span.start + pos, span.start + pos + "..".len());
        let next_span = Span::new(next_op_span.end, range_op_span.start);

        (
            Some(parse_value(working_set, next_span, &SyntaxShape::Number)),
            next_op_span,
        )
    } else {
        (None, span)
    };

    if working_set.parse_errors.len() != starting_error_count {
        return None;
    }

    let operator = RangeOperator {
        inclusion,
        span: range_op_span,
        next_op_span,
    };

    let mut range = Range {
        from,
        next,
        to,
        operator,
    };

    check_range_types(working_set, &mut range);

    Some(Expression::new(
        working_set,
        Expr::Range(Box::new(range)),
        span,
        Type::Range,
    ))
}

pub(crate) fn parse_dollar_expr(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    trace!("parsing: dollar expression");
    let contents = working_set.get_span_contents(span);

    if contents.starts_with(b"$\"") || contents.starts_with(b"$'") {
        parse_string_interpolation(working_set, span)
    } else if contents.starts_with(b"$.") {
        parse_simple_cell_path(working_set, Span::new(span.start + 2, span.end))
    } else {
        let starting_error_count = working_set.parse_errors.len();

        if let Some(expr) = parse_range(working_set, span) {
            expr
        } else {
            working_set.parse_errors.truncate(starting_error_count);
            parse_full_cell_path(working_set, None, span)
        }
    }
}

pub fn parse_raw_string(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    trace!("parsing: raw-string, with required delimiters");

    let bytes = working_set.get_span_contents(span);

    let prefix_sharp_cnt = if bytes.starts_with(b"r#") {
        // actually `sharp_cnt` is always `index - 1`
        // but create a variable here to make it clearer.
        let mut sharp_cnt = 1;
        let mut index = 2;
        while index < bytes.len() && bytes[index] == b'#' {
            index += 1;
            sharp_cnt += 1;
        }
        sharp_cnt
    } else {
        working_set.error(ParseError::Expected("r#", span));
        return garbage(working_set, span);
    };
    let expect_postfix_sharp_cnt = prefix_sharp_cnt;
    // check the length of whole raw string.
    // the whole raw string should contains at least
    // 1(r) + prefix_sharp_cnt + 1(') + 1(') + postfix_sharp characters
    if bytes.len() < prefix_sharp_cnt + expect_postfix_sharp_cnt + 3 {
        working_set.error(ParseError::Unclosed('\''.into(), span));
        return garbage(working_set, span);
    }

    // check for unbalanced # and single quotes.
    let postfix_bytes = &bytes[bytes.len() - expect_postfix_sharp_cnt..bytes.len()];
    if postfix_bytes.iter().any(|b| *b != b'#') {
        working_set.error(ParseError::Unbalanced(
            "prefix #".to_string(),
            "postfix #".to_string(),
            span,
        ));
        return garbage(working_set, span);
    }
    // check for unblanaced single quotes.
    if bytes[1 + prefix_sharp_cnt] != b'\''
        || bytes[bytes.len() - expect_postfix_sharp_cnt - 1] != b'\''
    {
        working_set.error(ParseError::Unclosed('\''.into(), span));
        return garbage(working_set, span);
    }

    let bytes = &bytes[prefix_sharp_cnt + 1 + 1..bytes.len() - 1 - prefix_sharp_cnt];
    if let Ok(token) = String::from_utf8(bytes.into()) {
        Expression::new(working_set, Expr::RawString(token), span, Type::String)
    } else {
        working_set.error(ParseError::Expected("utf8 raw-string", span));
        garbage(working_set, span)
    }
}

pub fn parse_paren_expr(
    working_set: &mut StateWorkingSet,
    span: Span,
    shape: &SyntaxShape,
) -> Expression {
    let starting_error_count = working_set.parse_errors.len();

    if let Some(expr) = parse_range(working_set, span) {
        return expr;
    }

    working_set.parse_errors.truncate(starting_error_count);

    if let SyntaxShape::Signature = shape {
        return parse_signature(working_set, span, false);
    }

    if let SyntaxShape::ExternalSignature = shape {
        return parse_signature(working_set, span, true);
    }

    let fcp_expr = parse_full_cell_path(working_set, None, span);
    let fcp_error_count = working_set.parse_errors.len();
    if fcp_error_count > starting_error_count {
        let malformed_subexpr = working_set.parse_errors[starting_error_count..]
            .first()
            .is_some_and(|e| match e {
                ParseError::Unclosed(right, _) if (right == ")") => true,
                ParseError::Unbalanced(left, right, _) if left == "(" && right == ")" => true,
                _ => false,
            });
        if malformed_subexpr {
            working_set.parse_errors.truncate(starting_error_count);
            parse_string_interpolation(working_set, span)
        } else {
            fcp_expr
        }
    } else {
        fcp_expr
    }
}

pub fn parse_brace_expr(
    working_set: &mut StateWorkingSet,
    span: Span,
    shape: &SyntaxShape,
) -> Expression {
    // Try to detect what kind of value we're about to parse
    // FIXME: In the future, we should work over the token stream so we only have to do this once
    // before parsing begins

    // FIXME: we're still using the shape because we rely on it to know how to handle syntax where
    // the parse is ambiguous. We'll need to update the parts of the grammar where this is ambiguous
    // and then revisit the parsing.

    if span.end <= (span.start + 1) {
        working_set.error(ParseError::ExpectedWithStringMsg(
            format!("non-block value: {shape}"),
            span,
        ));
        return Expression::garbage(working_set, span);
    }
    let bytes = working_set.get_span_contents(Span::new(span.start + 1, span.end - 1));
    let (tokens, _) = lex(bytes, span.start + 1, &[b'\r', b'\n', b'\t'], &[b':'], true);

    match tokens.as_slice() {
        // If we're empty, that means an empty record or closure
        [] => match shape {
            SyntaxShape::Closure(_) => parse_closure_expression(working_set, shape, span),
            SyntaxShape::Block => parse_block_expression(working_set, span),
            SyntaxShape::MatchBlock => parse_match_block_expression(working_set, span),
            _ => parse_record(working_set, span),
        },
        [
            Token {
                contents: TokenContents::Pipe | TokenContents::PipePipe,
                ..
            },
            ..,
        ] => {
            if let SyntaxShape::Block = shape {
                working_set.error(ParseError::Mismatch("block".into(), "closure".into(), span));
                return Expression::garbage(working_set, span);
            }
            parse_closure_expression(working_set, shape, span)
        }
        [_, third, ..] if working_set.get_span_contents(third.span) == b":" => {
            parse_full_cell_path(working_set, None, span)
        }
        [second, ..] => {
            let second_bytes = working_set.get_span_contents(second.span);
            match shape {
                SyntaxShape::Closure(_) => parse_closure_expression(working_set, shape, span),
                SyntaxShape::Block => parse_block_expression(working_set, span),
                SyntaxShape::MatchBlock => parse_match_block_expression(working_set, span),
                // For edge case of `{}.foo?`, #17896
                _ if second_bytes == b"}" => parse_full_cell_path(working_set, None, span),
                _ if second_bytes.starts_with(b"...")
                    && second_bytes.get(3).is_some_and(|c| b"${(".contains(c)) =>
                {
                    parse_record(working_set, span)
                }
                SyntaxShape::Any => parse_closure_expression(working_set, shape, span),
                _ => {
                    working_set.error(ParseError::ExpectedWithStringMsg(
                        format!("non-block value: {shape}"),
                        span,
                    ));

                    Expression::garbage(working_set, span)
                }
            }
        }
    }
}

pub fn parse_string_interpolation(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    #[derive(PartialEq, Eq, Debug)]
    enum InterpolationMode {
        String,
        Expression,
    }

    let contents = working_set.get_span_contents(span);

    let mut double_quote = false;

    let (start, end) = if contents.starts_with(b"$\"") {
        double_quote = true;

        if let Err(err) = check_string_no_trailing_tokens(contents, span, 1, b'\"') {
            working_set.error(err);
            return garbage(working_set, span);
        }

        let end = if contents.ends_with(b"\"") && contents.len() > 2 {
            span.end - 1
        } else {
            span.end
        };
        (span.start + 2, end)
    } else if contents.starts_with(b"$'") {
        if let Err(err) = check_string_no_trailing_tokens(contents, span, 1, b'\'') {
            working_set.error(err);
            return garbage(working_set, span);
        }

        let end = if contents.ends_with(b"'") && contents.len() > 2 {
            span.end - 1
        } else {
            span.end
        };
        (span.start + 2, end)
    } else {
        (span.start, span.end)
    };

    let inner_span = Span::new(start, end);
    let contents = working_set.get_span_contents(inner_span).to_vec();

    let mut output = vec![];
    let mut mode = InterpolationMode::String;
    let mut token_start = start;
    let mut delimiter_stack = vec![];

    let mut consecutive_backslashes: usize = 0;

    let mut b = start;

    while b != end {
        let current_byte = contents[b - start];

        if mode == InterpolationMode::String {
            let preceding_consecutive_backslashes = consecutive_backslashes;

            let is_backslash = current_byte == b'\\';
            consecutive_backslashes = if is_backslash {
                preceding_consecutive_backslashes + 1
            } else {
                0
            };

            if current_byte == b'('
                && (!double_quote || preceding_consecutive_backslashes.is_multiple_of(2))
            {
                mode = InterpolationMode::Expression;
                if token_start < b {
                    let span = Span::new(token_start, b);
                    let str_contents = working_set.get_span_contents(span);

                    let (str_contents, err) = if double_quote {
                        unescape_string(str_contents, span)
                    } else {
                        (str_contents.to_vec(), None)
                    };
                    if let Some(err) = err {
                        working_set.error(err);
                    }

                    output.push(Expression::new(
                        working_set,
                        Expr::String(String::from_utf8_lossy(&str_contents).to_string()),
                        span,
                        Type::String,
                    ));
                    token_start = b;
                }
            }
        }

        if mode == InterpolationMode::Expression {
            let byte = current_byte;
            if let Some(b'\'') = delimiter_stack.last() {
                if byte == b'\'' {
                    delimiter_stack.pop();
                }
            } else if let Some(b'"') = delimiter_stack.last() {
                if byte == b'"' {
                    delimiter_stack.pop();
                }
            } else if let Some(b'`') = delimiter_stack.last() {
                if byte == b'`' {
                    delimiter_stack.pop();
                }
            } else if byte == b'\'' {
                delimiter_stack.push(b'\'')
            } else if byte == b'"' {
                delimiter_stack.push(b'"');
            } else if byte == b'`' {
                delimiter_stack.push(b'`')
            } else if byte == b'(' {
                delimiter_stack.push(b')');
            } else if byte == b')' {
                if let Some(b')') = delimiter_stack.last() {
                    delimiter_stack.pop();
                }
                if delimiter_stack.is_empty() {
                    mode = InterpolationMode::String;

                    if token_start < b {
                        let span = Span::new(token_start, b + 1);

                        let expr = parse_full_cell_path(working_set, None, span);
                        output.push(expr);
                    }

                    token_start = b + 1;
                    continue;
                }
            }
        }
        b += 1;
    }

    match mode {
        InterpolationMode::String => {
            if token_start < end {
                let span = Span::new(token_start, end);
                let str_contents = working_set.get_span_contents(span);

                let (str_contents, err) = if double_quote {
                    unescape_string(str_contents, span)
                } else {
                    (str_contents.to_vec(), None)
                };
                if let Some(err) = err {
                    working_set.error(err);
                }

                output.push(Expression::new(
                    working_set,
                    Expr::String(String::from_utf8_lossy(&str_contents).to_string()),
                    span,
                    Type::String,
                ));
            }
        }
        InterpolationMode::Expression => {
            if token_start < end {
                let span = Span::new(token_start, end);
                let expr = parse_full_cell_path(working_set, None, span);
                output.push(expr);
            }
        }
    }

    Expression::new(
        working_set,
        Expr::StringInterpolation(output),
        span,
        Type::String,
    )
}

pub fn parse_variable_expr(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let contents = working_set.get_span_contents(span);

    if contents == b"$nu" {
        return Expression::new(
            working_set,
            Expr::Var(nu_protocol::NU_VARIABLE_ID),
            span,
            Type::Any,
        );
    } else if contents == b"$in" {
        return Expression::new(
            working_set,
            Expr::Var(nu_protocol::IN_VARIABLE_ID),
            span,
            Type::Any,
        );
    } else if contents == b"$env" {
        return Expression::new(
            working_set,
            Expr::Var(nu_protocol::ENV_VARIABLE_ID),
            span,
            Type::Any,
        );
    }

    let name = if contents.starts_with(b"$") {
        String::from_utf8_lossy(&contents[1..]).to_string()
    } else {
        String::from_utf8_lossy(contents).to_string()
    };

    let bytes = working_set.get_span_contents(span);
    let suggestion = || {
        DidYouMean::new(
            &working_set.list_variables(),
            working_set.get_span_contents(span),
        )
    };
    if !is_variable(bytes) {
        working_set.error(ParseError::ExpectedWithDidYouMean(
            "valid variable name",
            suggestion(),
            span,
        ));
        garbage(working_set, span)
    } else if let Some(id) = working_set.find_variable(bytes) {
        Expression::new(
            working_set,
            Expr::Var(id),
            span,
            working_set.get_variable(id).ty.clone(),
        )
    } else if working_set.get_env_var(&name).is_some() {
        working_set.error(ParseError::EnvVarNotVar(name, span));
        garbage(working_set, span)
    } else {
        working_set.error(ParseError::VariableNotFound(suggestion(), span));
        garbage(working_set, span)
    }
}

pub fn parse_cell_path(
    working_set: &mut StateWorkingSet,
    tokens: impl Iterator<Item = Token>,
    expect_dot: bool,
) -> Vec<PathMember> {
    enum TokenType {
        Dot,              // .
        DotOrSign,        // . or ? or !
        DotOrExclamation, // . or !
        DotOrQuestion,    // . or ?
        PathMember,       // an int or string, like `1` or `foo`
    }

    enum ModifyMember {
        No,
        Optional,
        Insensitive,
    }

    impl TokenType {
        fn expect(&mut self, byte: u8) -> Result<ModifyMember, &'static str> {
            match (&*self, byte) {
                (Self::PathMember, _) => {
                    *self = Self::DotOrSign;
                    Ok(ModifyMember::No)
                }
                (
                    Self::Dot | Self::DotOrSign | Self::DotOrExclamation | Self::DotOrQuestion,
                    b'.',
                ) => {
                    *self = Self::PathMember;
                    Ok(ModifyMember::No)
                }
                (Self::DotOrSign, b'!') => {
                    *self = Self::DotOrQuestion;
                    Ok(ModifyMember::Insensitive)
                }
                (Self::DotOrSign, b'?') => {
                    *self = Self::DotOrExclamation;
                    Ok(ModifyMember::Optional)
                }
                (Self::DotOrSign, _) => Err(". or ! or ?"),
                (Self::DotOrExclamation, b'!') => {
                    *self = Self::Dot;
                    Ok(ModifyMember::Insensitive)
                }
                (Self::DotOrExclamation, _) => Err(". or !"),
                (Self::DotOrQuestion, b'?') => {
                    *self = Self::Dot;
                    Ok(ModifyMember::Optional)
                }
                (Self::DotOrQuestion, _) => Err(". or ?"),
                (Self::Dot, _) => Err("."),
            }
        }
    }

    // Parsing a cell path is essentially a state machine, and this is the state
    let mut expected_token = if expect_dot {
        TokenType::Dot
    } else {
        TokenType::PathMember
    };

    let mut tail = vec![];

    for path_element in tokens {
        let bytes = working_set.get_span_contents(path_element.span);

        // both parse_int and parse_string require their source to be non-empty
        // all cases where `bytes` is empty is an error
        let Some((&first, rest)) = bytes.split_first() else {
            working_set.error(ParseError::Expected("string", path_element.span));
            return tail;
        };
        let single_char = rest.is_empty();

        if let TokenType::PathMember = expected_token {
            let starting_error_count = working_set.parse_errors.len();

            let expr = parse_int(working_set, path_element.span);
            working_set.parse_errors.truncate(starting_error_count);

            match expr {
                Expression {
                    expr: Expr::Int(val),
                    span,
                    ..
                } => tail.push(PathMember::Int {
                    val: val as usize,
                    span,
                    optional: false,
                }),
                _ => {
                    let result = parse_string(working_set, path_element.span);
                    match result {
                        Expression {
                            expr: Expr::String(string),
                            span,
                            ..
                        } => {
                            tail.push(PathMember::String {
                                val: string,
                                span,
                                optional: false,
                                casing: Casing::Sensitive,
                            });
                        }
                        _ => {
                            working_set.error(ParseError::Expected("string", path_element.span));
                            return tail;
                        }
                    }
                }
            }
            expected_token = TokenType::DotOrSign;
        } else {
            match expected_token.expect(if single_char { first } else { b' ' }) {
                Ok(modify) => {
                    if let Some(last) = tail.last_mut() {
                        match modify {
                            ModifyMember::No => {}
                            ModifyMember::Optional => last.make_optional(),
                            ModifyMember::Insensitive => last.make_insensitive(),
                        }
                    };
                }
                Err(expected) => {
                    working_set.error(ParseError::Expected(expected, path_element.span));
                    return tail;
                }
            }
        }
    }

    tail
}

pub fn parse_simple_cell_path(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let source = working_set.get_span_contents(span);

    let (tokens, err) = lex(
        source,
        span.start,
        &[b'\n', b'\r'],
        &[b'.', b'?', b'!'],
        true,
    );
    if let Some(err) = err {
        working_set.error(err)
    }

    let tokens = tokens.into_iter().peekable();

    let cell_path = parse_cell_path(working_set, tokens, false);

    Expression::new(
        working_set,
        Expr::CellPath(CellPath { members: cell_path }),
        span,
        Type::CellPath,
    )
}

pub fn parse_full_cell_path(
    working_set: &mut StateWorkingSet,
    implicit_head: Option<VarId>,
    span: Span,
) -> Expression {
    trace!("parsing: full cell path");
    let full_cell_span = span;
    let source = working_set.get_span_contents(span);

    let (tokens, err) = lex(
        source,
        span.start,
        &[b'\n', b'\r'],
        &[b'.', b'?', b'!'],
        true,
    );
    if let Some(err) = err {
        working_set.error(err)
    }

    let mut tokens = tokens.into_iter().peekable();
    if let Some(head) = tokens.peek() {
        let bytes = working_set.get_span_contents(head.span);
        let (head, expect_dot) = if bytes.starts_with(b"(") {
            trace!("parsing: paren-head of full cell path");

            let head_span = head.span;
            let mut start = head.span.start;
            let mut end = head.span.end;
            let mut is_closed = true;

            if bytes.starts_with(b"(") {
                start += 1;
            }
            if bytes.ends_with(b")") {
                end -= 1;
            } else {
                working_set.error(ParseError::Unclosed(")".into(), Span::new(end, end)));
                is_closed = false;
            }

            let span = Span::new(start, end);

            let source = working_set.get_span_contents(span);

            let (output, err) = lex(source, span.start, &[b'\n', b'\r'], &[], true);
            if let Some(err) = err {
                working_set.error(err)
            }

            // Creating a Type scope to parse the new block. This will keep track of
            // the previous input type found in that block
            let output = parse_block(working_set, &output, span, is_closed, true);

            let ty = output.output_type();

            let block_id = working_set.add_block(Arc::new(output));
            tokens.next();

            (
                Expression::new(working_set, Expr::Subexpression(block_id), head_span, ty),
                true,
            )
        } else if bytes.starts_with(b"[") {
            trace!("parsing: table head of full cell path");

            let output = parse_table_expression(working_set, head.span, &SyntaxShape::Any);

            tokens.next();

            (output, true)
        } else if bytes.starts_with(b"{") {
            trace!("parsing: record head of full cell path");
            let output = parse_record(working_set, head.span);

            tokens.next();

            (output, true)
        } else if bytes.starts_with(b"$") {
            trace!("parsing: $variable head of full cell path");

            let out = parse_variable_expr(working_set, head.span);

            tokens.next();

