yash_syntax/parser/lex/

core.rs

// This file is part of yash, an extended POSIX shell.
// Copyright (C) 2020 WATANABE Yuki
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.

//! Fundamental building blocks for the lexical analyzer.

use super::keyword::Keyword;
use super::op::Operator;
use crate::alias::Alias;
use crate::alias::EmptyGlossary;
use crate::input::Context;
use crate::input::InputObject;
use crate::input::Memory;
use crate::parser::core::Result;
use crate::parser::error::Error;
use crate::source::source_chars;
use crate::source::Code;
use crate::source::Location;
use crate::source::Source;
use crate::source::SourceChar;
use crate::syntax::Word;
use std::cell::RefCell;
use std::fmt;
use std::future::Future;
use std::num::NonZeroU64;
use std::ops::Deref;
use std::ops::DerefMut;
use std::ops::Range;
use std::pin::Pin;
use std::rc::Rc;

/// Returns true if the character is a blank character.
pub fn is_blank(c: char) -> bool {
    // TODO locale
    c != '\n' && c.is_whitespace()
}

/// Result of [`LexerCore::peek_char`].
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum PeekChar<'a> {
    Char(&'a SourceChar),
    EndOfInput(&'a Location),
}

impl<'a> PeekChar<'a> {
    /// Returns the location that was peeked.
    #[must_use]
    fn location<'b>(self: &'b PeekChar<'a>) -> &'a Location {
        match self {
            PeekChar::Char(c) => &c.location,
            PeekChar::EndOfInput(l) => l,
        }
    }
}

/// Token identifier, or classification of tokens.
///
/// This enum classifies a token as defined in POSIX XCU 2.10.1 Shell Grammar Lexical
/// Conventions, but does not exactly reflect further distinction defined in
/// POSIX XCU 2.10.2 Shell Grammar Rules.
///
/// For convenience, the special token identifier `EndOfInput` is included.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum TokenId {
    /// `TOKEN`
    ///
    /// If this token _looks like_ a reserved word, this variant has some
    /// associated `Keyword` that describes the word. However, it depends on
    /// context whether a token is actually regarded as a reserved word or
    /// just as an ordinary word. You must ensure that you're in an
    /// applicable context when examining the `Keyword` value.
    Token(Option<Keyword>),
    /// Operator
    Operator(Operator),
    /// `IO_NUMBER`
    IoNumber,
    /// Imaginary token identifier for the end of input.
    EndOfInput,
}

impl TokenId {
    /// Determines if this token can be a delimiter of a clause.
    ///
    /// This function delegates to [`Keyword::is_clause_delimiter`] if the token
    /// ID is a (possible) keyword, or to [`Operator::is_clause_delimiter`] if
    /// it is an operator. For `EndOfInput` the function returns true.
    /// Otherwise, the result is false.
    pub fn is_clause_delimiter(self) -> bool {
        use TokenId::*;
        match self {
            Token(Some(keyword)) => keyword.is_clause_delimiter(),
            Token(None) => false,
            Operator(operator) => operator.is_clause_delimiter(),
            IoNumber => false,
            EndOfInput => true,
        }
    }
}

/// Result of lexical analysis produced by the [`Lexer`].
#[derive(Debug)]
pub struct Token {
    /// Content of the token.
    ///
    /// The word value contains at least one [unit](crate::syntax::WordUnit),
    /// regardless of whether the token is an operator. The only exception is
    /// when `id` is `EndOfInput`, in which case the word is empty.
    pub word: Word,
    /// Token identifier.
    pub id: TokenId,
    /// Position of the first character of the word.
    pub index: usize,
}

impl fmt::Display for Token {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.word)
    }
}

/// State of the input function in a lexer.
#[derive(Clone, Debug)]
enum InputState {
    Alive,
    EndOfInput(Location),
    Error(Error),
}

/// Source character with additional attribute
#[derive(Clone, Debug, Eq, PartialEq)]
struct SourceCharEx {
    value: SourceChar,
    is_line_continuation: bool,
}

fn ex<I: IntoIterator<Item = SourceChar>>(i: I) -> impl Iterator<Item = SourceCharEx> {
    i.into_iter().map(|sc| SourceCharEx {
        value: sc,
        is_line_continuation: false,
    })
}

