nu_engine/

documentation.rs

use crate::eval_call;
use fancy_regex::{Captures, Regex};
use nu_protocol::{
    Category, Config, IntoPipelineData, PipelineData, PositionalArg, Signature, Span, SpanId,
    Spanned, SyntaxShape, Type, Value,
    ast::{Argument, Call, Expr, Expression, RecordItem},
    debugger::WithoutDebug,
    engine::CommandType,
    engine::{Command, EngineState, Stack, UNKNOWN_SPAN_ID},
    record,
};
use nu_utils::terminal_size;
use std::{
    borrow::Cow,
    collections::HashMap,
    fmt::Write,
    sync::{Arc, LazyLock},
};

/// ANSI style reset
const RESET: &str = "\x1b[0m";
/// ANSI set default color (as set in the terminal)
const DEFAULT_COLOR: &str = "\x1b[39m";
/// ANSI set default dimmed
const DEFAULT_DIMMED: &str = "\x1b[2;39m";
/// ANSI set default italic
const DEFAULT_ITALIC: &str = "\x1b[3;39m";

pub fn get_full_help(
    command: &dyn Command,
    engine_state: &EngineState,
    stack: &mut Stack,
) -> String {
    // Precautionary step to capture any command output generated during this operation. We
    // internally call several commands (`table`, `ansi`, `nu-highlight`) and get their
    // `PipelineData` using this `Stack`, any other output should not be redirected like the main
    // execution.
    let stack = &mut stack.start_collect_value();

    let nu_config = stack.get_config(engine_state);

    let sig = engine_state
        .get_signature(command)
        .update_from_command(command);

    // Create ansi colors
    let mut help_style = HelpStyle::default();
    help_style.update_from_config(engine_state, &nu_config);

    let mut long_desc = String::new();

    let desc = &sig.description;
    if !desc.is_empty() {
        long_desc.push_str(&highlight_code(desc, engine_state, stack));
        long_desc.push_str("\n\n");
    }

    let extra_desc = &sig.extra_description;
    if !extra_desc.is_empty() {
        long_desc.push_str(&highlight_code(extra_desc, engine_state, stack));
        long_desc.push_str("\n\n");
    }

    match command.command_type() {
        CommandType::Alias => get_alias_documentation(
            &mut long_desc,
            command,
            &sig,
            &help_style,
            engine_state,
            stack,
        ),
        _ => get_command_documentation(
            &mut long_desc,
            command,
            &sig,
            &nu_config,
            &help_style,
            engine_state,
            stack,
        ),
    };

    if !nu_config.use_ansi_coloring.get(engine_state) {
        nu_utils::strip_ansi_string_likely(long_desc)
    } else {
        long_desc
    }
}

/// Syntax highlight code using the `nu-highlight` command if available
fn try_nu_highlight(
    code_string: &str,
    reject_garbage: bool,
    engine_state: &EngineState,
    stack: &mut Stack,
) -> Option<String> {
    let highlighter = engine_state.find_decl(b"nu-highlight", &[])?;

    let decl = engine_state.get_decl(highlighter);
    let mut call = Call::new(Span::unknown());
    if reject_garbage {
        call.add_named((
            Spanned {
                item: "reject-garbage".into(),
                span: Span::unknown(),
            },
            None,
            None,
        ));
    }

    decl.run(
        engine_state,
        stack,
        &(&call).into(),
        Value::string(code_string, Span::unknown()).into_pipeline_data(),
    )
    .and_then(|pipe| pipe.into_value(Span::unknown()))
    .and_then(|val| val.coerce_into_string())
    .ok()
}

/// Syntax highlight code using the `nu-highlight` command if available, falling back to the given string
fn nu_highlight_string(code_string: &str, engine_state: &EngineState, stack: &mut Stack) -> String {
    try_nu_highlight(code_string, false, engine_state, stack)
        .unwrap_or_else(|| code_string.to_string())
}

/// Apply code highlighting to code in a capture group
fn highlight_capture_group(
    captures: &Captures,
    engine_state: &EngineState,
    stack: &mut Stack,
) -> String {
    let Some(content) = captures.get(1) else {
        // this shouldn't happen
        return String::new();
    };

    // Save current color config
    let config_old = stack.get_config(engine_state);
    let mut config = (*config_old).clone();