            (out, true)
        } else if let Some(var_id) = implicit_head {
            trace!("parsing: implicit head of full cell path");
            (
                Expression::new(working_set, Expr::Var(var_id), head.span, Type::Any),
                false,
            )
        } else {
            working_set.error(ParseError::Mismatch(
                "variable or subexpression".into(),
                String::from_utf8_lossy(bytes).to_string(),
                span,
            ));
            return garbage(working_set, span);
        };

        let tail = parse_cell_path(working_set, tokens, expect_dot);
        let ty = if !tail.is_empty() {
            if nu_experimental::CELL_PATH_TYPES.get() {
                head.ty
                    .follow_cell_path(&tail)
                    .map(|ty| ty.into_owned())
                    .unwrap_or(Type::Any)
            } else {
                Type::Any
            }
        } else {
            head.ty.clone()
        };

        Expression::new(
            working_set,
            Expr::FullCellPath(Box::new(FullCellPath { head, tail })),
            full_cell_span,
            ty,
        )
    } else {
        garbage(working_set, span)
    }
}

pub fn parse_directory(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let bytes = working_set.get_span_contents(span);
    trace!("parsing: directory");

    // Check for bare word interpolation
    if !bytes.is_empty()
        && bytes[0] != b'\''
        && bytes[0] != b'"'
        && bytes[0] != b'`'
        && bytes.contains(&b'(')
    {
        return parse_string_interpolation(working_set, span);
    }

    let quoted = is_quoted(bytes);
    let (token, err) = unescape_unquote_string(bytes, span);

    if err.is_none() {
        trace!("-- found {token}");

        Expression::new(
            working_set,
            Expr::Directory(token, quoted),
            span,
            Type::String,
        )
    } else {
        working_set.error(ParseError::Expected("directory", span));

        garbage(working_set, span)
    }
}

pub fn parse_filepath(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let bytes = working_set.get_span_contents(span);
    trace!("parsing: filepath");

    // Check for bare word interpolation
    if !bytes.is_empty()
        && bytes[0] != b'\''
        && bytes[0] != b'"'
        && bytes[0] != b'`'
        && bytes.contains(&b'(')
    {
        return parse_string_interpolation(working_set, span);
    }

    let quoted = is_quoted(bytes);
    let (token, err) = unescape_unquote_string(bytes, span);

    if err.is_none() {
        trace!("-- found {token}");

        Expression::new(
            working_set,
            Expr::Filepath(token, quoted),
            span,
            Type::String,
        )
    } else {
        working_set.error(ParseError::Expected("filepath", span));

        garbage(working_set, span)
    }
}

/// Parse a datetime type, eg '2022-02-02'
pub fn parse_datetime(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    trace!("parsing: datetime");

    let bytes = working_set.get_span_contents(span);

    if bytes.len() < 6
        || !bytes[0].is_ascii_digit()
        || !bytes[1].is_ascii_digit()
        || !bytes[2].is_ascii_digit()
        || !bytes[3].is_ascii_digit()
        || bytes[4] != b'-'
    {
        working_set.error(ParseError::Expected("datetime", span));
        return garbage(working_set, span);
    }

    let token = String::from_utf8_lossy(bytes).to_string();

    if let Ok(datetime) = chrono::DateTime::parse_from_rfc3339(&token) {
        return Expression::new(working_set, Expr::DateTime(datetime), span, Type::Date);
    }

    // Just the date
    let just_date = token.clone() + "T00:00:00+00:00";
    if let Ok(datetime) = chrono::DateTime::parse_from_rfc3339(&just_date) {
        return Expression::new(working_set, Expr::DateTime(datetime), span, Type::Date);
    }

    // Date and time, assume UTC
    let datetime = token + "+00:00";
    if let Ok(datetime) = chrono::DateTime::parse_from_rfc3339(&datetime) {
        return Expression::new(working_set, Expr::DateTime(datetime), span, Type::Date);
    }

    working_set.error(ParseError::Expected("datetime", span));

    garbage(working_set, span)
}

/// Parse a duration type, eg '10day'
pub fn parse_duration(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    trace!("parsing: duration");

    let bytes = working_set.get_span_contents(span);

    match parse_unit_value(bytes, span, DURATION_UNIT_GROUPS, Type::Duration, |x| x) {
        Some(Ok(expr)) => {
            let span_id = working_set.add_span(span);
            expr.with_span_id(span_id)
        }
        Some(Err(mk_err_for)) => {
            working_set.error(mk_err_for("duration"));
            garbage(working_set, span)
        }
        None => {
            working_set.error(ParseError::Expected("duration with valid units", span));
            garbage(working_set, span)
        }
    }
}

/// Parse a unit type, eg '10kb'
pub fn parse_filesize(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    trace!("parsing: filesize");

    let bytes = working_set.get_span_contents(span);

    // the hex digit `b` might be mistaken for the unit `b`, so check that first
    if bytes.starts_with(b"0x") {
        working_set.error(ParseError::Expected("filesize with valid units", span));
        return garbage(working_set, span);
    }

    match parse_unit_value(bytes, span, FILESIZE_UNIT_GROUPS, Type::Filesize, |x| {
        x.to_ascii_uppercase()
    }) {
        Some(Ok(expr)) => {
            let span_id = working_set.add_span(span);
            expr.with_span_id(span_id)
        }
        Some(Err(mk_err_for)) => {
            working_set.error(mk_err_for("filesize"));
            garbage(working_set, span)
        }
        None => {
            working_set.error(ParseError::Expected("filesize with valid units", span));
            garbage(working_set, span)
        }
    }
}

type ParseUnitResult<'res> = Result<Expression, Box<dyn Fn(&'res str) -> ParseError>>;
type UnitGroup<'unit> = (Unit, &'unit str, Option<(Unit, i64)>);

pub fn parse_unit_value<'res>(
    bytes: &[u8],
    span: Span,
    unit_groups: &[UnitGroup],
    ty: Type,
    transform: fn(String) -> String,
) -> Option<ParseUnitResult<'res>> {
    if bytes.len() < 2
        || !(bytes[0].is_ascii_digit()
            || bytes[0] == b'.'
            || (bytes[0] == b'-' && bytes[1].is_ascii_digit()))
    {
        return None;
    }

    // Bail if not UTF-8
    let value = transform(str::from_utf8(bytes).ok()?.into());

    if let Some((unit, name, convert)) = unit_groups.iter().find(|x| value.ends_with(x.1)) {
        let lhs_len = value.len() - name.len();
        let lhs = strip_underscores(&value.as_bytes()[..lhs_len]);
        let lhs_span = Span::new(span.start, span.start + lhs_len);
        let unit_span = Span::new(span.start + lhs_len, span.end);
        if lhs.ends_with('$') {
            // If `parse_unit_value` has higher precedence over `parse_range`,
            // a variable with the name of a unit could otherwise not be used as the end of a range.
            return None;
        }

        let (decimal_part, number_part) = modf(match lhs.parse::<f64>() {
            Ok(it) => it,
            Err(_) => {
                let mk_err = move |name| {
                    ParseError::LabeledError(
                        format!("{name} value must be a number"),
                        "not a number".into(),
                        lhs_span,
                    )
                };
                return Some(Err(Box::new(mk_err)));
            }
        });

        let mut unit = match convert {
            Some(convert_to) => convert_to.0,
            None => *unit,
        };

        let num_float = match convert {
            Some(convert_to) => {
                (number_part * convert_to.1 as f64) + (decimal_part * convert_to.1 as f64)
            }
            None => number_part,
        };

        // Convert all durations to nanoseconds, and filesizes to bytes,
        // to minimize loss of precision
        let factor = match ty {
            Type::Filesize => unit_to_byte_factor(&unit),
            Type::Duration => unit_to_ns_factor(&unit),
            _ => None,
        };

        let num = match factor {
            Some(factor) => {
                let num_base = num_float * factor;
                if i64::MIN as f64 <= num_base && num_base <= i64::MAX as f64 {
                    unit = if ty == Type::Filesize {
                        Unit::Filesize(FilesizeUnit::B)
                    } else {
                        Unit::Nanosecond
                    };
                    num_base as i64
                } else {
                    // not safe to convert, because of the overflow
                    num_float as i64
                }
            }
            None => num_float as i64,
        };

        trace!("-- found {num} {unit:?}");
        let value = ValueWithUnit {
            expr: Expression::new_unknown(Expr::Int(num), lhs_span, Type::Number),
            unit: Spanned {
                item: unit,
                span: unit_span,
            },
        };
        let expr = Expression::new_unknown(Expr::ValueWithUnit(Box::new(value)), span, ty);

        Some(Ok(expr))
    } else {
        None
    }
}

pub const FILESIZE_UNIT_GROUPS: &[UnitGroup] = &[
    (
        Unit::Filesize(FilesizeUnit::KB),
        "KB",
        Some((Unit::Filesize(FilesizeUnit::B), 1000)),
    ),
    (
        Unit::Filesize(FilesizeUnit::MB),
        "MB",
        Some((Unit::Filesize(FilesizeUnit::KB), 1000)),
    ),
    (
        Unit::Filesize(FilesizeUnit::GB),
        "GB",
        Some((Unit::Filesize(FilesizeUnit::MB), 1000)),
    ),
    (
        Unit::Filesize(FilesizeUnit::TB),
        "TB",
        Some((Unit::Filesize(FilesizeUnit::GB), 1000)),
    ),
    (
        Unit::Filesize(FilesizeUnit::PB),
        "PB",
        Some((Unit::Filesize(FilesizeUnit::TB), 1000)),
    ),
    (
        Unit::Filesize(FilesizeUnit::EB),
        "EB",
        Some((Unit::Filesize(FilesizeUnit::PB), 1000)),
    ),
    (
        Unit::Filesize(FilesizeUnit::KiB),
        "KIB",
        Some((Unit::Filesize(FilesizeUnit::B), 1024)),
    ),
    (
        Unit::Filesize(FilesizeUnit::MiB),
        "MIB",
        Some((Unit::Filesize(FilesizeUnit::KiB), 1024)),
    ),
    (
        Unit::Filesize(FilesizeUnit::GiB),
        "GIB",
        Some((Unit::Filesize(FilesizeUnit::MiB), 1024)),
    ),
    (
        Unit::Filesize(FilesizeUnit::TiB),
        "TIB",
        Some((Unit::Filesize(FilesizeUnit::GiB), 1024)),
    ),
    (
        Unit::Filesize(FilesizeUnit::PiB),
        "PIB",
        Some((Unit::Filesize(FilesizeUnit::TiB), 1024)),
    ),
    (
        Unit::Filesize(FilesizeUnit::EiB),
        "EIB",
        Some((Unit::Filesize(FilesizeUnit::PiB), 1024)),
    ),
    (Unit::Filesize(FilesizeUnit::B), "B", None),
];

pub const DURATION_UNIT_GROUPS: &[UnitGroup] = &[
    (Unit::Nanosecond, "ns", None),
    // todo start adding aliases for duration units here
    (Unit::Microsecond, "us", Some((Unit::Nanosecond, 1000))),
    (
        // µ Micro Sign
        Unit::Microsecond,
        "\u{00B5}s",
        Some((Unit::Nanosecond, 1000)),
    ),
    (
        // μ Greek small letter Mu
        Unit::Microsecond,
        "\u{03BC}s",
        Some((Unit::Nanosecond, 1000)),
    ),
    (Unit::Millisecond, "ms", Some((Unit::Microsecond, 1000))),
    (Unit::Second, "sec", Some((Unit::Millisecond, 1000))),
    (Unit::Minute, "min", Some((Unit::Second, 60))),
    (Unit::Hour, "hr", Some((Unit::Minute, 60))),
    (Unit::Day, "day", Some((Unit::Minute, 1440))),
    (Unit::Week, "wk", Some((Unit::Day, 7))),
];

fn unit_to_ns_factor(unit: &Unit) -> Option<f64> {
    match unit {
        Unit::Nanosecond => Some(1.0),
        Unit::Microsecond => Some(1_000.0),
        Unit::Millisecond => Some(1_000_000.0),
        Unit::Second => Some(1_000_000_000.0),
        Unit::Minute => Some(60.0 * 1_000_000_000.0),
        Unit::Hour => Some(60.0 * 60.0 * 1_000_000_000.0),
        Unit::Day => Some(24.0 * 60.0 * 60.0 * 1_000_000_000.0),
        Unit::Week => Some(7.0 * 24.0 * 60.0 * 60.0 * 1_000_000_000.0),
        _ => None,
    }
}

fn unit_to_byte_factor(unit: &Unit) -> Option<f64> {
    match unit {
        Unit::Filesize(FilesizeUnit::B) => Some(1.0),
        Unit::Filesize(FilesizeUnit::KB) => Some(1_000.0),
        Unit::Filesize(FilesizeUnit::MB) => Some(1_000_000.0),
        Unit::Filesize(FilesizeUnit::GB) => Some(1_000_000_000.0),
        Unit::Filesize(FilesizeUnit::TB) => Some(1_000_000_000_000.0),
        Unit::Filesize(FilesizeUnit::PB) => Some(1_000_000_000_000_000.0),
        Unit::Filesize(FilesizeUnit::EB) => Some(1_000_000_000_000_000_000.0),
        Unit::Filesize(FilesizeUnit::KiB) => Some(1024.0),
        Unit::Filesize(FilesizeUnit::MiB) => Some(1024.0 * 1024.0),
        Unit::Filesize(FilesizeUnit::GiB) => Some(1024.0 * 1024.0 * 1024.0),
        Unit::Filesize(FilesizeUnit::TiB) => Some(1024.0 * 1024.0 * 1024.0 * 1024.0),
        Unit::Filesize(FilesizeUnit::PiB) => Some(1024.0 * 1024.0 * 1024.0 * 1024.0 * 1024.0),
        Unit::Filesize(FilesizeUnit::EiB) => {
            Some(1024.0 * 1024.0 * 1024.0 * 1024.0 * 1024.0 * 1024.0)
        }
        _ => None,
    }
}

// Borrowed from libm at https://github.com/rust-lang/libm/blob/master/src/math/modf.rs
fn modf(x: f64) -> (f64, f64) {
    let rv2: f64;
    let mut u = x.to_bits();
    let e = (((u >> 52) & 0x7ff) as i32) - 0x3ff;

    /* no fractional part */
    if e >= 52 {
        rv2 = x;
        if e == 0x400 && (u << 12) != 0 {
            /* nan */
            return (x, rv2);
        }
        u &= 1 << 63;
        return (f64::from_bits(u), rv2);
    }

    /* no integral part*/
    if e < 0 {
        u &= 1 << 63;
        rv2 = f64::from_bits(u);
        return (x, rv2);
    }

    let mask = ((!0) >> 12) >> e;
    if (u & mask) == 0 {
        rv2 = x;
        u &= 1 << 63;
        return (f64::from_bits(u), rv2);
    }
    u &= !mask;
    rv2 = f64::from_bits(u);
    (x - rv2, rv2)
}

pub fn parse_glob_pattern(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let bytes = working_set.get_span_contents(span);
    let quoted = is_quoted(bytes);
    trace!("parsing: glob pattern");

    // Check for bare word interpolation
    if !bytes.is_empty()
        && bytes[0] != b'\''
        && bytes[0] != b'"'
        && bytes[0] != b'`'
        && bytes.contains(&b'(')
    {
        let interpolation_expr = parse_string_interpolation(working_set, span);

        // Convert StringInterpolation to GlobInterpolation
        if let Expr::StringInterpolation(exprs) = interpolation_expr.expr {
            return Expression::new(
                working_set,
                Expr::GlobInterpolation(exprs, quoted),
                span,
                Type::Glob,
            );
        }

        return interpolation_expr;
    }

    let (token, err) = unescape_unquote_string(bytes, span);

    if err.is_none() {
        trace!("-- found {token}");

        Expression::new(
            working_set,
            Expr::GlobPattern(token, quoted),
            span,
            Type::Glob,
        )
    } else {
        working_set.error(ParseError::Expected("glob pattern string", span));

        garbage(working_set, span)
    }
}

pub fn unescape_string(bytes: &[u8], span: Span) -> (Vec<u8>, Option<ParseError>) {
    let mut output = Vec::new();
    let mut error = None;

    let mut idx = 0;

    if !bytes.contains(&b'\\') {
        return (bytes.to_vec(), None);
    }

    'us_loop: while idx < bytes.len() {
        if bytes[idx] == b'\\' {
            // We're in an escape
            idx += 1;

            match bytes.get(idx) {
                Some(b'"') => {
                    output.push(b'"');
                    idx += 1;
                }
                Some(b'\'') => {
                    output.push(b'\'');
                    idx += 1;
                }
                Some(b'\\') => {
                    output.push(b'\\');
                    idx += 1;
                }
                Some(b'/') => {
                    output.push(b'/');
                    idx += 1;
                }
                Some(b'(') => {
                    output.push(b'(');
                    idx += 1;
                }
                Some(b')') => {
                    output.push(b')');
                    idx += 1;
                }
                Some(b'{') => {
                    output.push(b'{');
                    idx += 1;
                }
                Some(b'}') => {
                    output.push(b'}');
                    idx += 1;
                }
                Some(b'$') => {
                    output.push(b'$');
                    idx += 1;
                }
                Some(b'^') => {
                    output.push(b'^');
                    idx += 1;
                }
                Some(b'#') => {
                    output.push(b'#');
                    idx += 1;
                }
                Some(b'|') => {
                    output.push(b'|');
                    idx += 1;
                }
                Some(b'~') => {
                    output.push(b'~');
                    idx += 1;
                }
                Some(b'a') => {
                    output.push(0x7);
                    idx += 1;
                }
                Some(b'b') => {
                    output.push(0x8);
                    idx += 1;
                }
                Some(b'e') => {
                    output.push(0x1b);
                    idx += 1;
                }
                Some(b'f') => {
                    output.push(0xc);
                    idx += 1;
                }
                Some(b'n') => {
                    output.push(b'\n');
                    idx += 1;
                }
                Some(b'r') => {
                    output.push(b'\r');
                    idx += 1;
                }
                Some(b't') => {
                    output.push(b'\t');
                    idx += 1;
                }
                Some(b'u') => {
                    let mut digits = String::with_capacity(10);
                    let mut cur_idx = idx + 1; // index of first beyond current end of token

                    if let Some(b'{') = bytes.get(idx + 1) {
                        cur_idx = idx + 2;
                        loop {
                            match bytes.get(cur_idx) {
                                Some(b'}') => {
                                    cur_idx += 1;
                                    break;
                                }
                                Some(c) => {
                                    digits.push(*c as char);
                                    cur_idx += 1;
                                }
                                _ => {
                                    error = error.or(Some(ParseError::InvalidLiteral(
                                        "missing '}' for unicode escape '\\u{X...}'".into(),
                                        "string".into(),
                                        Span::new(span.start + idx, span.end),
                                    )));
                                    break 'us_loop;
                                }
                            }
                        }
                    }

                    if (1..=6).contains(&digits.len()) {
                        let int = u32::from_str_radix(&digits, 16);

                        if let Ok(int) = int
                            && int <= 0x10ffff
                        {
                            let result = char::from_u32(int);

                            if let Some(result) = result {
                                let mut buffer = [0; 4];
                                let result = result.encode_utf8(&mut buffer);

                                for elem in result.bytes() {
                                    output.push(elem);
                                }

                                idx = cur_idx;
                                continue 'us_loop;
                            }
                        }
                    }
                    // fall through -- escape not accepted above, must be error.
                    error = error.or(Some(ParseError::InvalidLiteral(
                            "invalid unicode escape '\\u{X...}', must be 1-6 hex digits, max value 10FFFF".into(),
                            "string".into(),
                            Span::new(span.start + idx, span.end),
                    )));
                    break 'us_loop;
                }

                _ => {
                    error = error.or(Some(ParseError::InvalidLiteral(
                        "unrecognized escape after '\\'".into(),
                        "string".into(),
                        Span::new(span.start + idx, span.end),
                    )));
                    break 'us_loop;
                }
            }
        } else {
            output.push(bytes[idx]);
            idx += 1;
        }
    }