/// Core part of the lexical analyzer.
struct LexerCore<'a> {
    input: Box<dyn InputObject + 'a>,
    state: InputState,
    raw_code: Rc<Code>,
    source: Vec<SourceCharEx>,
    index: usize,
}

impl<'a> LexerCore<'a> {
    /// Creates a new lexer core that reads using the given input function.
    #[must_use]
    fn new(
        input: Box<dyn InputObject + 'a>,
        start_line_number: NonZeroU64,
        source: Rc<Source>,
    ) -> LexerCore<'a> {
        LexerCore {
            input,
            raw_code: Rc::new(Code {
                value: RefCell::new(String::new()),
                start_line_number,
                source,
            }),
            state: InputState::Alive,
            source: Vec::new(),
            index: 0,
        }
    }

    /// Computes the start index of the location at the current position.
    #[must_use]
    fn next_index(&self) -> usize {
        let Some(last) = self.source.last() else {
            return 0;
        };

        let mut location = &last.value.location;
        while let Source::Alias { original, .. } = &*location.code.source {
            location = original;
        }
        location.range.end
    }

    /// Peeks the next character, reading the next line if necessary.
    async fn peek_char(&mut self) -> Result<PeekChar<'_>> {
        loop {
            // if let Some(sc) = self.source.get(self.index) {
            //     return Ok(PeekChar::Char(&sc.value));
            if self.index < self.source.len() {
                return Ok(PeekChar::Char(&self.source[self.index].value));
            }

            match self.state {
                InputState::Alive => (),
                InputState::EndOfInput(ref location) => return Ok(PeekChar::EndOfInput(location)),
                InputState::Error(ref error) => return Err(error.clone()),
            }

            // Read more input
            let index = self.next_index();
            match self.input.next_line(&self.input_context()).await {
                Ok(line) => {
                    if line.is_empty() {
                        // End of input
                        self.state = InputState::EndOfInput(Location {
                            code: Rc::clone(&self.raw_code),
                            range: index..index,
                        });
                    } else {
                        // Successful read
                        self.raw_code.value.borrow_mut().push_str(&line);
                        self.source
                            .extend(ex(source_chars(&line, &self.raw_code, index)));
                    }
                }
                Err(io_error) => {
                    self.state = InputState::Error(Error {
                        cause: io_error.into(),
                        location: Location {
                            code: Rc::clone(&self.raw_code),
                            range: index..index,
                        },
                    });
                }
            }
        }
    }

    /// Returns the input context for the next character.
    fn input_context(&self) -> Context {
        let mut context = Context::default();
        context.set_is_first_line(self.raw_code.value.borrow().is_empty());
        context
    }

    /// Consumes the next character.
    ///
    /// This function must be called after [`peek_char`](Lexer::peek_char) has successfully
    /// returned the character. Consuming a character that has not yet been peeked would result
    /// in a panic!
    fn consume_char(&mut self) {
        assert!(
            self.index < self.source.len(),
            "A character must have been peeked before being consumed: index={}",
            self.index
        );
        self.index += 1;
    }

    /// Returns a reference to the character at the given index.
    #[must_use]
    fn peek_char_at(&self, index: usize) -> &SourceChar {
        assert!(
            index <= self.index,
            "The index {} must not be larger than the current index {}",
            index,
            self.index
        );
        &self.source[index].value
    }

    /// Returns the current index.
    #[must_use]
    fn index(&self) -> usize {
        self.index
    }

    /// Rewinds the index to the given value.
    fn rewind(&mut self, index: usize) {
        assert!(
            index <= self.index,
            "The new index {} must not be larger than the current index {}",
            index,
            self.index
        );
        self.index = index;
    }

    /// Checks if there is any character that has been read from the input
    /// source but not yet consumed.
    #[must_use]
    fn pending(&self) -> bool {
        self.index < self.source.len()
    }