    // Style externals and external arguments with fallback style,
    // so nu-highlight styles code which is technically valid syntax,
    // but not an internal command is highlighted with the fallback style
    let code_style = Value::record(
        record! {
            "attr" => Value::string("di", Span::unknown()),
        },
        Span::unknown(),
    );
    let color_config = &mut config.color_config;
    color_config.insert("shape_external".into(), code_style.clone());
    color_config.insert("shape_external_resolved".into(), code_style.clone());
    color_config.insert("shape_externalarg".into(), code_style);

    // Apply config with external argument style
    stack.config = Some(Arc::new(config));

    // Highlight and reject invalid syntax
    let highlighted = try_nu_highlight(content.into(), true, engine_state, stack)
        // // Make highlighted string italic
        .map(|text| {
            let resets = text.match_indices(RESET).count();
            // replace resets with reset + italic, so the whole string is italicized, excluding the final reset
            let text = text.replacen(RESET, &format!("{RESET}{DEFAULT_ITALIC}"), resets - 1);
            // start italicized
            format!("{DEFAULT_ITALIC}{text}")
        });

    // Restore original config
    stack.config = Some(config_old);

    // Use fallback style if highlight failed/syntax was invalid
    highlighted.unwrap_or_else(|| highlight_fallback(content.into()))
}

/// Apply fallback code style
fn highlight_fallback(text: &str) -> String {
    format!("{DEFAULT_DIMMED}{DEFAULT_ITALIC}{text}{RESET}")
}

/// Highlight code within backticks
///
/// Will attempt to use nu-highlight, falling back to dimmed and italic on invalid syntax
fn highlight_code<'a>(
    text: &'a str,
    engine_state: &EngineState,
    stack: &mut Stack,
) -> Cow<'a, str> {
    let config = stack.get_config(engine_state);
    if !config.use_ansi_coloring.get(engine_state) {
        return Cow::Borrowed(text);
    }

    // See [`tests::test_code_formatting`] for examples
    static PATTERN: &str = r"(?x)     # verbose mode
        (?<![\p{Letter}\d])    # negative look-behind for alphanumeric: ensure backticks are not directly preceded by letter/number.
        `
        ([^`\n]+?)           # capture characters inside backticks, excluding backticks and newlines. ungreedy.
        `
        (?![\p{Letter}\d])     # negative look-ahead for alphanumeric: ensure backticks are not directly followed by letter/number.
    ";
    static RE: LazyLock<Regex> = LazyLock::new(|| Regex::new(PATTERN).expect("valid regex"));

    let do_try_highlight =
        |captures: &Captures| highlight_capture_group(captures, engine_state, stack);
    RE.replace_all(text, do_try_highlight)
}

fn get_alias_documentation(
    long_desc: &mut String,
    command: &dyn Command,
    sig: &Signature,
    help_style: &HelpStyle,
    engine_state: &EngineState,
    stack: &mut Stack,
) {
    let help_section_name = &help_style.section_name;
    let help_subcolor_one = &help_style.subcolor_one;

    let alias_name = &sig.name;

    long_desc.push_str(&format!(
        "{help_section_name}Alias{RESET}: {help_subcolor_one}{alias_name}{RESET}"
    ));
    long_desc.push_str("\n\n");

    let Some(alias) = command.as_alias() else {
        // this is already checked in `help alias`, but just omit the expansion if this is somehow not actually an alias
        return;
    };

    let alias_expansion =
        String::from_utf8_lossy(engine_state.get_span_contents(alias.wrapped_call.span));

    long_desc.push_str(&format!(
        "{help_section_name}Expansion{RESET}:\n  {}",
        nu_highlight_string(&alias_expansion, engine_state, stack)
    ));
}

fn get_command_documentation(
    long_desc: &mut String,
    command: &dyn Command,
    sig: &Signature,
    nu_config: &Config,
    help_style: &HelpStyle,
    engine_state: &EngineState,
    stack: &mut Stack,
) {
    let help_section_name = &help_style.section_name;
    let help_subcolor_one = &help_style.subcolor_one;

    let cmd_name = &sig.name;

    if !sig.search_terms.is_empty() {
        let _ = write!(
            long_desc,
            "{help_section_name}Search terms{RESET}: {help_subcolor_one}{}{RESET}\n\n",
            sig.search_terms.join(", "),
        );
    }

    let _ = write!(
        long_desc,
        "{help_section_name}Usage{RESET}:\n  > {}\n",
        sig.call_signature()
    );