    (output, error)
}

pub fn unescape_unquote_string(bytes: &[u8], span: Span) -> (String, Option<ParseError>) {
    if bytes.starts_with(b"\"") {
        // Needs unescaping
        let bytes = trim_quotes(bytes);

        let (bytes, err) = unescape_string(bytes, span);

        if let Ok(token) = String::from_utf8(bytes) {
            (token, err)
        } else {
            (String::new(), Some(ParseError::Expected("string", span)))
        }
    } else {
        let bytes = trim_quotes(bytes);

        if let Ok(token) = String::from_utf8(bytes.into()) {
            (token, None)
        } else {
            (String::new(), Some(ParseError::Expected("string", span)))
        }
    }
}

fn check_string_no_trailing_tokens(
    bytes: &[u8],
    span: Span,
    opening_quote_pos: usize,
    quote: u8,
) -> Result<(), ParseError> {
    let pos = bytes
        .iter()
        .rposition(|ch| *ch == quote)
        .expect("string begins with quote");
    if pos == bytes.len() - 1 {
        Ok(())
    } else if pos == opening_quote_pos {
        // this may look like an error, but it's not:
        // some code, like completions, requires allowing
        // unterminated strings at this stage.
        Ok(())
    } else {
        let span = Span::new(span.start + pos + 1, span.end);
        Err(ParseError::ExtraTokensAfterClosingDelimiter(span))
    }
}

pub fn parse_string(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    trace!("parsing: string");

    let bytes = working_set.get_span_contents(span);

    if bytes.is_empty() {
        working_set.error(ParseError::Expected("String", span));
        return Expression::garbage(working_set, span);
    }

    // Check for bare word interpolation
    if bytes[0] != b'\'' && bytes[0] != b'"' && bytes[0] != b'`' && bytes.contains(&b'(') {
        return parse_string_interpolation(working_set, span);
    }

    // Check for unbalanced quotes:
    for quote in [b'\"', b'\''] {
        if bytes[0] == quote
            && let Err(err) = check_string_no_trailing_tokens(bytes, span, 0, quote)
        {
            working_set.error(err);
            return garbage(working_set, span);
        }
    }

    let (s, err) = unescape_unquote_string(bytes, span);
    if let Some(err) = err {
        working_set.error(err);
    }

    Expression::new(working_set, Expr::String(s), span, Type::String)
}

fn is_quoted(bytes: &[u8]) -> bool {
    (bytes.starts_with(b"\"") && bytes.ends_with(b"\"") && bytes.len() > 1)
        || (bytes.starts_with(b"\'") && bytes.ends_with(b"\'") && bytes.len() > 1)
}

pub fn parse_string_strict(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    trace!("parsing: string, with required delimiters");

    let bytes = working_set.get_span_contents(span);

    // Check for unbalanced quotes:
    {
        let bytes = if bytes.starts_with(b"$") {
            &bytes[1..]
        } else {
            bytes
        };
        if bytes.starts_with(b"\"") && (bytes.len() == 1 || !bytes.ends_with(b"\"")) {
            working_set.error(ParseError::Unclosed("\"".into(), span));
            return garbage(working_set, span);
        }
        if bytes.starts_with(b"\'") && (bytes.len() == 1 || !bytes.ends_with(b"\'")) {
            working_set.error(ParseError::Unclosed("\'".into(), span));
            return garbage(working_set, span);
        }
        if bytes.starts_with(b"r#") && (bytes.len() == 1 || !bytes.ends_with(b"#")) {
            working_set.error(ParseError::Unclosed("r#".into(), span));
            return garbage(working_set, span);
        }
    }

    let (bytes, quoted) = if (bytes.starts_with(b"\"") && bytes.ends_with(b"\"") && bytes.len() > 1)
        || (bytes.starts_with(b"\'") && bytes.ends_with(b"\'") && bytes.len() > 1)
    {
        (&bytes[1..(bytes.len() - 1)], true)
    } else if (bytes.starts_with(b"$\"") && bytes.ends_with(b"\"") && bytes.len() > 2)
        || (bytes.starts_with(b"$\'") && bytes.ends_with(b"\'") && bytes.len() > 2)
    {
        (&bytes[2..(bytes.len() - 1)], true)
    } else {
        (bytes, false)
    };

    if let Ok(token) = String::from_utf8(bytes.into()) {
        trace!("-- found {token}");

        if quoted {
            Expression::new(working_set, Expr::String(token), span, Type::String)
        } else if token.contains(' ') {
            working_set.error(ParseError::Expected("string", span));

            garbage(working_set, span)
        } else {
            Expression::new(working_set, Expr::String(token), span, Type::String)
        }
    } else {
        working_set.error(ParseError::Expected("string", span));
        garbage(working_set, span)
    }
}

pub fn parse_import_pattern<'a>(
    working_set: &mut StateWorkingSet,
    mut arg_iter: impl Iterator<Item = &'a Expression>,
    spans: &[Span],
) -> Expression {
    let Some(head_expr) = arg_iter.next() else {
        working_set.error(ParseError::WrongImportPattern(
            "needs at least one component of import pattern".to_string(),
            Span::concat(spans),
        ));
        return garbage(working_set, Span::concat(spans));
    };

    let (maybe_module_id, head_name) = match eval_constant(working_set, head_expr) {
        Ok(Value::Nothing { .. }) => {
            return Expression::new(
                working_set,
                Expr::Nothing,
                Span::concat(spans),
                Type::Nothing,
            );
        }
        Ok(val) => match val.coerce_into_string() {
            Ok(s) => (working_set.find_module(s.as_bytes()), s.into_bytes()),
            Err(err) => {
                working_set.error(err.wrap(working_set, Span::concat(spans)));
                return garbage(working_set, Span::concat(spans));
            }
        },
        Err(err) => {
            working_set.error(err.wrap(working_set, Span::concat(spans)));
            return garbage(working_set, Span::concat(spans));
        }
    };

    let mut import_pattern = ImportPattern {
        head: ImportPatternHead {
            name: head_name,
            id: maybe_module_id,
            span: head_expr.span,
        },
        members: vec![],
        hidden: HashSet::new(),
        constants: vec![],
    };

    let mut leaf_member_expr: Option<(&str, Span)> = None;

    // TODO: box pattern syntax is experimental @rust v1.89.0
    let handle_list_items =
        |items: &Vec<ListItem>,
         span,
         working_set: &mut StateWorkingSet<'_>,
         import_pattern: &mut ImportPattern,
         leaf_member_expr: &mut Option<(&str, Span)>| {
            let mut output = vec![];

            for item in items.iter() {
                match item {
                    ListItem::Item(expr) => {
                        if let Some(name) = expr.as_string() {
                            output.push((name.as_bytes().to_vec(), expr.span));
                        }
                    }
                    ListItem::Spread(_, spread) => {
                        working_set.error(ParseError::WrongImportPattern(
                            "cannot spread in an import pattern".into(),
                            spread.span,
                        ))
                    }
                }
            }

            import_pattern
                .members
                .push(ImportPatternMember::List { names: output });

            *leaf_member_expr = Some(("list", span));
        };

    for tail_expr in arg_iter {
        if let Some((what, prev_span)) = leaf_member_expr {
            working_set.error(ParseError::WrongImportPattern(
                format!("{what} member can be only at the end of an import pattern"),
                prev_span,
            ));
            return Expression::new(
                working_set,
                Expr::ImportPattern(Box::new(import_pattern)),
                prev_span,
                Type::List(Box::new(Type::String)),
            );
        }

        match &tail_expr.expr {
            Expr::String(name) => {
                let span = tail_expr.span;
                if name == "*" {
                    import_pattern
                        .members
                        .push(ImportPatternMember::Glob { span });

                    leaf_member_expr = Some(("glob", span));
                } else {
                    import_pattern.members.push(ImportPatternMember::Name {
                        name: name.as_bytes().to_vec(),
                        span,
                    });
                }
            }
            Expr::FullCellPath(fcp) => {
                if let Expr::List(items) = &fcp.head.expr {
                    handle_list_items(
                        items,
                        fcp.head.span,
                        working_set,
                        &mut import_pattern,
                        &mut leaf_member_expr,
                    );
                }
            }
            Expr::List(items) => {
                handle_list_items(
                    items,
                    tail_expr.span,
                    working_set,
                    &mut import_pattern,
                    &mut leaf_member_expr,
                );
            }
            _ => {
                working_set.error(ParseError::WrongImportPattern(
                    "Wrong type of import pattern, only String and List<String> are allowed."
                        .into(),
                    tail_expr.span,
                ));
            }
        };
    }

    Expression::new(
        working_set,
        Expr::ImportPattern(Box::new(import_pattern)),
        Span::concat(&spans[1..]),
        Type::List(Box::new(Type::String)),
    )
}

/// Parse `spans[spans_idx..]` into a variable, with optional type annotation.
/// If the name of the variable ends with a colon (no space in-between allowed), then a type annotation
/// can appear after the variable, in which case the colon is stripped from the name of the variable.
/// `spans_idx` is updated to point to the last span that has been parsed.
pub fn parse_var_with_opt_type(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
    spans_idx: &mut usize,
    mutable: bool,
) -> (Expression, Option<Type>) {
    let name_span = spans[*spans_idx];
    let bytes = working_set.get_span_contents(name_span).to_vec();

    if bytes.contains(&b' ')
        || bytes.contains(&b'"')
        || bytes.contains(&b'\'')
        || bytes.contains(&b'`')
    {
        working_set.error(ParseError::VariableNotValid(spans[*spans_idx]));
        return (garbage(working_set, spans[*spans_idx]), None);
    }

    if bytes.ends_with(b":") {
        let name_span = Span::new(name_span.start, name_span.end - 1);
        let var_name = bytes[0..(bytes.len() - 1)].to_vec();

        // We end with colon, so the next span should be the type
        if *spans_idx + 1 < spans.len() {
            *spans_idx += 1;
            // signature like record<a: int b: int> is broken into multiple spans due to
            // whitespaces. Collect the rest into one span and work on it
            let full_span = Span::concat(&spans[*spans_idx..]);
            let type_bytes = working_set.get_span_contents(full_span).to_vec();

            let (tokens, parse_error) =
                lex_signature(&type_bytes, full_span.start, &[], &[b','], true);

            if let Some(parse_error) = parse_error {
                working_set.error(parse_error);
            }

            let ty = parse_type(working_set, &type_bytes, tokens[0].span);
            *spans_idx = spans.len() - 1;

            if !is_variable(&var_name) {
                working_set.error(ParseError::Expected(
                    "valid variable name",
                    spans[*spans_idx - 1],
                ));
                return (garbage(working_set, spans[*spans_idx - 1]), None);
            }

            ensure_not_reserved_variable_name(working_set, &var_name, name_span);

            let id = working_set.add_variable(var_name, spans[*spans_idx - 1], ty.clone(), mutable);

            (
                Expression::new(working_set, Expr::VarDecl(id), name_span, ty.clone()),
                Some(ty),
            )
        } else {
            if !is_variable(&var_name) {
                working_set.error(ParseError::Expected(
                    "valid variable name",
                    spans[*spans_idx],
                ));
                return (garbage(working_set, spans[*spans_idx]), None);
            }

            ensure_not_reserved_variable_name(working_set, &var_name, name_span);

            let id = working_set.add_variable(var_name, spans[*spans_idx], Type::Any, mutable);

            working_set.error(ParseError::MissingType(spans[*spans_idx]));
            (
                Expression::new(working_set, Expr::VarDecl(id), spans[*spans_idx], Type::Any),
                None,
            )
        }
    } else {
        let var_name = bytes;

        if !is_variable(&var_name) {
            working_set.error(ParseError::Expected(
                "valid variable name",
                spans[*spans_idx],
            ));
            return (garbage(working_set, spans[*spans_idx]), None);
        }

        ensure_not_reserved_variable_name(working_set, &var_name, name_span);

        let id = working_set.add_variable(
            var_name,
            Span::concat(&spans[*spans_idx..*spans_idx + 1]),
            Type::Any,
            mutable,
        );

        (
            Expression::new(working_set, Expr::VarDecl(id), spans[*spans_idx], Type::Any),
            None,
        )
    }
}

const RESERVED_VARIABLE_NAMES: [&[u8]; 3] = [b"in", b"nu", b"env"];

pub(crate) fn ensure_not_reserved_variable_name(
    working_set: &mut StateWorkingSet,
    name: &[u8],
    span: Span,
) {
    let var_name = name.strip_prefix(b"$").unwrap_or(name);

    if RESERVED_VARIABLE_NAMES.contains(&var_name) {
        working_set.error(ParseError::NameIsBuiltinVar(
            String::from_utf8_lossy(var_name).to_string(),
            span,
        ))
    }
}

pub fn expand_to_cell_path(
    working_set: &mut StateWorkingSet,
    expression: &mut Expression,
    var_id: VarId,
) {
    trace!("parsing: expanding to cell path");
    if let Expression {
        expr: Expr::String(_),
        span,
        ..
    } = expression
    {
        // Re-parse the string as if it were a cell-path
        let new_expression = parse_full_cell_path(working_set, Some(var_id), *span);

        *expression = new_expression;
    }

    if let Expression {
        expr: Expr::UnaryNot(inner),
        ..
    } = expression
    {
        expand_to_cell_path(working_set, inner, var_id);
    }
}

pub fn parse_input_output_types(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
) -> Vec<(Type, Type)> {
    let mut full_span = Span::concat(spans);

    let mut bytes = working_set.get_span_contents(full_span);

    if bytes.starts_with(b"[") {
        bytes = &bytes[1..];
        full_span.start += 1;
    }

    if bytes.ends_with(b"]") {
        bytes = &bytes[..(bytes.len() - 1)];
        full_span.end -= 1;
    }

    let (tokens, parse_error) =
        lex_signature(bytes, full_span.start, &[b'\n', b'\r', b','], &[], true);

    if let Some(parse_error) = parse_error {
        working_set.error(parse_error);
    }

    let mut output = vec![];

    let mut idx = 0;
    while idx < tokens.len() {
        let type_bytes = working_set.get_span_contents(tokens[idx].span).to_vec();
        let input_type = parse_type(working_set, &type_bytes, tokens[idx].span);

        idx += 1;
        if idx >= tokens.len() {
            working_set.error(ParseError::Expected(
                "arrow (->)",
                Span::new(tokens[idx - 1].span.end, tokens[idx - 1].span.end),
            ));
            break;
        }

        let arrow = working_set.get_span_contents(tokens[idx].span);
        if arrow != b"->" {
            working_set.error(ParseError::Expected("arrow (->)", tokens[idx].span));
        }

        idx += 1;
        if idx >= tokens.len() {
            working_set.error(ParseError::MissingType(Span::new(
                tokens[idx - 1].span.end,
                tokens[idx - 1].span.end,
            )));
            break;
        }

        let type_bytes = working_set.get_span_contents(tokens[idx].span).to_vec();
        let output_type = parse_type(working_set, &type_bytes, tokens[idx].span);

        output.push((input_type, output_type));

        idx += 1;
    }

    output
}

pub fn parse_full_signature(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
    is_external: bool,
) -> Expression {
    match spans.len() {
        // This case should never happen. It corresponds to declarations like `def foo {}`,
        // which should throw a 'Missing required positional argument.' before getting to this point
        0 => {
            working_set.error(ParseError::InternalError(
                "failed to catch missing positional arguments".to_string(),
                Span::concat(spans),
            ));
            garbage(working_set, Span::concat(spans))
        }

        // e.g. `[ b"[foo: string]" ]`
        1 => parse_signature(working_set, spans[0], is_external),

        // This case is needed to distinguish between e.g.
        // `[ b"[]", b"{ true }" ]` vs `[ b"[]:", b"int" ]`
        2 if working_set.get_span_contents(spans[1]).starts_with(b"{") => {
            parse_signature(working_set, spans[0], is_external)
        }

        // This should handle every other case, e.g.
        // `[ b"[]:", b"int" ]`
        // `[ b"[]", b":", b"int" ]`
        // `[ b"[]", b":", b"int", b"->", b"bool" ]`
        _ => {
            let (mut arg_signature, input_output_types_pos) =
                if working_set.get_span_contents(spans[0]).ends_with(b":") {
                    (
                        parse_signature(
                            working_set,
                            Span::new(spans[0].start, spans[0].end.saturating_sub(1)),
                            is_external,
                        ),
                        1,
                    )
                } else if working_set.get_span_contents(spans[1]) == b":" {
                    (parse_signature(working_set, spans[0], is_external), 2)
                } else {
                    // This should be an error case, but we call parse_signature anyway
                    // so it can handle the various possible errors
                    // e.g. `[ b"[]", b"int" ]` or `[
                    working_set.error(ParseError::Expected(
                        "colon (:) before type signature",
                        Span::concat(&spans[1..]),
                    ));
                    // (garbage(working_set, Span::concat(spans)), 1)

                    (parse_signature(working_set, spans[0], is_external), 1)
                };

            let input_output_types =
                parse_input_output_types(working_set, &spans[input_output_types_pos..]);

            if let Expression {
                expr: Expr::Signature(sig),
                span: expr_span,
                ..
            } = &mut arg_signature
            {
                sig.input_output_types = input_output_types;
                expr_span.end = Span::concat(&spans[input_output_types_pos..]).end;
            }
            arg_signature
        }
    }
}

pub fn parse_row_condition(working_set: &mut StateWorkingSet, spans: &[Span]) -> Expression {
    let pos = spans.first().map(|s| s.start).unwrap_or(0);
    // New scope in case where there's already a variable named `$it`
    working_set.enter_scope();
    let var_id = working_set.add_variable(b"$it".to_vec(), Span::new(pos, pos), Type::Any, false);
    let expression = parse_math_expression(working_set, spans, Some(var_id));
    let span = Span::concat(spans);

    let block_id = match expression.expr {
        Expr::Block(block_id) => block_id,
        Expr::Closure(block_id) => block_id,
        Expr::FullCellPath(ref box_fcp) if box_fcp.head.as_var().is_some_and(|id| id != var_id) => {
            let mut expression = expression;
            expression.ty = Type::Any;
            working_set.exit_scope();
            return expression;
        }
        Expr::Var(arg_var_id) if arg_var_id != var_id => {
            let mut expression = expression;
            expression.ty = Type::Any;
            working_set.exit_scope();
            return expression;
        }
        _ => {
            // We have an expression, check that it's compatible with bool
            if !type_compatible(&Type::Bool, &expression.ty) {
                working_set.error(ParseError::TypeMismatch(
                    Type::Bool,
                    expression.ty.clone(),
                    expression.span,
                ));
                working_set.exit_scope();
                return Expression::garbage(working_set, expression.span);
            }

            // Convert this expression into a block.
            let mut block = Block::new();
            let mut pipeline = Pipeline::new();
            pipeline.elements.push(PipelineElement {
                pipe: None,
                expr: expression,
                redirection: None,
            });

            block.pipelines.push(pipeline);

            block.signature.required_positional.push(PositionalArg {
                name: "$it".into(),
                desc: "row condition".into(),
                shape: SyntaxShape::Any,
                var_id: Some(var_id),
                default_value: None,
                completion: None,
            });

            compile_block(working_set, &mut block);

            working_set.add_block(Arc::new(block))
        }
    };
    working_set.exit_scope();

    Expression::new(working_set, Expr::RowCondition(block_id), span, Type::Bool)
}

pub fn parse_signature(
    working_set: &mut StateWorkingSet,
    span: Span,
    is_external: bool,
) -> Expression {
    let bytes = working_set.get_span_contents(span);

    let mut start = span.start;
    let mut end = span.end;

    let mut has_paren = false;

    if bytes.starts_with(b"[") {
        start += 1;
    } else if bytes.starts_with(b"(") {
        has_paren = true;
        start += 1;
    } else {
        working_set.error(ParseError::Expected("[ or (", Span::new(start, start + 1)));
        return garbage(working_set, span);
    }

    if (has_paren && bytes.ends_with(b")")) || (!has_paren && bytes.ends_with(b"]")) {
        end -= 1;
    } else {
        working_set.error(ParseError::Unclosed("] or )".into(), Span::new(end, end)));
    }

    let sig = parse_signature_helper(working_set, Span::new(start, end), is_external);