    /// Clears the internal buffer.
    fn flush(&mut self) {
        let start_line_number = self.raw_code.line_number(usize::MAX);
        self.raw_code = Rc::new(Code {
            value: RefCell::new(String::new()),
            start_line_number,
            source: self.raw_code.source.clone(),
        });
        self.source.clear();
        self.index = 0;
    }

    /// Clears an end-of-input or error status so that the lexer can resume
    /// parsing.
    fn reset(&mut self) {
        self.state = InputState::Alive;
        self.flush();
    }

    /// Extracts a string from the source code range.
    fn source_string(&self, range: Range<usize>) -> String {
        self.source[range].iter().map(|c| c.value.value).collect()
    }

    /// Returns a location for a given range of the source code.
    #[must_use]
    fn location_range(&self, range: Range<usize>) -> Location {
        if range.start == self.source.len() {
            if let InputState::EndOfInput(ref location) = self.state {
                return location.clone();
            }
        }
        let start = &self.peek_char_at(range.start).location;
        let code = start.code.clone();
        let end = range
            .map(|index| &self.peek_char_at(index).location)
            .take_while(|location| location.code == code)
            .last()
            .map(|location| location.range.end)
            .unwrap_or(start.range.start);
        let range = start.range.start..end;
        Location { code, range }
    }

    /// Marks the characters in the given range as line continuation.
    ///
    /// This function sets the `is_line_continuation` flag of the characters in
    /// the range to true. The characters must have been read before calling
    /// this function.
    fn mark_line_continuation(&mut self, range: Range<usize>) {
        assert!(
            range.end <= self.index,
            "characters must have been read (range = {:?}, current index = {})",
            range,
            self.index
        );
        for sc in &mut self.source[range] {
            sc.is_line_continuation = true;
        }
    }

    /// Performs alias substitution.
    ///
    /// This function replaces the characters starting from the `begin` index up
    /// to the current position with the alias value. The resulting part of code
    /// will be characters with a [`Source::Alias`] origin.
    fn substitute_alias(&mut self, begin: usize, alias: &Rc<Alias>) {
        let end = self.index;
        assert!(
            begin < end,
            "begin index {begin} should be less than end index {end}"
        );

        let source = Rc::new(Source::Alias {
            original: self.location_range(begin..end),
            alias: alias.clone(),
        });
        let code = Rc::new(Code {
            value: RefCell::new(alias.replacement.clone()),
            start_line_number: NonZeroU64::new(1).unwrap(),
            source,
        });
        let repl = ex(source_chars(&alias.replacement, &code, 0));

        self.source.splice(begin..end, repl);
        self.index = begin;
    }

    /// Tests if the given index is after the replacement string of alias
    /// substitution that ends with a blank.
    ///
    /// # Panics
    ///
    /// If `index` is larger than the currently read index.
    fn is_after_blank_ending_alias(&self, index: usize) -> bool {
        fn ends_with_blank(s: &str) -> bool {
            s.chars().next_back().map_or(false, is_blank)
        }
        fn is_same_alias(alias: &Alias, sc: Option<&SourceCharEx>) -> bool {
            sc.is_some_and(|sc| sc.value.location.code.source.is_alias_for(&alias.name))
        }

        for index in (0..index).rev() {
            let sc = &self.source[index];

            if !sc.is_line_continuation && !is_blank(sc.value.value) {
                return false;
            }

            if let Source::Alias { ref alias, .. } = *sc.value.location.code.source {
                #[allow(clippy::collapsible_if)]
                if ends_with_blank(&alias.replacement) {
                    if !is_same_alias(alias, self.source.get(index + 1)) {
                        return true;
                    }
                }
            }
        }

        false
    }
}

impl fmt::Debug for LexerCore<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("LexerCore")
            .field("state", &self.state)
            .field("source", &self.source)
            .field("index", &self.index)
            .finish_non_exhaustive()
    }
}