    // TODO: improve the subcommand name resolution
    // issues:
    // - Aliases are included
    //   - https://github.com/nushell/nushell/issues/11657
    // - Subcommands are included violating module scoping
    //   - https://github.com/nushell/nushell/issues/11447
    //   - https://github.com/nushell/nushell/issues/11625
    let mut subcommands = vec![];
    let signatures = engine_state.get_signatures_and_declids(true);
    for (sig, decl_id) in signatures {
        let command_type = engine_state.get_decl(decl_id).command_type();

        // Don't display removed/deprecated commands in the Subcommands list
        if sig.name.starts_with(&format!("{cmd_name} "))
            && !matches!(sig.category, Category::Removed)
        {
            // If it's a plugin, alias, or custom command, display that information in the help
            if command_type == CommandType::Plugin
                || command_type == CommandType::Alias
                || command_type == CommandType::Custom
            {
                subcommands.push(format!(
                    "  {help_subcolor_one}{} {help_section_name}({}){RESET} - {}",
                    sig.name,
                    command_type,
                    highlight_code(&sig.description, engine_state, stack)
                ));
            } else {
                subcommands.push(format!(
                    "  {help_subcolor_one}{}{RESET} - {}",
                    sig.name,
                    highlight_code(&sig.description, engine_state, stack)
                ));
            }
        }
    }

    if !subcommands.is_empty() {
        let _ = write!(long_desc, "\n{help_section_name}Subcommands{RESET}:\n");
        subcommands.sort();
        long_desc.push_str(&subcommands.join("\n"));
        long_desc.push('\n');
    }

    if !sig.named.is_empty() {
        long_desc.push_str(&get_flags_section(sig, help_style, |v| match v {
            FormatterValue::DefaultValue(value) => nu_highlight_string(
                &value.to_parsable_string(", ", nu_config),
                engine_state,
                stack,
            ),
            FormatterValue::CodeString(text) => {
                highlight_code(text, engine_state, stack).to_string()
            }
        }))
    }

    if !sig.required_positional.is_empty()
        || !sig.optional_positional.is_empty()
        || sig.rest_positional.is_some()
    {
        let _ = write!(long_desc, "\n{help_section_name}Parameters{RESET}:\n");
        for positional in &sig.required_positional {
            write_positional(
                long_desc,
                positional,
                PositionalKind::Required,
                help_style,
                nu_config,
                engine_state,
                stack,
            );
        }
        for positional in &sig.optional_positional {
            write_positional(
                long_desc,
                positional,
                PositionalKind::Optional,
                help_style,
                nu_config,
                engine_state,
                stack,
            );
        }

        if let Some(rest_positional) = &sig.rest_positional {
            write_positional(
                long_desc,
                rest_positional,
                PositionalKind::Rest,
                help_style,
                nu_config,
                engine_state,
                stack,
            );
        }
    }

    fn get_term_width() -> usize {
        if let Ok((w, _h)) = terminal_size() {
            w as usize
        } else {
            80
        }
    }

    if !command.is_keyword() && !sig.input_output_types.is_empty() {
        if let Some(decl_id) = engine_state.find_decl(b"table", &[]) {
            // FIXME: we may want to make this the span of the help command in the future
            let span = Span::unknown();
            let mut vals = vec![];
            for (input, output) in &sig.input_output_types {
                vals.push(Value::record(
                    record! {
                        "input" => Value::string(input.to_string(), span),
                        "output" => Value::string(output.to_string(), span),
                    },
                    span,
                ));
            }

            let caller_stack = &mut Stack::new().collect_value();
            if let Ok(result) = eval_call::<WithoutDebug>(
                engine_state,
                caller_stack,
                &Call {
                    decl_id,
                    head: span,
                    arguments: vec![Argument::Named((
                        Spanned {
                            item: "width".to_string(),
                            span: Span::unknown(),
                        },
                        None,
                        Some(Expression::new_unknown(
                            Expr::Int(get_term_width() as i64 - 2), // padding, see below
                            Span::unknown(),
                            Type::Int,
                        )),
                    ))],
                    parser_info: HashMap::new(),
                },
                PipelineData::value(Value::list(vals, span), None),
            ) {
                if let Ok((str, ..)) = result.collect_string_strict(span) {
                    let _ = writeln!(long_desc, "\n{help_section_name}Input/output types{RESET}:");
                    for line in str.lines() {
                        let _ = writeln!(long_desc, "  {line}");
                    }
                }
            }
        }
    }