    Expression::new(working_set, Expr::Signature(sig), span, Type::Any)
}

pub fn parse_signature_helper(
    working_set: &mut StateWorkingSet,
    span: Span,
    is_external: bool,
) -> Box<Signature> {
    enum ParseMode {
        Arg,
        AfterCommaArg,
        Type,
        AfterType,
        DefaultValue,
    }

    #[derive(Debug)]
    enum Arg {
        Positional {
            arg: PositionalArg,
            required: bool,
            type_annotated: bool,
        },
        RestPositional(PositionalArg),
        Flag {
            flag: Flag,
            type_annotated: bool,
        },
    }

    let source = working_set.get_span_contents(span);

    let (output, err) = lex_signature(
        source,
        span.start,
        &[b'\n', b'\r'],
        &[b':', b'=', b','],
        false,
    );
    if let Some(err) = err {
        working_set.error(err);
    }

    let mut args: Vec<Arg> = vec![];
    let mut parse_mode = ParseMode::Arg;
    // Track variables whose name→VarId mappings have not yet been inserted
    // into the overlay scope
    //
    // We defer all insertions until the entire signature is parsed so that
    // default value expressions always resolve to outer scope variables,
    // not to sibling parameters
    //
    // See #15306
    let mut pending_scope_inserts: Vec<(Vec<u8>, VarId)> = vec![];

    for (index, token) in output.iter().enumerate() {
        let last_token = index == output.len() - 1;

        match token {
            Token {
                contents: crate::TokenContents::Item | crate::TokenContents::AssignmentOperator,
                span,
            } => {
                let span = *span;
                let contents = working_set.get_span_contents(span).to_vec();

                // The : symbol separates types
                if contents == b":" {
                    match parse_mode {
                        ParseMode::Arg if last_token => working_set
                            .error(ParseError::Expected("type", Span::new(span.end, span.end))),
                        ParseMode::Arg => {
                            parse_mode = ParseMode::Type;
                        }
                        ParseMode::AfterCommaArg | ParseMode::AfterType => {
                            working_set.error(ParseError::Expected("parameter or flag", span));
                        }
                        ParseMode::Type | ParseMode::DefaultValue => {
                            // We're seeing two types for the same thing for some reason, error
                            working_set.error(ParseError::Expected("type", span));
                        }
                    }
                }
                // The = symbol separates a variable from its default value
                else if contents == b"=" {
                    match parse_mode {
                        ParseMode::Arg | ParseMode::AfterType if last_token => working_set.error(
                            ParseError::Expected("default value", Span::new(span.end, span.end)),
                        ),
                        ParseMode::Arg | ParseMode::AfterType => {
                            parse_mode = ParseMode::DefaultValue;
                        }
                        ParseMode::Type => {
                            working_set.error(ParseError::Expected("type", span));
                        }
                        ParseMode::AfterCommaArg => {
                            working_set.error(ParseError::Expected("parameter or flag", span));
                        }
                        ParseMode::DefaultValue => {
                            // We're seeing two default values for some reason, error
                            working_set.error(ParseError::Expected("default value", span));
                        }
                    }
                }
                // The , symbol separates params only
                else if contents == b"," {
                    match parse_mode {
                        ParseMode::Arg | ParseMode::AfterType => {
                            parse_mode = ParseMode::AfterCommaArg
                        }
                        ParseMode::AfterCommaArg => {
                            working_set.error(ParseError::Expected("parameter or flag", span));
                        }
                        ParseMode::Type => {
                            working_set.error(ParseError::Expected("type", span));
                        }
                        ParseMode::DefaultValue => {
                            working_set.error(ParseError::Expected("default value", span));
                        }
                    }
                } else {
                    let mut check_and_add_variable =
                        |working_set: &mut StateWorkingSet,
                         var_name: Vec<u8>,
                         ty: Type,
                         span: Span| {
                            if is_external {
                                None
                            } else {
                                ensure_not_reserved_variable_name(working_set, &var_name, span);
                                let var_id =
                                    working_set.add_variable_without_scope(span, ty, false);
                                pending_scope_inserts.push((var_name, var_id));
                                Some(var_id)
                            }
                        };

                    match parse_mode {
                        ParseMode::Arg | ParseMode::AfterCommaArg | ParseMode::AfterType => {
                            // Long flag with optional short form following with no whitespace, e.g. --output, --age(-a)
                            if contents.starts_with(b"--") && contents.len() > 2 {
                                // Split the long flag from the short flag with the ( character as delimiter.
                                // The trailing ) is removed further down.
                                let flags: Vec<_> = contents.split(|x| x == &b'(').collect();

                                let long = String::from_utf8_lossy(&flags[0][2..]).to_string();
                                let mut variable_name = flags[0][2..].to_vec();
                                // Replace the '-' in a variable name with '_'
                                for byte in variable_name.iter_mut() {
                                    if *byte == b'-' {
                                        *byte = b'_';
                                    }
                                }

                                if !is_variable(&variable_name) {
                                    working_set.error(ParseError::Expected(
                                        "valid variable name for this long flag",
                                        span,
                                    ))
                                }

                                let var_id = check_and_add_variable(
                                    working_set,
                                    variable_name,
                                    Type::Bool,
                                    span,
                                );

                                // If there's no short flag, exit now. Otherwise, parse it.
                                if flags.len() == 1 {
                                    args.push(Arg::Flag {
                                        flag: Flag {
                                            arg: None,
                                            desc: String::new(),
                                            long,
                                            short: None,
                                            required: false,
                                            var_id,
                                            default_value: None,
                                            completion: None,
                                        },
                                        type_annotated: false,
                                    });
                                } else if flags.len() >= 3 {
                                    working_set.error(ParseError::Expected(
                                        "only one short flag alternative",
                                        span,
                                    ));
                                } else {
                                    let short_flag = &flags[1];
                                    let short_flag = if !short_flag.starts_with(b"-")
                                        || !short_flag.ends_with(b")")
                                    {
                                        working_set.error(ParseError::Expected(
                                            "short flag alternative for the long flag",
                                            span,
                                        ));
                                        short_flag
                                    } else {
                                        // Obtain the flag's name by removing the starting - and trailing )
                                        &short_flag[1..(short_flag.len() - 1)]
                                    };
                                    // Note that it is currently possible to make a short flag with non-alphanumeric characters,
                                    // like -).

                                    let short_flag =
                                        String::from_utf8_lossy(short_flag).to_string();
                                    let chars: Vec<char> = short_flag.chars().collect();

                                    if chars.len() == 1 {
                                        args.push(Arg::Flag {
                                            flag: Flag {
                                                arg: None,
                                                desc: String::new(),
                                                long,
                                                short: Some(chars[0]),
                                                required: false,
                                                var_id,
                                                default_value: None,
                                                completion: None,
                                            },
                                            type_annotated: false,
                                        });
                                    } else {
                                        working_set.error(ParseError::Expected("short flag", span));
                                    }
                                }
                                parse_mode = ParseMode::Arg;
                            }
                            // Mandatory short flag, e.g. -e (must be one character)
                            else if contents.starts_with(b"-") && contents.len() > 1 {
                                let short_flag = &contents[1..];
                                let short_flag = String::from_utf8_lossy(short_flag).to_string();
                                let chars: Vec<char> = short_flag.chars().collect();

                                if chars.len() > 1 {
                                    working_set.error(ParseError::Expected("short flag", span));
                                }

                                let mut encoded_var_name = [0u8; 4];
                                let len = chars[0].encode_utf8(&mut encoded_var_name).len();
                                let variable_name = encoded_var_name[0..len].to_vec();

                                if !is_variable(&variable_name) {
                                    working_set.error(ParseError::Expected(
                                        "valid variable name for this short flag",
                                        span,
                                    ))
                                }

                                let var_id = check_and_add_variable(
                                    working_set,
                                    variable_name,
                                    Type::Bool,
                                    span,
                                );

                                args.push(Arg::Flag {
                                    flag: Flag {
                                        arg: None,
                                        desc: String::new(),
                                        long: String::new(),
                                        short: Some(chars[0]),
                                        required: false,
                                        var_id,
                                        default_value: None,
                                        completion: None,
                                    },
                                    type_annotated: false,
                                });
                                parse_mode = ParseMode::Arg;
                            }
                            // Short flag alias for long flag, e.g. --b (-a)
                            // This is the same as the short flag in --b(-a)
                            else if let Some(short_flag) = contents.strip_prefix(b"(-") {
                                if let ParseMode::AfterCommaArg = parse_mode {
                                    working_set
                                        .error(ParseError::Expected("parameter or flag", span));
                                }

                                let short_flag = if !short_flag.ends_with(b")") {
                                    working_set.error(ParseError::Expected("short flag", span));
                                    short_flag
                                } else {
                                    &short_flag[..(short_flag.len() - 1)]
                                };

                                let short_flag = String::from_utf8_lossy(short_flag).to_string();
                                let chars: Vec<char> = short_flag.chars().collect();

                                if chars.len() == 1 {
                                    match args.last_mut() {
                                        Some(Arg::Flag { flag, .. }) => {
                                            if flag.short.is_some() {
                                                working_set.error(ParseError::Expected(
                                                    "one short flag",
                                                    span,
                                                ));
                                            } else {
                                                flag.short = Some(chars[0]);
                                            }
                                        }
                                        _ => {
                                            working_set
                                                .error(ParseError::Expected("unknown flag", span));
                                        }
                                    }
                                } else {
                                    working_set.error(ParseError::Expected("short flag", span));
                                }
                            }
                            // Positional arg, optional
                            else if let Some(optional_param) = contents.strip_suffix(b"?") {
                                let name = String::from_utf8_lossy(optional_param).to_string();

                                if !is_variable(optional_param) {
                                    working_set.error(ParseError::Expected(
                                        "valid variable name for this optional parameter",
                                        span,
                                    ))
                                }

                                let var_id = check_and_add_variable(
                                    working_set,
                                    optional_param.to_vec(),
                                    Type::Any,
                                    span,
                                );

                                args.push(Arg::Positional {
                                    arg: PositionalArg {
                                        desc: String::new(),
                                        name,
                                        shape: SyntaxShape::Any,
                                        var_id,
                                        default_value: None,
                                        completion: None,
                                    },
                                    required: false,
                                    type_annotated: false,
                                });
                                parse_mode = ParseMode::Arg;
                            }
                            // Rest param
                            else if let Some(contents) = contents.strip_prefix(b"...") {
                                let name = String::from_utf8_lossy(contents).to_string();
                                let contents_vec: Vec<u8> = contents.to_vec();

                                if !is_variable(&contents_vec) {
                                    working_set.error(ParseError::Expected(
                                        "valid variable name for this rest parameter",
                                        span,
                                    ))
                                }

                                let var_id = check_and_add_variable(
                                    working_set,
                                    contents_vec,
                                    Type::Any,
                                    span,
                                );

                                args.push(Arg::RestPositional(PositionalArg {
                                    desc: String::new(),
                                    name,
                                    shape: SyntaxShape::Any,
                                    var_id,
                                    default_value: None,
                                    completion: None,
                                }));
                                parse_mode = ParseMode::Arg;
                            }
                            // Normal param
                            else {
                                let name = String::from_utf8_lossy(&contents).to_string();
                                let contents_vec = contents.to_vec();

                                if !is_variable(&contents_vec) {
                                    working_set.error(ParseError::Expected(
                                        "valid variable name for this parameter",
                                        span,
                                    ))
                                }

                                let var_id = check_and_add_variable(
                                    working_set,
                                    contents_vec,
                                    Type::Any,
                                    span,
                                );

                                // Positional arg, required
                                args.push(Arg::Positional {
                                    arg: PositionalArg {
                                        desc: String::new(),
                                        name,
                                        shape: SyntaxShape::Any,
                                        var_id,
                                        default_value: None,
                                        completion: None,
                                    },
                                    required: true,
                                    type_annotated: false,
                                });
                                parse_mode = ParseMode::Arg;
                            }
                        }
                        ParseMode::Type => {
                            if let Some(last) = args.last_mut() {
                                let (syntax_shape, completer) = if contents.contains(&b'@') {
                                    let mut split = contents.splitn(2, |b| b == &b'@');

                                    let shape_name = split
                                        .next()
                                        .expect("If `bytes` contains `@` splitn returns 2 slices");
                                    let shape_span =
                                        Span::new(span.start, span.start + shape_name.len());
                                    let cmd_span =
                                        Span::new(span.start + shape_name.len() + 1, span.end);
                                    let cmd_name = split
                                        .next()
                                        .expect("If `bytes` contains `@` splitn returns 2 slices");
                                    (
                                        parse_shape_name(working_set, shape_name, shape_span),
                                        parse_completer(working_set, cmd_name, cmd_span),
                                    )
                                } else {
                                    (parse_shape_name(working_set, &contents, span), None)
                                };
                                //TODO check if we're replacing a custom parameter already
                                match last {
                                    Arg::Positional {
                                        arg:
                                            PositionalArg {
                                                shape,
                                                var_id,
                                                completion,
                                                ..
                                            },
                                        required: _,
                                        type_annotated,
                                    } => {
                                        if !is_external {
                                            working_set.set_variable_type(
                                                var_id.expect(
                                                    "internal error: all custom parameters must have \
                                                    var_ids",
                                                ),
                                                syntax_shape.to_type(),
                                            );
                                        }
                                        *completion = completer;
                                        *shape = syntax_shape;
                                        *type_annotated = true;
                                    }
                                    Arg::RestPositional(PositionalArg {
                                        shape,
                                        var_id,
                                        completion,
                                        ..
                                    }) => {
                                        if !is_external {
                                            working_set.set_variable_type(
                                                var_id.expect(
                                                    "internal error: all custom parameters must have \
                                                    var_ids",
                                                ),
                                                Type::List(Box::new(syntax_shape.to_type())),
                                            );
                                        }
                                        *completion = completer;
                                        *shape = syntax_shape;
                                    }
                                    Arg::Flag {
                                        flag:
                                            Flag {
                                                arg,
                                                var_id,
                                                completion,
                                                ..
                                            },
                                        type_annotated,
                                    } => {
                                        if !is_external {
                                            working_set.set_variable_type(var_id.expect("internal error: all custom parameters must have var_ids"), syntax_shape.to_type());
                                        }
                                        if syntax_shape == SyntaxShape::Boolean {
                                            working_set.error(ParseError::LabeledError(
                                                "Type annotations are not allowed for boolean switches.".to_string(),
                                                "Remove the `: bool` type annotation.".to_string(),
                                                span,
                                            ));
                                        }
                                        *completion = completer;
                                        *arg = Some(syntax_shape);
                                        *type_annotated = true;
                                    }
                                }
                            }
                            parse_mode = ParseMode::AfterType;
                        }
                        ParseMode::DefaultValue => {
                            if !is_external && let Some(last) = args.last_mut() {
                                let expression = parse_value(working_set, span, &SyntaxShape::Any);

                                //TODO check if we're replacing a custom parameter already
                                match last {
                                    Arg::Positional {
                                        arg:
                                            PositionalArg {
                                                shape,
                                                var_id,
                                                default_value,
                                                ..
                                            },
                                        required,
                                        type_annotated,
                                    } => {
                                        let var_id = var_id.expect("internal error: all custom parameters must have var_ids");
                                        let var_type = &working_set.get_variable(var_id).ty;
                                        match var_type {
                                            Type::Any => {
                                                if !*type_annotated {
                                                    working_set.set_variable_type(
                                                        var_id,
                                                        expression.ty.clone(),
                                                    );
                                                }
                                            }
                                            _ => {
                                                if !type_compatible(var_type, &expression.ty) {
                                                    working_set.error(
                                                        ParseError::AssignmentMismatch(
                                                            "Default value wrong type".into(),
                                                            format!(
                                                            "expected default value to be `{var_type}`"
                                                        ),
                                                            expression.span,
                                                        ),
                                                    )
                                                }
                                            }
                                        }

                                        *default_value = if let Ok(constant) =
                                            eval_constant(working_set, &expression)
                                        {
                                            Some(constant)
                                        } else {
                                            working_set.error(ParseError::NonConstantDefaultValue(
                                                expression.span,
                                            ));
                                            None
                                        };

                                        if !*type_annotated {
                                            *shape = expression.ty.to_shape();
                                        }
                                        *required = false;
                                    }
                                    Arg::RestPositional(..) => {
                                        working_set.error(ParseError::AssignmentMismatch(
                                            "Rest parameter was given a default value".into(),
                                            "can't have default value".into(),
                                            expression.span,
                                        ))
                                    }
                                    Arg::Flag {
                                        flag:
                                            Flag {
                                                arg,
                                                var_id,
                                                default_value,
                                                ..
                                            },
                                        type_annotated,
                                    } => {
                                        let expression_span = expression.span;

                                        *default_value = if let Ok(value) =
                                            eval_constant(working_set, &expression)
                                        {
                                            Some(value)
                                        } else {
                                            working_set.error(ParseError::NonConstantDefaultValue(
                                                expression_span,
                                            ));
                                            None
                                        };

                                        let var_id = var_id.expect("internal error: all custom parameters must have var_ids");
                                        let var_type = &working_set.get_variable(var_id).ty;
                                        let expression_ty = expression.ty.clone();

                                        // Flags without type annotations are present/not-present
                                        // switches *except* when they have a default value
                                        // assigned. In that case they are regular flags and take
                                        // on the type of their default value.
                                        if !*type_annotated {
                                            *arg = Some(expression_ty.to_shape());
                                            working_set.set_variable_type(var_id, expression_ty);
                                        } else if !type_compatible(var_type, &expression_ty) {
                                            working_set.error(ParseError::AssignmentMismatch(
                                                "Default value is the wrong type".into(),
                                                format!(
                                                    "expected default value to be `{var_type}`"
                                                ),
                                                expression_span,
                                            ))
                                        }
                                    }
                                }
                            }
                            parse_mode = ParseMode::Arg;
                        }
                    }
                }
            }
            Token {
                contents: crate::TokenContents::Comment,
                span,
            } => {
                let contents = working_set.get_span_contents(Span::new(span.start + 1, span.end));

                let mut contents = String::from_utf8_lossy(contents).to_string();
                contents = contents.trim().into();

                if let Some(last) = args.last_mut() {
                    match last {
                        Arg::Flag { flag, .. } => {
                            if !flag.desc.is_empty() {
                                flag.desc.push('\n');
                            }
                            flag.desc.push_str(&contents);
                        }
                        Arg::Positional {
                            arg: positional, ..
                        } => {
                            if !positional.desc.is_empty() {
                                positional.desc.push('\n');
                            }
                            positional.desc.push_str(&contents);
                        }
                        Arg::RestPositional(positional) => {
                            if !positional.desc.is_empty() {
                                positional.desc.push('\n');
                            }
                            positional.desc.push_str(&contents);
                        }
                    }
                }
            }
            _ => {}
        }
    }

    for (name, var_id) in pending_scope_inserts {
        working_set.insert_variable_into_scope(name, var_id);
    }

    let mut sig = Signature::new(String::new());

    for arg in args {
        match arg {
            Arg::Positional {
                arg: positional,
                required,
                ..
            } => {
                if required {
                    if !sig.optional_positional.is_empty() {
                        working_set.error(ParseError::RequiredAfterOptional(
                            positional.name.clone(),
                            span,
                        ))
                    }
                    sig.required_positional.push(positional)
                } else {
                    sig.optional_positional.push(positional)
                }
            }
            Arg::Flag { flag, .. } => sig.named.push(flag),
            Arg::RestPositional(positional) => {
                if positional.name.is_empty() {
                    working_set.error(ParseError::RestNeedsName(span))
                } else if sig.rest_positional.is_none() {
                    sig.rest_positional = Some(PositionalArg {
                        name: positional.name,
                        ..positional
                    })
                } else {
                    // Too many rest params
                    working_set.error(ParseError::MultipleRestParams(span))
                }
            }
        }
    }