/// Lexical analyzer.
///
/// A lexer reads lines using an input function and parses the characters into tokens. It has an
/// internal buffer containing the characters that have been read and the position (or the
/// index) of the character that is to be parsed next.
///
/// `Lexer` has primitive functions such as [`peek_char`](Lexer::peek_char) that provide access
/// to the character at the current position. Derived functions such as
/// [`skip_blanks_and_comment`](Lexer::skip_blanks_and_comment) depend on those primitives to
/// parse more complex structures in the source code.
#[derive(Debug)]
pub struct Lexer<'a> {
    // `Lexer` is a thin wrapper around `LexerCore`. `Lexer` delegates most
    // functions to `LexerCore`. `Lexer` adds automatic line-continuation
    // skipping to `LexerCore`.
    core: LexerCore<'a>,
    line_continuation_enabled: bool,
}

impl<'a> Lexer<'a> {
    /// Creates a new lexer that reads using the given input function.
    #[must_use]
    pub fn new(
        input: Box<dyn InputObject + 'a>,
        start_line_number: NonZeroU64,
        source: Rc<Source>,
    ) -> Lexer<'a> {
        Lexer {
            core: LexerCore::new(input, start_line_number, source),
            line_continuation_enabled: true,
        }
    }

    /// Creates a new lexer with a fixed source code.
    ///
    /// This is a convenience function that creates a lexer that reads from a
    /// string using a [`Memory`] input function. The line number starts from 1.
    #[must_use]
    pub fn from_memory<S: Into<Rc<Source>>>(code: &'a str, source: S) -> Lexer<'a> {
        fn inner(code: &str, source: Rc<Source>) -> Lexer {
            let line = NonZeroU64::new(1).unwrap();
            Lexer::new(Box::new(Memory::new(code)), line, source)
        }
        inner(code, source.into())
    }

    /// Disables line continuation recognition onward.
    ///
    /// By default, [`peek_char`](Self::peek_char) silently skips line
    /// continuation sequences. When line continuation is disabled, however,
    /// `peek_char` returns characters literally.
    ///
    /// Call [`enable_line_continuation`](Self::enable_line_continuation) to
    /// switch line continuation recognition on.
    ///
    /// This function will panic if line continuation has already been disabled.
    pub fn disable_line_continuation<'b>(&'b mut self) -> PlainLexer<'b, 'a> {
        assert!(
            self.line_continuation_enabled,
            "line continuation already disabled"
        );
        self.line_continuation_enabled = false;
        PlainLexer { lexer: self }
    }

    /// Re-enables line continuation.
    ///
    /// You can pass the `PlainLexer` returned from
    /// [`disable_line_continuation`](Self::disable_line_continuation) to this
    /// function to re-enable line continuation. That is equivalent to dropping
    /// the `PlainLexer` instance, but the code will be more descriptive.
    pub fn enable_line_continuation<'b>(_: PlainLexer<'a, 'b>) {}

    /// Skips line continuation, i.e., a backslash followed by a newline.
    ///
    /// If there is a line continuation at the current position, this function
    /// consumes the backslash and the newline and returns `Ok(true)`. The
    /// characters are marked as line continuation.
    ///
    /// If there is no line continuation, this function does nothing and returns
    /// `Ok(false)`.
    ///
    /// This function does nothing if line continuation has been
    /// [disabled](Self::disable_line_continuation).
    async fn line_continuation(&mut self) -> Result<bool> {
        if !self.line_continuation_enabled {
            return Ok(false);
        }

        let index = self.core.index();
        match self.core.peek_char().await? {
            PeekChar::Char(c) if c.value == '\\' => self.core.consume_char(),
            _ => return Ok(false),
        }

        match self.core.peek_char().await? {
            PeekChar::Char(c) if c.value == '\n' => self.core.consume_char(),
            _ => {
                self.core.rewind(index);
                return Ok(false);
            }
        }

        self.core.mark_line_continuation(index..index + 2);

        Ok(true)
    }

    /// Peeks the next character.
    ///
    /// If the end of input is reached, `Ok(None)` is returned. On error,
    /// `Err(_)` is returned.
    ///
    /// If line continuation recognition is enabled, combinations of a backslash
    /// and a newline are silently skipped before returning the next character.
    /// Call [`disable_line_continuation`](Self::disable_line_continuation) to
    /// switch off line continuation recognition.
    ///
    /// This function requires a mutable reference to `self` since it may need
    /// to read the next line if needed.
    pub async fn peek_char(&mut self) -> Result<Option<char>> {
        while self.line_continuation().await? {}

        match self.core.peek_char().await? {
            PeekChar::Char(source_char) => Ok(Some(source_char.value)),
            PeekChar::EndOfInput(_) => Ok(None),
        }
    }