    let examples = command.examples();

    if !examples.is_empty() {
        let _ = write!(long_desc, "\n{help_section_name}Examples{RESET}:");
    }

    for example in examples {
        long_desc.push('\n');
        long_desc.push_str("  ");
        long_desc.push_str(&highlight_code(example.description, engine_state, stack));

        if !nu_config.use_ansi_coloring.get(engine_state) {
            let _ = write!(long_desc, "\n  > {}\n", example.example);
        } else {
            let code_string = nu_highlight_string(example.example, engine_state, stack);
            let _ = write!(long_desc, "\n  > {code_string}\n");
        };

        if let Some(result) = &example.result {
            let mut table_call = Call::new(Span::unknown());
            if example.example.ends_with("--collapse") {
                // collapse the result
                table_call.add_named((
                    Spanned {
                        item: "collapse".to_string(),
                        span: Span::unknown(),
                    },
                    None,
                    None,
                ))
            } else {
                // expand the result
                table_call.add_named((
                    Spanned {
                        item: "expand".to_string(),
                        span: Span::unknown(),
                    },
                    None,
                    None,
                ))
            }
            table_call.add_named((
                Spanned {
                    item: "width".to_string(),
                    span: Span::unknown(),
                },
                None,
                Some(Expression::new_unknown(
                    Expr::Int(get_term_width() as i64 - 2),
                    Span::unknown(),
                    Type::Int,
                )),
            ));

            let table = engine_state
                .find_decl("table".as_bytes(), &[])
                .and_then(|decl_id| {
                    engine_state
                        .get_decl(decl_id)
                        .run(
                            engine_state,
                            stack,
                            &(&table_call).into(),
                            PipelineData::value(result.clone(), None),
                        )
                        .ok()
                });

            for item in table.into_iter().flatten() {
                let _ = writeln!(
                    long_desc,
                    "  {}",
                    item.to_expanded_string("", nu_config)
                        .trim_end()
                        .trim_start_matches(|c: char| c.is_whitespace() && c != ' ')
                        .replace('\n', "\n  ")
                );
            }
        }
    }

    long_desc.push('\n');
}

fn update_ansi_from_config(
    ansi_code: &mut String,
    engine_state: &EngineState,
    nu_config: &Config,
    theme_component: &str,
) {
    if let Some(color) = &nu_config.color_config.get(theme_component) {
        let caller_stack = &mut Stack::new().collect_value();
        let span = Span::unknown();
        let span_id = UNKNOWN_SPAN_ID;

        let argument_opt = get_argument_for_color_value(nu_config, color, span, span_id);

        // Call ansi command using argument
        if let Some(argument) = argument_opt {
            if let Some(decl_id) = engine_state.find_decl(b"ansi", &[]) {
                if let Ok(result) = eval_call::<WithoutDebug>(
                    engine_state,
                    caller_stack,
                    &Call {
                        decl_id,
                        head: span,
                        arguments: vec![argument],
                        parser_info: HashMap::new(),
                    },
                    PipelineData::empty(),
                ) {
                    if let Ok((str, ..)) = result.collect_string_strict(span) {
                        *ansi_code = str;
                    }
                }
            }
        }
    }
}

fn get_argument_for_color_value(
    nu_config: &Config,
    color: &Value,
    span: Span,
    span_id: SpanId,
) -> Option<Argument> {
    match color {
        Value::Record { val, .. } => {
            let record_exp: Vec<RecordItem> = (**val)
                .iter()
                .map(|(k, v)| {
                    RecordItem::Pair(
                        Expression::new_existing(
                            Expr::String(k.clone()),
                            span,
                            span_id,
                            Type::String,
                        ),
                        Expression::new_existing(
                            Expr::String(v.clone().to_expanded_string("", nu_config)),
                            span,
                            span_id,
                            Type::String,
                        ),
                    )
                })
                .collect();

            Some(Argument::Positional(Expression::new_existing(
                Expr::Record(record_exp),
                Span::unknown(),
                UNKNOWN_SPAN_ID,
                Type::Record(
                    [
                        ("fg".to_string(), Type::String),
                        ("attr".to_string(), Type::String),
                    ]
                    .into(),
                ),
            )))
        }
        Value::String { val, .. } => Some(Argument::Positional(Expression::new_existing(
            Expr::String(val.clone()),
            Span::unknown(),
            UNKNOWN_SPAN_ID,
            Type::String,
        ))),
        _ => None,
    }
}