    Box::new(sig)
}

pub fn parse_list_expression(
    working_set: &mut StateWorkingSet,
    span: Span,
    element_shape: &SyntaxShape,
) -> Expression {
    let bytes = working_set.get_span_contents(span);

    let mut start = span.start;
    let mut end = span.end;

    if bytes.starts_with(b"[") {
        start += 1;
    }
    if bytes.ends_with(b"]") {
        end -= 1;
    } else {
        working_set.error(ParseError::Unclosed("]".into(), Span::new(end, end)));
    }

    let inner_span = Span::new(start, end);
    let source = working_set.get_span_contents(inner_span);

    let (output, err) = lex(source, inner_span.start, &[b'\n', b'\r', b','], &[], true);
    if let Some(err) = err {
        working_set.error(err)
    }

    let (mut output, err) = lite_parse(&output, working_set);
    if let Some(err) = err {
        working_set.error(err)
    }

    let mut args = vec![];

    let mut contained_type: Option<Type> = None;

    if !output.block.is_empty() {
        for mut command in output.block.remove(0).commands {
            let mut spans_idx = 0;

            while spans_idx < command.parts.len() {
                let curr_span = command.parts[spans_idx];
                let curr_tok = working_set.get_span_contents(curr_span);
                let (arg, ty) = if curr_tok.starts_with(b"...")
                    && curr_tok.len() > 3
                    && (curr_tok[3] == b'$' || curr_tok[3] == b'[' || curr_tok[3] == b'(')
                {
                    // Parse the spread operator
                    // Remove "..." before parsing argument to spread operator
                    command.parts[spans_idx] = Span::new(curr_span.start + 3, curr_span.end);
                    let spread_arg = parse_multispan_value(
                        working_set,
                        &command.parts,
                        &mut spans_idx,
                        &SyntaxShape::List(Box::new(element_shape.clone())),
                    );
                    let elem_ty = match &spread_arg.ty {
                        Type::List(elem_ty) => *elem_ty.clone(),
                        _ => Type::Any,
                    };
                    let span = Span::new(curr_span.start, curr_span.start + 3);
                    (ListItem::Spread(span, spread_arg), elem_ty)
                } else {
                    let arg = parse_multispan_value(
                        working_set,
                        &command.parts,
                        &mut spans_idx,
                        element_shape,
                    );
                    let ty = arg.ty.clone();
                    (ListItem::Item(arg), ty)
                };

                contained_type = match contained_type {
                    Some(ctype) => Some(ctype.widen(ty)),
                    None => Some(ty),
                };

                args.push(arg);

                spans_idx += 1;
            }
        }
    }

    Expression::new(
        working_set,
        Expr::List(args),
        span,
        Type::List(Box::new(if let Some(ty) = contained_type {
            ty
        } else {
            Type::Any
        })),
    )
}

fn parse_table_row(
    working_set: &mut StateWorkingSet,
    span: Span,
) -> Result<(Vec<Expression>, Span), Span> {
    let list = parse_list_expression(working_set, span, &SyntaxShape::Any);
    let Expression {
        expr: Expr::List(list),
        span,
        ..
    } = list
    else {
        unreachable!("the item must be a list")
    };

    list.into_iter()
        .map(|item| match item {
            ListItem::Item(expr) => Ok(expr),
            ListItem::Spread(_, spread) => Err(spread.span),
        })
        .collect::<Result<_, _>>()
        .map(|exprs| (exprs, span))
}

fn parse_table_expression(
    working_set: &mut StateWorkingSet,
    span: Span,
    list_element_shape: &SyntaxShape,
) -> Expression {
    let bytes = working_set.get_span_contents(span);
    let inner_span = {
        let start = if bytes.starts_with(b"[") {
            span.start + 1
        } else {
            span.start
        };

        let end = if bytes.ends_with(b"]") {
            span.end - 1
        } else {
            let end = span.end;
            working_set.error(ParseError::Unclosed("]".into(), Span::new(end, end)));
            span.end
        };

        Span::new(start, end)
    };

    let source = working_set.get_span_contents(inner_span);
    let (tokens, err) = lex(source, inner_span.start, &[b'\n', b'\r', b','], &[], true);
    if let Some(err) = err {
        working_set.error(err);
    }

    // Check that we have all arguments first, before trying to parse the first
    // in order to avoid exponential parsing time
    let [first, second, rest @ ..] = &tokens[..] else {
        return parse_list_expression(working_set, span, list_element_shape);
    };
    if !working_set.get_span_contents(first.span).starts_with(b"[")
        || second.contents != TokenContents::Semicolon
        || rest.is_empty()
    {
        return parse_list_expression(working_set, span, list_element_shape);
    };
    let head = parse_table_row(working_set, first.span);

    let errors = working_set.parse_errors.len();

    let (head, rows) = match head {
        Ok((head, _)) => {
            let rows = rest
                .iter()
                .filter_map(|it| {
                    use std::cmp::Ordering;

                    match working_set.get_span_contents(it.span) {
                        b"," => None,
                        text if !text.starts_with(b"[") => {
                            let err = ParseError::LabeledErrorWithHelp {
                                error: String::from("Table item not list"),
                                label: String::from("not a list"),
                                span: it.span,
                                help: String::from("All table items must be lists"),
                            };
                            working_set.error(err);
                            None
                        }
                        _ => match parse_table_row(working_set, it.span) {
                            Ok((list, span)) => {
                                match list.len().cmp(&head.len()) {
                                    Ordering::Less => {
                                        let err = ParseError::MissingColumns(head.len(), span);
                                        working_set.error(err);
                                    }
                                    Ordering::Greater => {
                                        let span = {
                                            let start = list[head.len()].span.start;
                                            let end = span.end;
                                            Span::new(start, end)
                                        };
                                        let err = ParseError::ExtraColumns(head.len(), span);
                                        working_set.error(err);
                                    }
                                    Ordering::Equal => {}
                                }
                                Some(list)
                            }
                            Err(span) => {
                                let err = ParseError::LabeledError(
                                    String::from("Cannot spread in a table row"),
                                    String::from("invalid spread here"),
                                    span,
                                );
                                working_set.error(err);
                                None
                            }
                        },
                    }
                })
                .collect();

            (head, rows)
        }
        Err(span) => {
            let err = ParseError::LabeledError(
                String::from("Cannot spread in a table row"),
                String::from("invalid spread here"),
                span,
            );
            working_set.error(err);
            (Vec::new(), Vec::new())
        }
    };

    let ty = if working_set.parse_errors.len() == errors {
        let (ty, errs) = table_type(&head, &rows);
        working_set.parse_errors.extend(errs);
        ty
    } else {
        Type::table()
    };

    let table = Table {
        columns: head.into(),
        rows: rows.into_iter().map(Into::into).collect(),
    };

    Expression::new(working_set, Expr::Table(table), span, ty)
}

fn table_type(head: &[Expression], rows: &[Vec<Expression>]) -> (Type, Vec<ParseError>) {
    let mut errors = vec![];
    let mut rows: Vec<_> = rows.iter().map(|row| row.iter()).collect();

    let column_types = std::iter::from_fn(move || {
        let column = rows
            .iter_mut()
            .filter_map(|row| row.next())
            .map(|col| col.ty.clone());
        Some(Type::supertype_of(column).unwrap_or(Type::Any))
    });

    let mk_error = |span| ParseError::LabeledErrorWithHelp {
        error: "Table column name not string".into(),
        label: "must be a string".into(),
        help: "Table column names should be able to be converted into strings".into(),
        span,
    };

    let ty: Box<[(String, Type)]> = head
        .iter()
        .zip(column_types)
        .filter_map(|(expr, col_ty)| {
            if !Type::String.is_subtype_of(&expr.ty) {
                errors.push(mk_error(expr.span));
                None
            } else {
                expr.as_string().zip(Some(col_ty))
            }
        })
        .collect();

    (Type::Table(ty), errors)
}

pub fn parse_block_expression(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    trace!("parsing: block expression");

    let bytes = working_set.get_span_contents(span);

    let mut start = span.start;
    let mut end = span.end;
    let mut is_closed = true;

    if bytes.starts_with(b"{") {
        start += 1;
    } else {
        working_set.error(ParseError::Expected("block", span));
        return garbage(working_set, span);
    }
    if bytes.ends_with(b"}") {
        end -= 1;
    } else {
        working_set.error(ParseError::Unclosed("}".into(), Span::new(end, end)));
        is_closed = false;
    }

    let inner_span = Span::new(start, end);

    let source = working_set.get_span_contents(inner_span);

    let (output, err) = lex(source, start, &[], &[], false);
    if let Some(err) = err {
        working_set.error(err);
    }

    working_set.enter_scope();

    // Check to see if we have parameters
    let (signature, amt_to_skip): (Option<(Box<Signature>, Span)>, usize) = match output.first() {
        Some(Token {
            contents: TokenContents::Pipe,
            span,
        }) => {
            working_set.error(ParseError::Expected("block but found closure", *span));
            (None, 0)
        }
        _ => (None, 0),
    };

    let mut output = parse_block(working_set, &output[amt_to_skip..], span, false, false);

    if let Some(signature) = signature {
        output.signature = signature.0;
    }

    output.span = Some(span);

    if is_closed {
        working_set.exit_scope();
    }

    let block_id = working_set.add_block(Arc::new(output));

    Expression::new(working_set, Expr::Block(block_id), span, Type::Block)
}

pub fn parse_match_block_expression(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let bytes = working_set.get_span_contents(span);

    let mut start = span.start;
    let mut end = span.end;
    let mut is_closed = true;

    if bytes.starts_with(b"{") {
        start += 1;
    } else {
        working_set.error(ParseError::Expected("closure", span));
        return garbage(working_set, span);
    }
    if bytes.ends_with(b"}") {
        end -= 1;
    } else {
        working_set.error(ParseError::Unclosed("}".into(), Span::new(end, end)));
        is_closed = false;
    }

    let inner_span = Span::new(start, end);

    let source = working_set.get_span_contents(inner_span);

    let (output, err) = lex(source, start, &[b' ', b'\r', b'\n', b',', b'|'], &[], true);
    if let Some(err) = err {
        working_set.error(err);
    }

    let mut position = 0;

    let mut output_matches = vec![];

    while position < output.len() {
        // Each match gets its own scope

        working_set.enter_scope();

        // First parse the pattern
        let mut pattern = parse_pattern(working_set, output[position].span);

        position += 1;

        if position >= output.len() {
            working_set.error(ParseError::Mismatch(
                "=>".into(),
                "end of input".into(),
                Span::new(output[position - 1].span.end, output[position - 1].span.end),
            ));

            working_set.exit_scope();
            break;
        }

        let mut connector = working_set.get_span_contents(output[position].span);

        // Multiple patterns connected by '|'
        if connector == b"|" && position < output.len() {
            let mut or_pattern = vec![pattern];

            while connector == b"|" && position < output.len() {
                connector = b"";

                position += 1;

                if position >= output.len() {
                    working_set.error(ParseError::Mismatch(
                        "pattern".into(),
                        "end of input".into(),
                        Span::new(output[position - 1].span.end, output[position - 1].span.end),
                    ));
                    break;
                }

                let pattern = parse_pattern(working_set, output[position].span);
                or_pattern.push(pattern);

                position += 1;
                if position >= output.len() {
                    working_set.error(ParseError::Mismatch(
                        "=>".into(),
                        "end of input".into(),
                        Span::new(output[position - 1].span.end, output[position - 1].span.end),
                    ));
                    break;
                } else {
                    connector = working_set.get_span_contents(output[position].span);
                }
            }

            let start = or_pattern
                .first()
                .expect("internal error: unexpected state of or-pattern")
                .span
                .start;
            let end = or_pattern
                .last()
                .expect("internal error: unexpected state of or-pattern")
                .span
                .end;

            pattern = MatchPattern {
                pattern: Pattern::Or(or_pattern),
                guard: None,
                span: Span::new(start, end),
            }
        }
        // A match guard
        if connector == b"if" {
            let if_end = {
                let end = output[position].span.end;
                Span::new(end, end)
            };

            position += 1;

            let mk_err = || ParseError::LabeledErrorWithHelp {
                error: "Match guard without an expression".into(),
                label: "expected an expression".into(),
                help: "The `if` keyword must be followed with an expression".into(),
                span: if_end,
            };

            if output.get(position).is_none() {
                working_set.error(mk_err());
                return garbage(working_set, span);
            };

            let (tokens, found) = if let Some((pos, _)) = output[position..]
                .iter()
                .find_position(|t| working_set.get_span_contents(t.span) == b"=>")
            {
                if position + pos == position {
                    working_set.error(mk_err());
                    return garbage(working_set, span);
                }

                (&output[position..position + pos], true)
            } else {
                (&output[position..], false)
            };

            let mut start = 0;
            let guard = parse_multispan_value(
                working_set,
                &tokens.iter().map(|tok| tok.span).collect_vec(),
                &mut start,
                &SyntaxShape::MathExpression,
            );

            pattern.guard = Some(Box::new(guard));
            position += if found { start + 1 } else { start };
            connector = working_set.get_span_contents(output[position].span);
        }
        // Then the `=>` arrow
        if connector != b"=>" {
            working_set.error(ParseError::Mismatch(
                "=>".into(),
                "end of input".into(),
                Span::new(output[position - 1].span.end, output[position - 1].span.end),
            ));
        } else {
            position += 1;
        }

        // Finally, the value/expression/block that we will run to produce the result
        if position >= output.len() {
            working_set.error(ParseError::Mismatch(
                "match result".into(),
                "end of input".into(),
                Span::new(output[position - 1].span.end, output[position - 1].span.end),
            ));

            working_set.exit_scope();
            break;
        }

        let result = parse_multispan_value(
            working_set,
            &[output[position].span],
            &mut 0,
            &SyntaxShape::OneOf(vec![SyntaxShape::Block, SyntaxShape::Expression]),
        );
        position += 1;
        if is_closed {
            working_set.exit_scope();
        }

        output_matches.push((pattern, result));
    }

    Expression::new(
        working_set,
        Expr::MatchBlock(output_matches),
        span,
        Type::Any,
    )
}

pub fn parse_closure_expression(
    working_set: &mut StateWorkingSet,
    shape: &SyntaxShape,
    span: Span,
) -> Expression {
    trace!("parsing: closure expression");

    let bytes = working_set.get_span_contents(span);

    let mut start = span.start;
    let mut end = span.end;
    let mut is_closed = true;

    if bytes.starts_with(b"{") {
        start += 1;
    } else {
        working_set.error(ParseError::Expected("closure", span));
        return garbage(working_set, span);
    }
    if bytes.ends_with(b"}") {
        end -= 1;
    } else {
        working_set.error(ParseError::Unclosed("}".into(), Span::new(end, end)));
        is_closed = false;
    }

    let inner_span = Span::new(start, end);

    let source = working_set.get_span_contents(inner_span);

    let (output, err) = lex(source, start, &[], &[], false);
    if let Some(err) = err {
        working_set.error(err);
    }

    working_set.enter_scope();

    // Check to see if we have parameters
    let (signature, amt_to_skip): (Option<(Box<Signature>, Span)>, usize) = match output.first() {
        Some(Token {
            contents: TokenContents::Pipe,
            span,
        }) => {
            // We've found a parameter list
            let start_point = span.start;
            let mut token_iter = output.iter().enumerate().skip(1);
            let mut end_span = None;
            let mut amt_to_skip = 1;

            for token in &mut token_iter {
                if let Token {
                    contents: TokenContents::Pipe,
                    span,
                } = token.1
                {
                    end_span = Some(span);
                    amt_to_skip += token.0;
                    break;
                }
            }

            let end_point = if let Some(span) = end_span {
                span.end
            } else {
                working_set.error(ParseError::Unclosed("|".into(), Span::new(end, end)));
                end
            };

            let signature_span = Span::new(start_point, end_point);
            let signature = parse_signature_helper(working_set, signature_span, false);

            (Some((signature, signature_span)), amt_to_skip)
        }
        Some(Token {
            contents: TokenContents::PipePipe,
            span,
        }) => (
            Some((Box::new(Signature::new("closure".to_string())), *span)),
            1,
        ),
        _ => (None, 0),
    };

    // TODO: Finish this
    if let SyntaxShape::Closure(Some(v)) = shape
        && let Some((sig, sig_span)) = &signature
    {
        if sig.num_positionals() > v.len() {
            working_set.error(ParseError::ExpectedWithStringMsg(
                format!(
                    "{} closure parameter{}",
                    v.len(),
                    if v.len() > 1 { "s" } else { "" }
                ),
                *sig_span,
            ));
        }

        for (expected, PositionalArg { name, shape, .. }) in
            v.iter().zip(sig.required_positional.iter())
        {
            if expected != shape && *shape != SyntaxShape::Any {
                working_set.error(ParseError::ParameterMismatchType(
                    name.to_owned(),
                    expected.to_string(),
                    shape.to_string(),
                    *sig_span,
                ));
            }
        }
    }

    let mut output = parse_block(working_set, &output[amt_to_skip..], span, false, false);

    // NOTE: closures need to be compiled eagerly due to these reasons:
    //  - their `Block`s (which contains their `IrBlock`) are stored in the working_set
    //  - Ir compiler does not have mutable access to the working_set and can't attach `IrBlock`s
    //  to existing `Block`s
    // so they can't be compiled as part of their parent `Block`'s compilation
    //
    // If the compiler used a mechanism similar to the `EngineState`/`StateWorkingSet` divide, we
    // could defer all compilation and apply the generated delta to `StateWorkingSet` afterwards.
    if working_set.parse_errors.is_empty() {
        compile_block(working_set, &mut output);
    }

    if let Some(signature) = signature {
        output.signature = signature.0;
    }

    output.span = Some(span);

    if is_closed {
        working_set.exit_scope();
    }

    let block_id = working_set.add_block(Arc::new(output));

    Expression::new(working_set, Expr::Closure(block_id), span, Type::Closure)
}

pub fn parse_value(
    working_set: &mut StateWorkingSet,
    span: Span,
    shape: &SyntaxShape,
) -> Expression {
    trace!("parsing: value: {shape}");

    let bytes = working_set.get_span_contents(span);

    if bytes.is_empty() {
        working_set.error(ParseError::IncompleteParser(span));
        return garbage(working_set, span);
    }

    match bytes[0] {
        b'$' => return parse_dollar_expr(working_set, span),
        b'(' => return parse_paren_expr(working_set, span, shape),
        b'{' => return parse_brace_expr(working_set, span, shape),
        b'[' => match shape {
            SyntaxShape::Any
            | SyntaxShape::List(_)
            | SyntaxShape::Table(_)
            | SyntaxShape::Signature
            | SyntaxShape::ExternalSignature
            | SyntaxShape::Filepath
            | SyntaxShape::String
            | SyntaxShape::GlobPattern
            | SyntaxShape::ExternalArgument => {}
            SyntaxShape::OneOf(possible_shapes) => {
                if !possible_shapes
                    .iter()
                    .any(|s| matches!(s, SyntaxShape::List(_)))
                {
                    working_set.error(ParseError::ExpectedWithStringMsg(shape.to_string(), span));
                    return Expression::garbage(working_set, span);
                }
            }
            _ => {
                working_set.error(ParseError::ExpectedWithStringMsg(shape.to_string(), span));
                return Expression::garbage(working_set, span);
            }
        },
        b'r' if bytes.len() > 1 && bytes[1] == b'#' => {
            return parse_raw_string(working_set, span);
        }
        _ => {}
    }

    match shape {
        SyntaxShape::Number => parse_number(working_set, span),
        SyntaxShape::Float => parse_float(working_set, span),
        SyntaxShape::Int => parse_int(working_set, span),
        SyntaxShape::Duration => parse_duration(working_set, span),
        SyntaxShape::DateTime => parse_datetime(working_set, span),
        SyntaxShape::Filesize => parse_filesize(working_set, span),
        SyntaxShape::Range => {
            parse_range(working_set, span).unwrap_or_else(|| garbage(working_set, span))
        }
        // Check for reserved keyword values
        SyntaxShape::Nothing | SyntaxShape::Any if bytes == b"null" => {
            Expression::new(working_set, Expr::Nothing, span, Type::Nothing)
        }
        SyntaxShape::Boolean | SyntaxShape::Any if bytes == b"true" => {
            Expression::new(working_set, Expr::Bool(true), span, Type::Bool)
        }
        SyntaxShape::Boolean | SyntaxShape::Any if bytes == b"false" => {
            Expression::new(working_set, Expr::Bool(false), span, Type::Bool)
        }
        SyntaxShape::Filepath
        | SyntaxShape::Directory
        | SyntaxShape::GlobPattern
        // TODO: this serves for backward compatibility.
        // As a consequence, for commands like `def foo [foo: string] {}`,
        // it forbids usage like `foo true`, have to call it explicitly with `foo "true"`.
        // On the other hand, given current `SyntaxShape` based `parse_value`, `foo 10.0` doesn't raise any error.
        // We want to fix this discrepancy in the future.
        | SyntaxShape::String
            if matches!(bytes, b"true" | b"false" | b"null") =>
        {
            working_set.error(ParseError::ExpectedWithStringMsg(shape.to_string(), span));
            garbage(working_set, span)
        }
        SyntaxShape::Filepath => parse_filepath(working_set, span),
        SyntaxShape::Directory => parse_directory(working_set, span),
        SyntaxShape::GlobPattern => parse_glob_pattern(working_set, span),
        SyntaxShape::String => parse_string(working_set, span),
        SyntaxShape::Binary => parse_binary(working_set, span),
        SyntaxShape::Signature if bytes.starts_with(b"[") => parse_signature(working_set, span, false),
        SyntaxShape::ExternalSignature if bytes.starts_with(b"[") => parse_signature(working_set, span, true),
        SyntaxShape::List(elem) if bytes.starts_with(b"[") => {
            parse_table_expression(working_set, span, elem)
        }
        SyntaxShape::Table(_) if bytes.starts_with(b"[") => {
            parse_table_expression(working_set, span, &SyntaxShape::Any)
        }
        SyntaxShape::CellPath => parse_simple_cell_path(working_set, span),