/// Contains the settings for ANSI colors in help output
///
/// By default contains a fixed set of (4-bit) colors
///
/// Can reflect configuration using [`HelpStyle::update_from_config`]
pub struct HelpStyle {
    section_name: String,
    subcolor_one: String,
    subcolor_two: String,
}

impl Default for HelpStyle {
    fn default() -> Self {
        HelpStyle {
            // default: green
            section_name: "\x1b[32m".to_string(),
            // default: cyan
            subcolor_one: "\x1b[36m".to_string(),
            // default: light blue
            subcolor_two: "\x1b[94m".to_string(),
        }
    }
}

impl HelpStyle {
    /// Pull colors from the [`Config`]
    ///
    /// Uses some arbitrary `shape_*` settings, assuming they are well visible in the terminal theme.
    ///
    /// Implementation detail: currently executes `ansi` command internally thus requiring the
    /// [`EngineState`] for execution.
    /// See <https://github.com/nushell/nushell/pull/10623> for details
    pub fn update_from_config(&mut self, engine_state: &EngineState, nu_config: &Config) {
        update_ansi_from_config(
            &mut self.section_name,
            engine_state,
            nu_config,
            "shape_string",
        );
        update_ansi_from_config(
            &mut self.subcolor_one,
            engine_state,
            nu_config,
            "shape_external",
        );
        update_ansi_from_config(
            &mut self.subcolor_two,
            engine_state,
            nu_config,
            "shape_block",
        );
    }
}

#[derive(PartialEq)]
enum PositionalKind {
    Required,
    Optional,
    Rest,
}

fn write_positional(
    long_desc: &mut String,
    positional: &PositionalArg,
    arg_kind: PositionalKind,
    help_style: &HelpStyle,
    nu_config: &Config,
    engine_state: &EngineState,
    stack: &mut Stack,
) {
    let help_subcolor_one = &help_style.subcolor_one;
    let help_subcolor_two = &help_style.subcolor_two;

    // Indentation
    long_desc.push_str("  ");
    if arg_kind == PositionalKind::Rest {
        long_desc.push_str("...");
    }
    match &positional.shape {
        SyntaxShape::Keyword(kw, shape) => {
            let _ = write!(
                long_desc,
                "{help_subcolor_one}\"{}\" + {RESET}<{help_subcolor_two}{}{RESET}>",
                String::from_utf8_lossy(kw),
                shape,
            );
        }
        _ => {
            let _ = write!(
                long_desc,
                "{help_subcolor_one}{}{RESET} <{help_subcolor_two}{}{RESET}>",
                positional.name, &positional.shape,
            );
        }
    };
    if !positional.desc.is_empty() || arg_kind == PositionalKind::Optional {
        let _ = write!(
            long_desc,
            ": {}",
            highlight_code(&positional.desc, engine_state, stack)
        );
    }
    if arg_kind == PositionalKind::Optional {
        if let Some(value) = &positional.default_value {
            let _ = write!(
                long_desc,
                " (optional, default: {})",
                nu_highlight_string(
                    &value.to_parsable_string(", ", nu_config),
                    engine_state,
                    stack
                )
            );
        } else {
            long_desc.push_str(" (optional)");
        };
    }
    long_desc.push('\n');
}

/// Helper for `get_flags_section`
///
/// The formatter with access to nu-highlight must be passed to `get_flags_section`, but it's not possible
/// to pass separate closures since they both need `&mut Stack`, so this enum lets us differentiate between
/// default values to be formatted and strings which might contain code in backticks to be highlighted.
pub enum FormatterValue<'a> {
    /// Default value to be styled
    DefaultValue(&'a Value),
    /// String which might have code in backticks to be highlighted
    CodeString(&'a str),
}

fn write_flag_to_long_desc<F>(
    flag: &nu_protocol::Flag,
    long_desc: &mut String,
    help_subcolor_one: &str,
    help_subcolor_two: &str,
    formatter: &mut F,
) where
    F: FnMut(FormatterValue) -> String,
{
    // Indentation
    long_desc.push_str("  ");
    // Short flag shown before long flag
    if let Some(short) = flag.short {
        let _ = write!(long_desc, "{help_subcolor_one}-{short}{RESET}");
        if !flag.long.is_empty() {
            let _ = write!(long_desc, "{DEFAULT_COLOR},{RESET} ");
        }
    }
    if !flag.long.is_empty() {
        let _ = write!(long_desc, "{help_subcolor_one}--{}{RESET}", flag.long);
    }
    if flag.required {
        long_desc.push_str(" (required parameter)")
    }
    // Type/Syntax shape info
    if let Some(arg) = &flag.arg {
        let _ = write!(long_desc, " <{help_subcolor_two}{arg}{RESET}>");
    }
    if !flag.desc.is_empty() {
        let _ = write!(
            long_desc,
            ": {}",
            &formatter(FormatterValue::CodeString(&flag.desc))
        );
    }
    if let Some(value) = &flag.default_value {
        let _ = write!(
            long_desc,
            " (default: {})",
            &formatter(FormatterValue::DefaultValue(value))
        );
    }
    long_desc.push('\n');
}

pub fn get_flags_section<F>(
    signature: &Signature,
    help_style: &HelpStyle,
    mut formatter: F, // format default Value or text with code (because some calls cant access config or nu-highlight)
) -> String
where
    F: FnMut(FormatterValue) -> String,
{
    let help_section_name = &help_style.section_name;
    let help_subcolor_one = &help_style.subcolor_one;
    let help_subcolor_two = &help_style.subcolor_two;

    let mut long_desc = String::new();
    let _ = write!(long_desc, "\n{help_section_name}Flags{RESET}:\n");

    let help = signature.named.iter().find(|flag| flag.long == "help");
    let required = signature.named.iter().filter(|flag| flag.required);
    let optional = signature
        .named
        .iter()
        .filter(|flag| !flag.required && flag.long != "help");

    let flags = required.chain(help).chain(optional);

    for flag in flags {
        write_flag_to_long_desc(
            flag,
            &mut long_desc,
            help_subcolor_one,
            help_subcolor_two,
            &mut formatter,
        );
    }

    long_desc
}

#[cfg(test)]
mod tests {
    use nu_protocol::UseAnsiColoring;

    use super::*;

    #[test]
    fn test_code_formatting() {
        let mut engine_state = EngineState::new();
        let mut stack = Stack::new();

        // force coloring on for test
        let mut config = (*engine_state.config).clone();
        config.use_ansi_coloring = UseAnsiColoring::True;
        engine_state.config = Arc::new(config);

        // using Cow::Owned here to mean a match, since the content changed,
        // and borrowed to mean not a match, since the content didn't change

        // match: typical example
        let haystack = "Run the `foo` command";
        assert!(matches!(
            highlight_code(haystack, &engine_state, &mut stack),
            Cow::Owned(_)
        ));

        // no match: backticks preceded by alphanum
        let haystack = "foo`bar`";
        assert!(matches!(
            highlight_code(haystack, &engine_state, &mut stack),
            Cow::Borrowed(_)
        ));

        // match: command at beginning of string is ok
        let haystack = "`my-command` is cool";
        assert!(matches!(
            highlight_code(haystack, &engine_state, &mut stack),
            Cow::Owned(_)
        ));

        // match: preceded and followed by newline is ok
        let haystack = r"
        `command`
        ";
        assert!(matches!(
            highlight_code(haystack, &engine_state, &mut stack),
            Cow::Owned(_)
        ));

        // no match: newline between backticks
        let haystack = "// hello `beautiful \n world`";
        assert!(matches!(
            highlight_code(haystack, &engine_state, &mut stack),
            Cow::Borrowed(_)
        ));

        // match: backticks followed by period, not letter/number
        let haystack = "try running `my cool command`.";
        assert!(matches!(
            highlight_code(haystack, &engine_state, &mut stack),
            Cow::Owned(_)
        ));

        // match: backticks enclosed by parenthesis, not letter/number
        let haystack = "a command (`my cool command`).";
        assert!(matches!(
            highlight_code(haystack, &engine_state, &mut stack),
            Cow::Owned(_)
        ));

        // no match: only characters inside backticks are backticks
        // (the regex sees two backtick pairs with a single backtick inside, which doesn't qualify)
        let haystack = "```\ncode block\n```";
        assert!(matches!(
            highlight_code(haystack, &engine_state, &mut stack),
            Cow::Borrowed(_)
        ));
    }
}