        // Be sure to return ParseError::Expected(..) if invoked for one of these shapes, but lex
        // stream doesn't start with '{'} -- parsing in SyntaxShape::Any arm depends on this error variant.
        SyntaxShape::Block | SyntaxShape::Closure(..) | SyntaxShape::Record(_) => {
            working_set.error(ParseError::Expected("block, closure or record", span));

            Expression::garbage(working_set, span)
        }

        SyntaxShape::ExternalArgument => parse_regular_external_arg(working_set, span),
        SyntaxShape::OneOf(possible_shapes) => {
            parse_oneof(working_set, &[span], &mut 0, possible_shapes, false)
        }

        SyntaxShape::Any => {
            if bytes.starts_with(b"[") {
                //parse_value(working_set, span, &SyntaxShape::Table)
                parse_full_cell_path(working_set, None, span)
            } else {
                let shapes = [
                    SyntaxShape::Binary,
                    SyntaxShape::Range,
                    SyntaxShape::Filesize,
                    SyntaxShape::Duration,
                    SyntaxShape::DateTime,
                    SyntaxShape::Int,
                    SyntaxShape::Number,
                    SyntaxShape::String,
                ];
                for shape in shapes.iter() {
                    let starting_error_count = working_set.parse_errors.len();

                    let s = parse_value(working_set, span, shape);

                    if starting_error_count == working_set.parse_errors.len() {
                        return s;
                    } else {
                        match working_set.parse_errors.get(starting_error_count) {
                            Some(
                                ParseError::Expected(_, _)
                                | ParseError::ExpectedWithStringMsg(_, _),
                            ) => {
                                working_set.parse_errors.truncate(starting_error_count);
                                continue;
                            }
                            _ => {
                                return s;
                            }
                        }
                    }
                }
                working_set.error(ParseError::Expected("any shape", span));
                garbage(working_set, span)
            }
        }
        _ => {
            working_set.error(ParseError::ExpectedWithStringMsg(shape.to_string(), span));
            garbage(working_set, span)
        }
    }
}

pub fn parse_assignment_operator(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let contents = working_set.get_span_contents(span);

    let operator = match contents {
        b"=" => Operator::Assignment(Assignment::Assign),
        b"+=" => Operator::Assignment(Assignment::AddAssign),
        b"-=" => Operator::Assignment(Assignment::SubtractAssign),
        b"*=" => Operator::Assignment(Assignment::MultiplyAssign),
        b"/=" => Operator::Assignment(Assignment::DivideAssign),
        b"++=" => Operator::Assignment(Assignment::ConcatenateAssign),
        _ => {
            working_set.error(ParseError::Expected("assignment operator", span));
            return garbage(working_set, span);
        }
    };

    Expression::new(working_set, Expr::Operator(operator), span, Type::Any)
}

pub fn parse_assignment_expression(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
) -> Expression {
    trace!("parsing: assignment expression");
    let expr_span = Span::concat(spans);

    // Assignment always has the most precedence, and its right-hand side can be a pipeline
    let Some(op_index) = spans
        .iter()
        .position(|span| is_assignment_operator(working_set.get_span_contents(*span)))
    else {
        working_set.error(ParseError::Expected("assignment expression", expr_span));
        return garbage(working_set, expr_span);
    };

    let lhs_spans = &spans[0..op_index];
    let op_span = spans[op_index];
    let rhs_spans = &spans[(op_index + 1)..];

    if lhs_spans.is_empty() {
        working_set.error(ParseError::Expected(
            "left hand side of assignment",
            op_span,
        ));
        return garbage(working_set, expr_span);
    }

    if rhs_spans.is_empty() {
        working_set.error(ParseError::Expected(
            "right hand side of assignment",
            op_span,
        ));
        return garbage(working_set, expr_span);
    }

    // Parse the lhs and operator as usual for a math expression
    let mut lhs = parse_expression(working_set, lhs_spans);
    // make sure that lhs is a mutable variable.
    match &lhs.expr {
        Expr::FullCellPath(p) => {
            if let Expr::Var(var_id) = p.head.expr
                && var_id != nu_protocol::ENV_VARIABLE_ID
                && !working_set.get_variable(var_id).mutable
            {
                working_set.error(ParseError::AssignmentRequiresMutableVar(lhs.span))
            }
        }
        _ => working_set.error(ParseError::AssignmentRequiresVar(lhs.span)),
    }

    let mut operator = parse_assignment_operator(working_set, op_span);

    // Re-parse the right-hand side as a subexpression
    let rhs_span = Span::concat(rhs_spans);

    let (rhs_tokens, rhs_error) = lex(
        working_set.get_span_contents(rhs_span),
        rhs_span.start,
        &[],
        &[],
        false,
    );
    working_set.parse_errors.extend(rhs_error);

    trace!("parsing: assignment right-hand side subexpression");
    let rhs_block = parse_block(working_set, &rhs_tokens, rhs_span, false, true);
    let rhs_ty = rhs_block.output_type();

    // TEMP: double-check that if the RHS block starts with an external call, it must start with a
    // caret. This is to mitigate the change in assignment parsing introduced in 0.97.0 which could
    // result in unintentional execution of commands.
    if let Some(Expr::ExternalCall(head, ..)) = rhs_block
        .pipelines
        .first()
        .and_then(|pipeline| pipeline.elements.first())
        .map(|element| &element.expr.expr)
    {
        let contents = working_set.get_span_contents(Span {
            start: head.span.start - 1,
            end: head.span.end,
        });
        if !contents.starts_with(b"^") {
            working_set.parse_errors.push(ParseError::LabeledErrorWithHelp {
                error: "External command calls must be explicit in assignments".into(),
                label: "add a caret (^) before the command name if you intended to run and capture its output".into(),
                help: "the parsing of assignments was changed in 0.97.0, and this would have previously been treated as a string. Alternatively, quote the string with single or double quotes to avoid it being interpreted as a command name. This restriction may be removed in a future release.".into(),
                span: head.span,
            });
        }
    }

    let rhs_block_id = working_set.add_block(Arc::new(rhs_block));
    let mut rhs = Expression::new(
        working_set,
        Expr::Subexpression(rhs_block_id),
        rhs_span,
        rhs_ty,
    );

    let (result_ty, err) = math_result_type(working_set, &mut lhs, &mut operator, &mut rhs);
    if let Some(err) = err {
        working_set.parse_errors.push(err);
    }

    Expression::new(
        working_set,
        Expr::BinaryOp(Box::new(lhs), Box::new(operator), Box::new(rhs)),
        expr_span,
        result_ty,
    )
}

pub fn parse_operator(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let contents = working_set.get_span_contents(span);

    let operator = match contents {
        b"==" => Operator::Comparison(Comparison::Equal),
        b"!=" => Operator::Comparison(Comparison::NotEqual),
        b"<" => Operator::Comparison(Comparison::LessThan),
        b"<=" => Operator::Comparison(Comparison::LessThanOrEqual),
        b">" => Operator::Comparison(Comparison::GreaterThan),
        b">=" => Operator::Comparison(Comparison::GreaterThanOrEqual),
        b"=~" | b"like" => Operator::Comparison(Comparison::RegexMatch),
        b"!~" | b"not-like" => Operator::Comparison(Comparison::NotRegexMatch),
        b"in" => Operator::Comparison(Comparison::In),
        b"not-in" => Operator::Comparison(Comparison::NotIn),
        b"has" => Operator::Comparison(Comparison::Has),
        b"not-has" => Operator::Comparison(Comparison::NotHas),
        b"starts-with" => Operator::Comparison(Comparison::StartsWith),
        b"not-starts-with" => Operator::Comparison(Comparison::NotStartsWith),
        b"ends-with" => Operator::Comparison(Comparison::EndsWith),
        b"not-ends-with" => Operator::Comparison(Comparison::NotEndsWith),
        b"+" => Operator::Math(Math::Add),
        b"-" => Operator::Math(Math::Subtract),
        b"*" => Operator::Math(Math::Multiply),
        b"/" => Operator::Math(Math::Divide),
        b"//" => Operator::Math(Math::FloorDivide),
        b"mod" => Operator::Math(Math::Modulo),
        b"**" => Operator::Math(Math::Pow),
        b"++" => Operator::Math(Math::Concatenate),
        b"bit-or" => Operator::Bits(Bits::BitOr),
        b"bit-xor" => Operator::Bits(Bits::BitXor),
        b"bit-and" => Operator::Bits(Bits::BitAnd),
        b"bit-shl" => Operator::Bits(Bits::ShiftLeft),
        b"bit-shr" => Operator::Bits(Bits::ShiftRight),
        b"or" => Operator::Boolean(Boolean::Or),
        b"xor" => Operator::Boolean(Boolean::Xor),
        b"and" => Operator::Boolean(Boolean::And),
        // WARNING: not actual operators below! Error handling only
        pow @ (b"^" | b"pow") => {
            working_set.error(ParseError::UnknownOperator(
                match pow {
                    b"^" => "^",
                    b"pow" => "pow",
                    _ => unreachable!(),
                },
                "Use '**' for exponentiation or 'bit-xor' for bitwise XOR.",
                span,
            ));
            return garbage(working_set, span);
        }
        equality @ (b"is" | b"===") => {
            working_set.error(ParseError::UnknownOperator(
                match equality {
                    b"is" => "is",
                    b"===" => "===",
                    _ => unreachable!(),
                },
                "Did you mean '=='?",
                span,
            ));
            return garbage(working_set, span);
        }
        b"contains" => {
            working_set.error(ParseError::UnknownOperator(
                "contains",
                "Did you mean 'has'?",
                span,
            ));
            return garbage(working_set, span);
        }
        b"%" => {
            working_set.error(ParseError::UnknownOperator(
                "%",
                "Did you mean 'mod'?",
                span,
            ));
            return garbage(working_set, span);
        }
        b"&" => {
            working_set.error(ParseError::UnknownOperator(
                "&",
                "Did you mean 'bit-and'?",
                span,
            ));
            return garbage(working_set, span);
        }
        b"<<" => {
            working_set.error(ParseError::UnknownOperator(
                "<<",
                "Did you mean 'bit-shl'?",
                span,
            ));
            return garbage(working_set, span);
        }
        b">>" => {
            working_set.error(ParseError::UnknownOperator(
                ">>",
                "Did you mean 'bit-shr'?",
                span,
            ));
            return garbage(working_set, span);
        }
        bits @ (b"bits-and" | b"bits-xor" | b"bits-or" | b"bits-shl" | b"bits-shr") => {
            working_set.error(ParseError::UnknownOperator(
                match bits {
                    b"bits-and" => "bits-and",
                    b"bits-xor" => "bits-xor",
                    b"bits-or" => "bits-or",
                    b"bits-shl" => "bits-shl",
                    b"bits-shr" => "bits-shr",
                    _ => unreachable!(),
                },
                match bits {
                    b"bits-and" => "Did you mean 'bit-and'?",
                    b"bits-xor" => "Did you mean 'bit-xor'?",
                    b"bits-or" => "Did you mean 'bit-or'?",
                    b"bits-shl" => "Did you mean 'bit-shl'?",
                    b"bits-shr" => "Did you mean 'bit-shr'?",
                    _ => unreachable!(),
                },
                span,
            ));
            return garbage(working_set, span);
        }
        op if is_assignment_operator(op) => {
            working_set.error(ParseError::Expected("a non-assignment operator", span));
            return garbage(working_set, span);
        }
        _ => {
            working_set.error(ParseError::Expected("operator", span));
            return garbage(working_set, span);
        }
    };

    Expression::new(working_set, Expr::Operator(operator), span, Type::Any)
}

pub fn parse_math_expression(
    working_set: &mut StateWorkingSet,
    spans: &[Span],
    lhs_row_var_id: Option<VarId>,
) -> Expression {
    trace!("parsing: math expression");

    // As the expr_stack grows, we increase the required precedence to grow larger
    // If, at any time, the operator we're looking at is the same or lower precedence
    // of what is in the expression stack, we collapse the expression stack.
    //
    // This leads to an expression stack that grows under increasing precedence and collapses
    // under decreasing/sustained precedence
    //
    // The end result is a stack that we can fold into binary operations as right associations
    // safely.

    let mut expr_stack: Vec<Expression> = vec![];

    let mut idx = 0;
    let mut last_prec = u8::MAX;

    let first_span = working_set.get_span_contents(spans[0]);

    let mut not_start_spans = vec![];

    if first_span == b"if" || first_span == b"match" {
        // If expression
        if spans.len() > 1 {
            return parse_call(working_set, spans, spans[0]);
        } else {
            working_set.error(ParseError::Expected(
                "expression",
                Span::new(spans[0].end, spans[0].end),
            ));
            return garbage(working_set, spans[0]);
        }
    } else if first_span == b"not" {
        not_start_spans.push(spans[idx].start);
        idx += 1;
        while idx < spans.len() {
            let next_value = working_set.get_span_contents(spans[idx]);

            if next_value == b"not" {
                not_start_spans.push(spans[idx].start);
                idx += 1;
            } else {
                break;
            }
        }

        if idx == spans.len() {
            working_set.error(ParseError::Expected(
                "expression",
                Span::new(spans[idx - 1].end, spans[idx - 1].end),
            ));
            return garbage(working_set, spans[idx - 1]);
        }
    }

    let mut lhs = parse_value(working_set, spans[idx], &SyntaxShape::Any);

    for not_start_span in not_start_spans.iter().rev() {
        lhs = Expression::new(
            working_set,
            Expr::UnaryNot(Box::new(lhs)),
            Span::new(*not_start_span, spans[idx].end),
            Type::Bool,
        );
    }
    not_start_spans.clear();

    idx += 1;

    if idx >= spans.len() {
        // We already found the one part of our expression, so let's expand
        if let Some(row_var_id) = lhs_row_var_id {
            expand_to_cell_path(working_set, &mut lhs, row_var_id);
        }
    }

    expr_stack.push(lhs);

    while idx < spans.len() {
        let op = parse_operator(working_set, spans[idx]);

        let op_prec = op.precedence();

        idx += 1;

        if idx == spans.len() {
            // Handle broken math expr `1 +` etc
            working_set.error(ParseError::IncompleteMathExpression(spans[idx - 1]));

            expr_stack.push(Expression::garbage(working_set, spans[idx - 1]));
            let missing_span = Span::new(spans[idx - 1].end, spans[idx - 1].end);
            expr_stack.push(Expression::garbage(working_set, missing_span));

            break;
        }

        let content = working_set.get_span_contents(spans[idx]);
        // allow `if` to be a special value for assignment.

        if content == b"if" || content == b"match" {
            let rhs = parse_call(working_set, &spans[idx..], spans[0]);
            expr_stack.push(op);
            expr_stack.push(rhs);
            break;
        } else if content == b"not" {
            not_start_spans.push(spans[idx].start);
            idx += 1;
            while idx < spans.len() {
                let next_value = working_set.get_span_contents(spans[idx]);

                if next_value == b"not" {
                    not_start_spans.push(spans[idx].start);
                    idx += 1;
                } else {
                    break;
                }
            }

            if idx == spans.len() {
                working_set.error(ParseError::Expected(
                    "expression",
                    Span::new(spans[idx - 1].end, spans[idx - 1].end),
                ));
                return garbage(working_set, spans[idx - 1]);
            }
        }
        let mut rhs = parse_value(working_set, spans[idx], &SyntaxShape::Any);

        for not_start_span in not_start_spans.iter().rev() {
            rhs = Expression::new(
                working_set,
                Expr::UnaryNot(Box::new(rhs)),
                Span::new(*not_start_span, spans[idx].end),
                Type::Bool,
            );
        }
        not_start_spans.clear();

        // Parsing power must be right-associative unlike most operations which are left
        // Hence, we should not collapse if the last and current operations are both power
        let is_left_associative =
            op.expr != Expr::Operator(Operator::Math(Math::Pow)) && op_prec <= last_prec;

        while is_left_associative && expr_stack.len() > 1 {
            // Collapse the right associated operations first
            // so that we can get back to a stack with a lower precedence
            let mut rhs = expr_stack
                .pop()
                .expect("internal error: expression stack empty");
            let mut op = expr_stack
                .pop()
                .expect("internal error: expression stack empty");

            last_prec = op.precedence();

            if last_prec < op_prec {
                expr_stack.push(op);
                expr_stack.push(rhs);
                break;
            }

            let mut lhs = expr_stack
                .pop()
                .expect("internal error: expression stack empty");

            if let Some(row_var_id) = lhs_row_var_id {
                expand_to_cell_path(working_set, &mut lhs, row_var_id);
            }

            let (result_ty, err) = math_result_type(working_set, &mut lhs, &mut op, &mut rhs);
            if let Some(err) = err {
                working_set.error(err);
            }

            let op_span = Span::append(lhs.span, rhs.span);
            expr_stack.push(Expression::new(
                working_set,
                Expr::BinaryOp(Box::new(lhs), Box::new(op), Box::new(rhs)),
                op_span,
                result_ty,
            ));
        }
        expr_stack.push(op);
        expr_stack.push(rhs);

        last_prec = op_prec;

        idx += 1;
    }

    while expr_stack.len() != 1 {
        let mut rhs = expr_stack
            .pop()
            .expect("internal error: expression stack empty");
        let mut op = expr_stack
            .pop()
            .expect("internal error: expression stack empty");
        let mut lhs = expr_stack
            .pop()
            .expect("internal error: expression stack empty");

        if let Some(row_var_id) = lhs_row_var_id {
            expand_to_cell_path(working_set, &mut lhs, row_var_id);
        }

        let (result_ty, err) = math_result_type(working_set, &mut lhs, &mut op, &mut rhs);
        if let Some(err) = err {
            working_set.error(err)
        }

        let binary_op_span = Span::append(lhs.span, rhs.span);
        expr_stack.push(Expression::new(
            working_set,
            Expr::BinaryOp(Box::new(lhs), Box::new(op), Box::new(rhs)),
            binary_op_span,
            result_ty,
        ));
    }

    expr_stack
        .pop()
        .expect("internal error: expression stack empty")
}

pub fn parse_expression(working_set: &mut StateWorkingSet, spans: &[Span]) -> Expression {
    trace!("parsing: expression");

    let mut pos = 0;
    let mut shorthand = vec![];

    while pos < spans.len() {
        // Check if there is any environment shorthand
        let name = working_set.get_span_contents(spans[pos]);

        let split: Vec<_> = name.splitn(2, |x| *x == b'=').collect();
        if split.len() != 2 || !is_env_variable_name(split[0]) {
            break;
        }

        let point = split[0].len() + 1;
        let starting_error_count = working_set.parse_errors.len();

        let rhs = if spans[pos].start + point < spans[pos].end {
            let rhs_span = Span::new(spans[pos].start + point, spans[pos].end);
            if split[1].starts_with(b"$") {
                parse_dollar_expr(working_set, rhs_span)
            } else {
                parse_string_strict(working_set, rhs_span)
            }
        } else {
            Expression::new(
                working_set,
                Expr::String(String::new()),
                Span::unknown(),
                Type::Nothing,
            )
        };

        let lhs_span = Span::new(spans[pos].start, spans[pos].start + point - 1);
        let lhs = parse_string_strict(working_set, lhs_span);

        if starting_error_count == working_set.parse_errors.len() {
            shorthand.push((lhs, rhs));
            pos += 1;
        } else {
            working_set.parse_errors.truncate(starting_error_count);
            break;
        }
    }

    if pos == spans.len() {
        working_set.error(ParseError::UnknownCommand(spans[0]));
        return garbage(working_set, Span::concat(spans));
    }

    let output = if spans[pos..]
        .iter()
        .any(|span| is_assignment_operator(working_set.get_span_contents(*span)))
    {
        parse_assignment_expression(working_set, &spans[pos..])
    } else if is_math_expression_like(working_set, spans[pos]) {
        parse_math_expression(working_set, &spans[pos..], None)
    } else {
        let bytes = working_set.get_span_contents(spans[pos]).to_vec();

        // For now, check for special parses of certain keywords
        match bytes.as_slice() {
            b"def" | b"extern" | b"for" | b"module" | b"use" | b"source" | b"alias" | b"export"
            | b"export-env" | b"hide" => {
                working_set.error(ParseError::BuiltinCommandInPipeline(
                    String::from_utf8(bytes)
                        .expect("builtin commands bytes should be able to convert to string"),
                    spans[0],
                ));

                parse_call(working_set, &spans[pos..], spans[0])
            }
            b"const" | b"mut" => {
                working_set.error(ParseError::AssignInPipeline(
                    String::from_utf8(bytes)
                        .expect("builtin commands bytes should be able to convert to string"),
                    String::from_utf8_lossy(match spans.len() {
                        1..=3 => b"value",
                        _ => working_set.get_span_contents(spans[3]),
                    })
                    .to_string(),
                    String::from_utf8_lossy(match spans.len() {
                        1 => b"variable",
                        _ => working_set.get_span_contents(spans[1]),
                    })
                    .to_string(),
                    spans[0],
                ));
                parse_call(working_set, &spans[pos..], spans[0])
            }
            b"overlay" => {
                if spans.len() > 1 && working_set.get_span_contents(spans[1]) == b"list" {
                    // whitelist 'overlay list'
                    parse_call(working_set, &spans[pos..], spans[0])
                } else {
                    working_set.error(ParseError::BuiltinCommandInPipeline(
                        "overlay".into(),
                        spans[0],
                    ));

                    parse_call(working_set, &spans[pos..], spans[0])
                }
            }
            b"where" => parse_where_expr(working_set, &spans[pos..]),
            #[cfg(feature = "plugin")]
            b"plugin" => {
                if spans.len() > 1 && working_set.get_span_contents(spans[1]) == b"use" {
                    // only 'plugin use' is banned
                    working_set.error(ParseError::BuiltinCommandInPipeline(
                        "plugin use".into(),
                        spans[0],
                    ));
                }

                parse_call(working_set, &spans[pos..], spans[0])
            }

            _ => parse_call(working_set, &spans[pos..], spans[0]),
        }
    };

    if !shorthand.is_empty() {
        let with_env = working_set.find_decl(b"with-env");
        if let Some(decl_id) = with_env {
            let mut block = Block::default();
            let ty = output.ty.clone();
            block.pipelines = vec![Pipeline::from_vec(vec![output])];
            block.span = Some(Span::concat(spans));

            compile_block(working_set, &mut block);

            let block_id = working_set.add_block(Arc::new(block));

            let mut env_vars = vec![];
            for sh in shorthand {
                env_vars.push(RecordItem::Pair(sh.0, sh.1));
            }

            let arguments = vec![
                Argument::Positional(Expression::new(
                    working_set,
                    Expr::Record(env_vars),
                    Span::concat(&spans[..pos]),
                    Type::Any,
                )),
                Argument::Positional(Expression::new(
                    working_set,
                    Expr::Closure(block_id),
                    Span::concat(&spans[pos..]),
                    Type::Closure,
                )),
            ];

            let expr = Expr::Call(Box::new(Call {
                head: Span::unknown(),
                decl_id,
                arguments,
                parser_info: HashMap::new(),
            }));

            Expression::new(working_set, expr, Span::concat(spans), ty)
        } else {
            output
        }
    } else {
        output
    }
}

pub fn parse_builtin_commands(
    working_set: &mut StateWorkingSet,
    lite_command: &LiteCommand,
) -> Pipeline {
    trace!("parsing: builtin commands");
    if !is_math_expression_like(working_set, lite_command.parts[0])
        && !is_unaliasable_parser_keyword(working_set, &lite_command.parts)
    {
        trace!("parsing: not math expression or unaliasable parser keyword");
        let name = working_set.get_span_contents(lite_command.parts[0]);
        if let Some(decl_id) = working_set.find_decl(name) {
            let cmd = working_set.get_decl(decl_id);
            if cmd.is_alias() {
                // Parse keywords that can be aliased. Note that we check for "unaliasable" keywords
                // because alias can have any name, therefore, we can't check for "aliasable" keywords.
                let call_expr = parse_call(working_set, &lite_command.parts, lite_command.parts[0]);

                if let Expression {
                    expr: Expr::Call(call),
                    ..
                } = call_expr
                {
                    // Apply parse keyword side effects
                    let cmd = working_set.get_decl(call.decl_id);
                    match cmd.name() {
                        "overlay hide" => return parse_overlay_hide(working_set, call),
                        "overlay new" => return parse_overlay_new(working_set, call),
                        "overlay use" => return parse_overlay_use(working_set, call),
                        _ => { /* this alias is not a parser keyword */ }
                    }
                }
            }
        }
    }

    trace!("parsing: checking for keywords");
    let name = lite_command
        .command_parts()
        .first()
        .map(|s| working_set.get_span_contents(*s))
        .unwrap_or(b"");

    match name {
        // `parse_def` and `parse_extern` work both with and without attributes
        b"def" => parse_def(working_set, lite_command, None).0,
        b"extern" => parse_extern(working_set, lite_command, None),
        // `parse_export_in_block` also handles attributes by itself
        b"export" => parse_export_in_block(working_set, lite_command),
        b"export-env" => parse_export_env(working_set, &lite_command.parts).0,
        // Other definitions can't have attributes, so we handle attributes here with parse_attribute_block
        _ if lite_command.has_attributes() => parse_attribute_block(working_set, lite_command),
        b"let" => parse_let(
            working_set,
            &lite_command
                .parts_including_redirection()
                .collect::<Vec<Span>>(),
        ),
        b"const" => parse_const(working_set, &lite_command.parts).0,
        b"mut" => parse_mut(
            working_set,
            &lite_command
                .parts_including_redirection()
                .collect::<Vec<Span>>(),
        ),
        b"for" => {
            let expr = parse_for(working_set, lite_command);
            Pipeline::from_vec(vec![expr])
        }
        b"alias" => parse_alias(working_set, lite_command, None),
        b"module" => parse_module(working_set, lite_command, None).0,
        b"use" => parse_use(working_set, lite_command, None).0,
        b"overlay" => {
            if let Some(redirection) = lite_command.redirection.as_ref() {
                working_set.error(redirecting_builtin_error("overlay", redirection));
                return garbage_pipeline(working_set, &lite_command.parts);
            }
            parse_keyword(working_set, lite_command)
        }
        b"source" | b"source-env" => parse_source(working_set, lite_command),
        b"hide" => parse_hide(working_set, lite_command),
        b"where" => parse_where(working_set, lite_command),
        // Only "plugin use" is a keyword
        #[cfg(feature = "plugin")]
        b"plugin"
            if lite_command
                .parts
                .get(1)
                .is_some_and(|span| working_set.get_span_contents(*span) == b"use") =>
        {
            if let Some(redirection) = lite_command.redirection.as_ref() {
                working_set.error(redirecting_builtin_error("plugin use", redirection));
                return garbage_pipeline(working_set, &lite_command.parts);
            }
            parse_keyword(working_set, lite_command)
        }
        _ => {
            let element = parse_pipeline_element(working_set, lite_command);

            // There is still a chance to make `parse_pipeline_element` parse into
            // some keyword that should apply side effects first, Example:
            //
            // module a { export alias b = overlay use first.nu };
            // use a
            // a b
            //
            // In this case, `a b` will be parsed as a pipeline element, which leads
            // to the `overlay use` command.
            // In this case, we need to ensure that the side effects of these keywords
            // are applied.
            if let Expression {
                expr: Expr::Call(call),
                ..
            } = &element.expr
            {
                // Apply parse keyword side effects
                let cmd = working_set.get_decl(call.decl_id);
                match cmd.name() {
                    "overlay hide" => return parse_overlay_hide(working_set, call.clone()),
                    "overlay new" => return parse_overlay_new(working_set, call.clone()),
                    "overlay use" => return parse_overlay_use(working_set, call.clone()),
                    _ => { /* this alias is not a parser keyword */ }
                }
            }
            Pipeline {
                elements: vec![element],
            }
        }
    }
}

fn check_record_key_or_value(
    working_set: &StateWorkingSet,
    expr: &Expression,
    position: &str,
) -> Option<ParseError> {
    let bareword_error = |string_value: &Expression| {
        working_set
            .get_span_contents(string_value.span)
            .iter()
            .find_position(|b| **b == b':')
            .map(|(i, _)| {
                let colon_position = i + string_value.span.start;
                ParseError::InvalidLiteral(
                    "colon".to_string(),
                    format!("bare word specifying record {position}"),
                    Span::new(colon_position, colon_position + 1),
                )
            })
    };
    let value_span = working_set.get_span_contents(expr.span);
    match expr.expr {
        Expr::String(_) => {
            if ![b'"', b'\'', b'`'].contains(&value_span[0]) {
                bareword_error(expr)
            } else {
                None
            }
        }
        Expr::StringInterpolation(ref expressions) => {
            if value_span[0] != b'$' {
                expressions
                    .iter()
                    .filter(|expr| matches!(expr.expr, Expr::String(_)))
                    .filter_map(bareword_error)
                    .next()
            } else {
                None
            }
        }
        _ => None,
    }
}

pub fn parse_record(working_set: &mut StateWorkingSet, span: Span) -> Expression {
    let bytes = working_set.get_span_contents(span);

    let mut start = span.start;
    let mut end = span.end;

    if bytes.starts_with(b"{") {
        start += 1;
    } else {
        working_set.error(ParseError::Expected("{", Span::new(start, start + 1)));
        return garbage(working_set, span);
    }

    let mut unclosed = false;
    let mut extra_tokens = false;
    if bytes.ends_with(b"}") {
        end -= 1;
    } else {
        unclosed = true;
    }

    let inner_span = Span::new(start, end);

    let mut lex_state = LexState {
        input: working_set.get_span_contents(inner_span),
        output: Vec::new(),
        error: None,
        span_offset: start,
    };
    while !lex_state.input.is_empty() {
        if let Some(ParseError::Unbalanced(left, right, _)) = lex_state.error.as_ref()
            && left == "{"
            && right == "}"
        {
            extra_tokens = true;
            unclosed = false;
            break;
        }
        let additional_whitespace = &[b'\n', b'\r', b','];
        if lex_n_tokens(&mut lex_state, additional_whitespace, &[b':'], true, 1) < 1 {
            break;
        };
        let span = lex_state
            .output
            .last()
            .expect("should have gotten 1 token")
            .span;
        let contents = working_set.get_span_contents(span);
        if contents.len() > 3
            && contents.starts_with(b"...")
            && (contents[3] == b'$' || contents[3] == b'{' || contents[3] == b'(')
        {
            // This was a spread operator, so there's no value
            continue;
        }
        // Get token for colon
        if lex_n_tokens(&mut lex_state, additional_whitespace, &[b':'], true, 1) < 1 {
            break;
        };
        // Get token for value
        if lex_n_tokens(&mut lex_state, additional_whitespace, &[], true, 1) < 1 {
            break;
        };
    }
    let (tokens, err) = (lex_state.output, lex_state.error);

    if unclosed {
        working_set.error(ParseError::Unclosed("}".into(), Span::new(end, end)));
    } else if extra_tokens {
        working_set.error(ParseError::ExtraTokensAfterClosingDelimiter(Span::new(
            lex_state.span_offset,
            end,
        )));
    }

    if let Some(err) = err {
        working_set.error(err);
    }

    let mut output = vec![];
    let mut idx = 0;

    let mut field_types = Some(vec![]);
    while idx < tokens.len() {
        let curr_span = tokens[idx].span;
        let curr_tok = working_set.get_span_contents(curr_span);
        if curr_tok.starts_with(b"...")
            && curr_tok.len() > 3
            && (curr_tok[3] == b'$' || curr_tok[3] == b'{' || curr_tok[3] == b'(')
        {
            // Parse spread operator
            let inner = parse_value(
                working_set,
                Span::new(curr_span.start + 3, curr_span.end),
                &SyntaxShape::Record(vec![]),
            );
            idx += 1;

            match &inner.ty {
                Type::Record(inner_fields) => {
                    if let Some(fields) = &mut field_types {
                        for (field, ty) in inner_fields.as_ref() {
                            fields.push((field.clone(), ty.clone()));
                        }
                    }
                }
                _ => {
                    // We can't properly see all the field types
                    // so fall back to the Any type later
                    field_types = None;
                }
            }
            output.push(RecordItem::Spread(
                Span::new(curr_span.start, curr_span.start + 3),
                inner,
            ));
        } else {
            // Normal key-value pair
            let field_token = &tokens[idx];
            let field = if field_token.contents != TokenContents::Item {
                working_set.error(ParseError::Expected(
                    "item in record key position",
                    Span::new(field_token.span.start, field_token.span.end),
                ));
                garbage(working_set, curr_span)
            } else {
                let field = parse_value(working_set, curr_span, &SyntaxShape::String);
                if let Some(error) = check_record_key_or_value(working_set, &field, "key") {
                    working_set.error(error);
                    garbage(working_set, field.span)
                } else {
                    field
                }
            };

            idx += 1;
            if idx == tokens.len() {
                working_set.error(ParseError::Expected(
                    "':'",
                    Span::new(curr_span.end, curr_span.end),
                ));
                output.push(RecordItem::Pair(
                    garbage(working_set, curr_span),
                    garbage(working_set, Span::new(curr_span.end, curr_span.end)),
                ));
                break;
            }
            let colon_span = tokens[idx].span;
            let colon = working_set.get_span_contents(colon_span);
            idx += 1;
            if colon != b":" {
                working_set.error(ParseError::Expected(
                    "':'",
                    Span::new(colon_span.start, colon_span.start),
                ));
                output.push(RecordItem::Pair(
                    field,
                    garbage(
                        working_set,
                        Span::new(colon_span.start, tokens[tokens.len() - 1].span.end),
                    ),
                ));
                break;
            }
            if idx == tokens.len() {
                working_set.error(ParseError::Expected(
                    "value for record field",
                    Span::new(colon_span.end, colon_span.end),
                ));
                output.push(RecordItem::Pair(
                    garbage(working_set, Span::new(curr_span.start, colon_span.end)),
                    garbage(
                        working_set,
                        Span::new(colon_span.end, tokens[tokens.len() - 1].span.end),
                    ),
                ));
                break;
            }

            let value_token = &tokens[idx];
            let value = if value_token.contents != TokenContents::Item {
                working_set.error(ParseError::Expected(
                    "item in record value position",
                    Span::new(value_token.span.start, value_token.span.end),
                ));
                garbage(
                    working_set,
                    Span::new(value_token.span.start, value_token.span.end),
                )
            } else {
                let value = parse_value(working_set, tokens[idx].span, &SyntaxShape::Any);
                if let Some(parse_error) = check_record_key_or_value(working_set, &value, "value") {
                    working_set.error(parse_error);
                    garbage(working_set, value.span)
                } else {
                    value
                }
            };
            idx += 1;

            if let Some(field) = field.as_string() {
                if let Some(fields) = &mut field_types {
                    fields.push((field, value.ty.clone()));
                }
            } else {
                // We can't properly see all the field types
                // so fall back to the Any type later
                field_types = None;
            }
            output.push(RecordItem::Pair(field, value));
        }
    }

    Expression::new(
        working_set,
        Expr::Record(output),
        span,
        if let Some(fields) = field_types {
            Type::Record(fields.into())
        } else {
            Type::Any
        },
    )
}

fn parse_redirection_target(
    working_set: &mut StateWorkingSet,
    target: &LiteRedirectionTarget,
) -> RedirectionTarget {
    match target {
        LiteRedirectionTarget::File {
            connector,
            file,
            append,
        } => RedirectionTarget::File {
            expr: parse_value(working_set, *file, &SyntaxShape::Any),
            append: *append,
            span: *connector,
        },
        LiteRedirectionTarget::Pipe { connector } => RedirectionTarget::Pipe { span: *connector },
    }
}

pub(crate) fn parse_redirection(
    working_set: &mut StateWorkingSet,
    target: &LiteRedirection,
) -> PipelineRedirection {
    match target {
        LiteRedirection::Single { source, target } => PipelineRedirection::Single {
            source: *source,
            target: parse_redirection_target(working_set, target),
        },
        LiteRedirection::Separate { out, err } => PipelineRedirection::Separate {
            out: parse_redirection_target(working_set, out),
            err: parse_redirection_target(working_set, err),
        },
    }
}

fn parse_pipeline_element(
    working_set: &mut StateWorkingSet,
    command: &LiteCommand,
) -> PipelineElement {
    trace!("parsing: pipeline element");

    let expr = parse_expression(working_set, &command.parts);

    let redirection = command
        .redirection
        .as_ref()
        .map(|r| parse_redirection(working_set, r));

    PipelineElement {
        pipe: command.pipe,
        expr,
        redirection,
    }
}

pub(crate) fn redirecting_builtin_error(
    name: &'static str,
    redirection: &LiteRedirection,
) -> ParseError {
    match redirection {
        LiteRedirection::Single { target, .. } => {
            ParseError::RedirectingBuiltinCommand(name, target.connector(), None)
        }
        LiteRedirection::Separate { out, err } => ParseError::RedirectingBuiltinCommand(
            name,
            out.connector().min(err.connector()),
            Some(out.connector().max(err.connector())),
        ),
    }
}

pub fn parse_pipeline(working_set: &mut StateWorkingSet, pipeline: &LitePipeline) -> Pipeline {
    if pipeline.commands.len() > 1 {
        // Parse a normal multi command pipeline
        let elements: Vec<_> = pipeline
            .commands
            .iter()
            .enumerate()
            .map(|(index, element)| {
                let element = parse_pipeline_element(working_set, element);
                // Handle $in for pipeline elements beyond the first one
                if index > 0 && element.has_in_variable(working_set) {
                    wrap_element_with_collect(working_set, element.clone())
                } else {
                    element
                }
            })
            .collect();

        Pipeline { elements }
    } else {
        // If there's only one command in the pipeline, this could be a builtin command
        parse_builtin_commands(working_set, &pipeline.commands[0])
    }
}

pub fn parse_block(
    working_set: &mut StateWorkingSet,
    tokens: &[Token],
    span: Span,
    scoped: bool,
    is_subexpression: bool,
) -> Block {
    let (lite_block, err) = lite_parse(tokens, working_set);
    if let Some(err) = err {
        working_set.error(err);
    }

    trace!("parsing block: {lite_block:?}");

    if scoped {
        working_set.enter_scope();
    }

    // Pre-declare any definition so that definitions
    // that share the same block can see each other
    for pipeline in &lite_block.block {
        if pipeline.commands.len() == 1 {
            parse_def_predecl(working_set, pipeline.commands[0].command_parts())
        }
    }

    let mut block = Block::new_with_capacity(lite_block.block.len());
    block.span = Some(span);

    for lite_pipeline in &lite_block.block {
        let pipeline = parse_pipeline(working_set, lite_pipeline);
        block.pipelines.push(pipeline);
    }

    // If this is not a subexpression and there are any pipelines where the first element has $in,
    // we can wrap the whole block in collect so that they all reference the same $in
    if !is_subexpression
        && block
            .pipelines
            .iter()
            .flat_map(|pipeline| pipeline.elements.first())
            .any(|element| element.has_in_variable(working_set))
    {
        // Move the block out to prepare it to become a subexpression
        let inner_block = std::mem::take(&mut block);
        block.span = inner_block.span;
        let ty = inner_block.output_type();
        let block_id = working_set.add_block(Arc::new(inner_block));

        // Now wrap it in a Collect expression, and put it in the block as the only pipeline
        let subexpression = Expression::new(working_set, Expr::Subexpression(block_id), span, ty);
        let collect = wrap_expr_with_collect(working_set, subexpression);

        block.pipelines.push(Pipeline {
            elements: vec![PipelineElement {
                pipe: None,
                expr: collect,
                redirection: None,
            }],
        });
    }

    if scoped {
        working_set.exit_scope();
    }

    let errors = type_check::check_block_input_output(working_set, &block);
    if !errors.is_empty() {
        working_set.parse_errors.extend_from_slice(&errors);
    }

    block
}

/// Compile an [IrBlock][nu_protocol::ir::IrBlock] for the [Block], adding a compile error on
/// failure.
///
/// To compile a block that's already in the [StateWorkingSet] use [compile_block_with_id]
pub fn compile_block(working_set: &mut StateWorkingSet<'_>, block: &mut Block) {
    if !working_set.parse_errors.is_empty() {
        // This means there might be a bug in the parser, since calling this function while parse
        // errors are present is a logic error. However, it's not fatal and it's best to continue
        // without doing anything.
        log::error!("compile_block called with parse errors");
        return;
    }

    match nu_engine::compile(working_set, block) {
        Ok(ir_block) => {
            block.ir_block = Some(ir_block);
        }
        Err(err) => working_set.compile_errors.push(err),
    }
}

/// Compile an [IrBlock][nu_protocol::ir::IrBlock] for a [Block] that's already in the
/// [StateWorkingSet] using its id, adding a compile error on failure.
pub fn compile_block_with_id(working_set: &mut StateWorkingSet<'_>, block_id: BlockId) {
    if !working_set.parse_errors.is_empty() {
        // This means there might be a bug in the parser, since calling this function while parse
        // errors are present is a logic error. However, it's not fatal and it's best to continue
        // without doing anything.
        log::error!("compile_block_with_id called with parse errors");
        return;
    }

    match nu_engine::compile(working_set, working_set.get_block(block_id)) {
        Ok(ir_block) => {
            working_set.get_block_mut(block_id).ir_block = Some(ir_block);
        }
        Err(err) => {
            working_set.compile_errors.push(err);
        }
    };
}

pub fn discover_captures_in_closure(
    working_set: &StateWorkingSet,
    block: &Block,
    seen: &mut Vec<VarId>,
    seen_blocks: &mut HashMap<BlockId, Vec<(VarId, Span)>>,
    output: &mut Vec<(VarId, Span)>,
) -> Result<(), ParseError> {
    for flag in &block.signature.named {
        if let Some(var_id) = flag.var_id {
            seen.push(var_id);
        }
    }

    for positional in &block.signature.required_positional {
        if let Some(var_id) = positional.var_id {
            seen.push(var_id);
        }
    }
    for positional in &block.signature.optional_positional {
        if let Some(var_id) = positional.var_id {
            seen.push(var_id);
        }
    }
    if let Some(positional) = &block.signature.rest_positional
        && let Some(var_id) = positional.var_id
    {
        seen.push(var_id);
    }

    for pipeline in &block.pipelines {
        discover_captures_in_pipeline(working_set, pipeline, seen, seen_blocks, output)?;
    }

    Ok(())
}

fn discover_captures_in_pipeline(
    working_set: &StateWorkingSet,
    pipeline: &Pipeline,
    seen: &mut Vec<VarId>,
    seen_blocks: &mut HashMap<BlockId, Vec<(VarId, Span)>>,
    output: &mut Vec<(VarId, Span)>,
) -> Result<(), ParseError> {
    for element in &pipeline.elements {
        discover_captures_in_pipeline_element(working_set, element, seen, seen_blocks, output)?;
    }

    Ok(())
}

// Closes over captured variables
pub fn discover_captures_in_pipeline_element(
    working_set: &StateWorkingSet,
    element: &PipelineElement,
    seen: &mut Vec<VarId>,
    seen_blocks: &mut HashMap<BlockId, Vec<(VarId, Span)>>,
    output: &mut Vec<(VarId, Span)>,
) -> Result<(), ParseError> {
    discover_captures_in_expr(working_set, &element.expr, seen, seen_blocks, output)?;

    if let Some(redirection) = element.redirection.as_ref() {
        match redirection {
            PipelineRedirection::Single { target, .. } => {
                if let Some(expr) = target.expr() {
                    discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
                }
            }
            PipelineRedirection::Separate { out, err } => {
                if let Some(expr) = out.expr() {
                    discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
                }
                if let Some(expr) = err.expr() {
                    discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
                }
            }
        }
    }

    Ok(())
}

pub fn discover_captures_in_pattern(pattern: &MatchPattern, seen: &mut Vec<VarId>) {
    match &pattern.pattern {
        Pattern::Variable(var_id) => seen.push(*var_id),
        Pattern::List(items) => {
            for item in items {
                discover_captures_in_pattern(item, seen)
            }
        }
        Pattern::Record(items) => {
            for item in items {
                discover_captures_in_pattern(&item.1, seen)
            }
        }
        Pattern::Or(patterns) => {
            for pattern in patterns {
                discover_captures_in_pattern(pattern, seen)
            }
        }
        Pattern::Rest(var_id) => seen.push(*var_id),
        Pattern::Expression(_)
        | Pattern::Value(_)
        | Pattern::IgnoreValue
        | Pattern::IgnoreRest
        | Pattern::Garbage => {}
    }
}

// Closes over captured variables
pub fn discover_captures_in_expr(
    working_set: &StateWorkingSet,
    expr: &Expression,
    seen: &mut Vec<VarId>,
    seen_blocks: &mut HashMap<BlockId, Vec<(VarId, Span)>>,
    output: &mut Vec<(VarId, Span)>,
) -> Result<(), ParseError> {
    match &expr.expr {
        Expr::AttributeBlock(ab) => {
            discover_captures_in_expr(working_set, &ab.item, seen, seen_blocks, output)?;
        }
        Expr::BinaryOp(lhs, _, rhs) => {
            discover_captures_in_expr(working_set, lhs, seen, seen_blocks, output)?;
            discover_captures_in_expr(working_set, rhs, seen, seen_blocks, output)?;
        }
        Expr::UnaryNot(expr) => {
            discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
        }
        Expr::Closure(block_id) => {
            let block = working_set.get_block(*block_id);
            let results = {
                let mut seen = vec![];
                let mut results = vec![];

                discover_captures_in_closure(
                    working_set,
                    block,
                    &mut seen,
                    seen_blocks,
                    &mut results,
                )?;

                for (var_id, span) in results.iter() {
                    if !seen.contains(var_id)
                        && let Some(variable) = working_set.get_variable_if_possible(*var_id)
                        && variable.mutable
                    {
                        return Err(ParseError::CaptureOfMutableVar(*span));
                    }
                }

                results
            };
            seen_blocks.insert(*block_id, results.clone());
            for (var_id, span) in results.into_iter() {
                if !seen.contains(&var_id) {
                    output.push((var_id, span))
                }
            }
        }
        Expr::Block(block_id) => {
            let block = working_set.get_block(*block_id);
            // FIXME: is this correct?
            let results = {
                let mut seen = vec![];
                let mut results = vec![];
                discover_captures_in_closure(
                    working_set,
                    block,
                    &mut seen,
                    seen_blocks,
                    &mut results,
                )?;
                results
            };

            seen_blocks.insert(*block_id, results.clone());
            for (var_id, span) in results.into_iter() {
                if !seen.contains(&var_id) {
                    output.push((var_id, span))
                }
            }
        }
        Expr::Binary(_) => {}
        Expr::Bool(_) => {}
        Expr::Call(call) => {
            let decl = working_set.get_decl(call.decl_id);
            if let Some(block_id) = decl.block_id() {
                match seen_blocks.get(&block_id) {
                    Some(capture_list) => {
                        // Push captures onto the outer closure that aren't created by that outer closure
                        for capture in capture_list {
                            if !seen.contains(&capture.0) {
                                output.push(*capture);
                            }
                        }
                    }
                    None => {
                        let block = working_set.get_block(block_id);
                        if !block.captures.is_empty() {
                            for (capture, span) in &block.captures {
                                if !seen.contains(capture) {
                                    output.push((*capture, *span));
                                }
                            }
                        } else {
                            let result = {
                                let mut seen = vec![];
                                seen_blocks.insert(block_id, vec![]);

                                let mut result = vec![];
                                discover_captures_in_closure(
                                    working_set,
                                    block,
                                    &mut seen,
                                    seen_blocks,
                                    &mut result,
                                )?;

                                result
                            };
                            // Push captures onto the outer closure that aren't created by that outer closure
                            for capture in &result {
                                if !seen.contains(&capture.0) {
                                    output.push(*capture);
                                }
                            }

                            seen_blocks.insert(block_id, result);
                        }
                    }
                }
            }

            for arg in &call.arguments {
                match arg {
                    Argument::Named(named) => {
                        if let Some(arg) = &named.2 {
                            discover_captures_in_expr(working_set, arg, seen, seen_blocks, output)?;
                        }
                    }
                    Argument::Positional(expr)
                    | Argument::Unknown(expr)
                    | Argument::Spread(expr) => {
                        discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
                    }
                }
            }
        }
        Expr::CellPath(_) => {}
        Expr::DateTime(_) => {}
        Expr::ExternalCall(head, args) => {
            discover_captures_in_expr(working_set, head, seen, seen_blocks, output)?;

            for ExternalArgument::Regular(expr) | ExternalArgument::Spread(expr) in args.as_ref() {
                discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
            }
        }
        Expr::Filepath(_, _) => {}
        Expr::Directory(_, _) => {}
        Expr::Float(_) => {}
        Expr::FullCellPath(cell_path) => {
            discover_captures_in_expr(working_set, &cell_path.head, seen, seen_blocks, output)?;
        }
        Expr::ImportPattern(_) => {}
        Expr::Overlay(_) => {}
        Expr::Garbage => {}
        Expr::Nothing => {}
        Expr::GlobPattern(_, _) => {}
        Expr::Int(_) => {}
        Expr::Keyword(kw) => {
            discover_captures_in_expr(working_set, &kw.expr, seen, seen_blocks, output)?;
        }
        Expr::List(list) => {
            for item in list {
                discover_captures_in_expr(working_set, item.expr(), seen, seen_blocks, output)?;
            }
        }
        Expr::Operator(_) => {}
        Expr::Range(range) => {
            if let Some(from) = &range.from {
                discover_captures_in_expr(working_set, from, seen, seen_blocks, output)?;
            }
            if let Some(next) = &range.next {
                discover_captures_in_expr(working_set, next, seen, seen_blocks, output)?;
            }
            if let Some(to) = &range.to {
                discover_captures_in_expr(working_set, to, seen, seen_blocks, output)?;
            }
        }
        Expr::Record(items) => {
            for item in items {
                match item {
                    RecordItem::Pair(field_name, field_value) => {
                        discover_captures_in_expr(
                            working_set,
                            field_name,
                            seen,
                            seen_blocks,
                            output,
                        )?;
                        discover_captures_in_expr(
                            working_set,
                            field_value,
                            seen,
                            seen_blocks,
                            output,
                        )?;
                    }
                    RecordItem::Spread(_, record) => {
                        discover_captures_in_expr(working_set, record, seen, seen_blocks, output)?;
                    }
                }
            }
        }
        Expr::Signature(sig) => {
            // Something with a declaration, similar to a var decl, will introduce more VarIds into the stack at eval
            for pos in &sig.required_positional {
                if let Some(var_id) = pos.var_id {
                    seen.push(var_id);
                }
            }
            for pos in &sig.optional_positional {
                if let Some(var_id) = pos.var_id {
                    seen.push(var_id);
                }
            }
            if let Some(rest) = &sig.rest_positional
                && let Some(var_id) = rest.var_id
            {
                seen.push(var_id);
            }
            for named in &sig.named {
                if let Some(var_id) = named.var_id {
                    seen.push(var_id);
                }
            }
        }
        Expr::String(_) => {}
        Expr::RawString(_) => {}
        Expr::StringInterpolation(exprs) | Expr::GlobInterpolation(exprs, _) => {
            for expr in exprs {
                discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
            }
        }
        Expr::MatchBlock(match_block) => {
            for match_ in match_block {
                discover_captures_in_pattern(&match_.0, seen);
                discover_captures_in_expr(working_set, &match_.1, seen, seen_blocks, output)?;
            }
        }
        Expr::Collect(var_id, expr) => {
            seen.push(*var_id);
            discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?
        }
        Expr::RowCondition(block_id) | Expr::Subexpression(block_id) => {
            let block = working_set.get_block(*block_id);

            let results = {
                let mut results = vec![];
                let mut seen = vec![];
                discover_captures_in_closure(
                    working_set,
                    block,
                    &mut seen,
                    seen_blocks,
                    &mut results,
                )?;
                results
            };

            seen_blocks.insert(*block_id, results.clone());
            for (var_id, span) in results.into_iter() {
                if !seen.contains(&var_id) {
                    output.push((var_id, span))
                }
            }
        }
        Expr::Table(table) => {
            for header in table.columns.as_ref() {
                discover_captures_in_expr(working_set, header, seen, seen_blocks, output)?;
            }
            for row in table.rows.as_ref() {
                for cell in row.as_ref() {
                    discover_captures_in_expr(working_set, cell, seen, seen_blocks, output)?;
                }
            }
        }
        Expr::ValueWithUnit(value) => {
            discover_captures_in_expr(working_set, &value.expr, seen, seen_blocks, output)?;
        }
        Expr::Var(var_id) => {
            if (*var_id > ENV_VARIABLE_ID || *var_id == IN_VARIABLE_ID) && !seen.contains(var_id) {
                output.push((*var_id, expr.span));
            }
        }
        Expr::VarDecl(var_id) => {
            seen.push(*var_id);
        }
    }
    Ok(())
}

fn wrap_redirection_with_collect(
    working_set: &mut StateWorkingSet,
    target: RedirectionTarget,
) -> RedirectionTarget {
    match target {
        RedirectionTarget::File { expr, append, span } => RedirectionTarget::File {
            expr: wrap_expr_with_collect(working_set, expr),
            span,
            append,
        },
        RedirectionTarget::Pipe { span } => RedirectionTarget::Pipe { span },
    }
}

fn wrap_element_with_collect(
    working_set: &mut StateWorkingSet,
    element: PipelineElement,
) -> PipelineElement {
    PipelineElement {
        pipe: element.pipe,
        expr: wrap_expr_with_collect(working_set, element.expr),
        redirection: element.redirection.map(|r| match r {
            PipelineRedirection::Single { source, target } => PipelineRedirection::Single {
                source,
                target: wrap_redirection_with_collect(working_set, target),
            },
            PipelineRedirection::Separate { out, err } => PipelineRedirection::Separate {
                out: wrap_redirection_with_collect(working_set, out),
                err: wrap_redirection_with_collect(working_set, err),
            },
        }),
    }
}

fn wrap_expr_with_collect(working_set: &mut StateWorkingSet, expr: Expression) -> Expression {
    let span = expr.span;

    // IN_VARIABLE_ID should get replaced with a unique variable, so that we don't have to
    // execute as a closure
    let var_id = working_set.add_variable(
        b"$in".into(),
        Span::new(span.start, span.start),
        Type::Any,
        false,
    );
    let mut expr = expr.clone();
    expr.replace_in_variable(working_set, var_id);

    // Bind the custom `$in` variable for that particular expression
    let ty = expr.ty.clone();
    Expression::new(
        working_set,
        Expr::Collect(var_id, Box::new(expr)),
        span,
        // We can expect it to have the same result type
        ty,
    )
}

// Parses a vector of u8 to create an AST Block. If a file name is given, then
// the name is stored in the working set. When parsing a source without a file
// name, the source of bytes is stored as "source"
pub fn parse(
    working_set: &mut StateWorkingSet,
    fname: Option<&str>,
    contents: &[u8],
    scoped: bool,
) -> Arc<Block> {
    trace!("parse");
    let name = match fname {
        Some(fname) => {
            // use the canonical name for this filename
            nu_path::expand_to_real_path(fname)
                .to_string_lossy()
                .to_string()
        }
        None => "source".to_string(),
    };

    let file_id = working_set.add_file(name, contents);
    let new_span = working_set.get_span_for_file(file_id);

    let previously_parsed_block = working_set.find_block_by_span(new_span);

    let mut output = {
        if let Some(block) = previously_parsed_block {
            return block;
        } else {
            let (output, err) = lex(contents, new_span.start, &[], &[], false);
            if let Some(err) = err {
                working_set.error(err)
            }

            Arc::new(parse_block(working_set, &output, new_span, scoped, false))
        }
    };

    // Top level `Block`s are compiled eagerly, as they don't have a parent which would cause them
    // to be compiled later.
    if working_set.parse_errors.is_empty() {
        compile_block(working_set, Arc::make_mut(&mut output));
    }

    let mut seen = vec![];
    let mut seen_blocks = HashMap::new();

    let mut captures = vec![];
    match discover_captures_in_closure(
        working_set,
        &output,
        &mut seen,
        &mut seen_blocks,
        &mut captures,
    ) {
        Ok(_) => {
            Arc::make_mut(&mut output).captures = captures;
        }
        Err(err) => working_set.error(err),
    }

    // Also check other blocks that might have been imported
    let mut errors = vec![];
    for (block_idx, block) in working_set.delta.blocks.iter().enumerate() {
        let block_id = block_idx + working_set.permanent_state.num_blocks();
        let block_id = BlockId::new(block_id);

        if !seen_blocks.contains_key(&block_id) {
            let mut captures = vec![];

            match discover_captures_in_closure(
                working_set,
                block,
                &mut seen,
                &mut seen_blocks,
                &mut captures,
            ) {
                Ok(_) => {
                    seen_blocks.insert(block_id, captures);
                }
                Err(err) => {
                    errors.push(err);
                }
            }
        }
    }
    for err in errors {
        working_set.error(err)
    }

    for (block_id, captures) in seen_blocks.into_iter() {
        // In theory, we should only be updating captures where we have new information
        // the only place where this is possible would be blocks that are newly created
        // by our working set delta. If we ever tried to modify the permanent state, we'd
        // panic (again, in theory, this shouldn't be possible)
        let block = working_set.get_block(block_id);
        let block_captures_empty = block.captures.is_empty();
        // need to check block_id >= working_set.permanent_state.num_blocks()
        // to avoid mutate a block that is in the permanent state.
        // this can happened if user defines a function with recursive call
        // and pipe a variable to the command, e.g:
        // def px [] { if true { 42 } else { px } };    # the block px is saved in permanent state.
        // let x = 3
        // $x | px
        // If we don't guard for `block_id`, it will change captures of `px`, which is
        // already saved in permanent state
        if !captures.is_empty()
            && block_captures_empty
            && block_id.get() >= working_set.permanent_state.num_blocks()
        {
            let block = working_set.get_block_mut(block_id);
            block.captures = captures;
        }
    }

    output
}