    /// Returns the location of the next character.
    ///
    /// If there is no more character (that is, it is the end of input), an imaginary location
    /// is returned that would be returned if a character existed.
    ///
    /// This function requires a mutable reference to `self` since it needs to
    /// [peek](Self::peek_char) the next character.
    pub async fn location(&mut self) -> Result<&Location> {
        self.core.peek_char().await.map(|p| p.location())
    }

    /// Consumes the next character.
    ///
    /// This function must be called after [`peek_char`](Lexer::peek_char) has successfully
    /// returned the character. Consuming a character that has not yet been peeked would result
    /// in a panic!
    pub fn consume_char(&mut self) {
        self.core.consume_char()
    }

    /// Returns the position of the next character, counted from zero.
    ///
    /// ```
    /// # use yash_syntax::parser::lex::Lexer;
    /// # use yash_syntax::source::Source;
    /// futures_executor::block_on(async {
    ///     let mut lexer = Lexer::from_memory("abc", Source::Unknown);
    ///     assert_eq!(lexer.index(), 0);
    ///     let _ = lexer.peek_char().await;
    ///     assert_eq!(lexer.index(), 0);
    ///     lexer.consume_char();
    ///     assert_eq!(lexer.index(), 1);
    /// })
    /// ```
    #[must_use]
    pub fn index(&self) -> usize {
        self.core.index()
    }

    /// Moves the current position back to the given index so that characters that have been
    /// consumed can be read again.
    ///
    /// The given index must not be larger than the [current index](Lexer::index), or this
    /// function would panic.
    ///
    /// ```
    /// # use yash_syntax::parser::lex::Lexer;
    /// # use yash_syntax::source::Source;
    /// futures_executor::block_on(async {
    ///     let mut lexer = Lexer::from_memory("abc", Source::Unknown);
    ///     let saved_index = lexer.index();
    ///     assert_eq!(lexer.peek_char().await, Ok(Some('a')));
    ///     lexer.consume_char();
    ///     assert_eq!(lexer.peek_char().await, Ok(Some('b')));
    ///     lexer.rewind(saved_index);
    ///     assert_eq!(lexer.peek_char().await, Ok(Some('a')));
    /// })
    /// ```
    pub fn rewind(&mut self, index: usize) {
        self.core.rewind(index)
    }

    /// Checks if there is any character that has been read from the input
    /// source but not yet consumed.
    #[must_use]
    pub fn pending(&self) -> bool {
        self.core.pending()
    }

    /// Clears the internal buffer of the lexer.
    ///
    /// Locations returned from [`location`](Self::location) share a single code
    /// instance that is also retained by the lexer. The code grows long as the
    /// lexer reads more input. To prevent the code from getting too large, you
    /// can call this function that replaces the retained code with a new empty
    /// one. The new code's `start_line_number` will be incremented by the
    /// number of lines in the previous.
    pub fn flush(&mut self) {
        self.core.flush()
    }

    /// Clears an end-of-input or error status so that the lexer can resume
    /// parsing.
    ///
    /// This function will be useful only in an interactive shell where the user
    /// can continue entering commands even after (s)he sends an end-of-input or
    /// is interrupted by a syntax error.
    pub fn reset(&mut self) {
        self.core.reset()
    }

    /// Peeks the next character and, if the given decider function returns true for it,
    /// advances the position.
    ///
    /// Returns the consumed character if the function returned true. Returns `Ok(None)` if it
    /// returned false or there is no more character.
    pub async fn consume_char_if<F>(&mut self, mut f: F) -> Result<Option<&SourceChar>>
    where
        F: FnMut(char) -> bool,
    {
        self.consume_char_if_dyn(&mut f).await
    }

    /// Dynamic version of [`Self::consume_char_if`].
    pub(crate) async fn consume_char_if_dyn(
        &mut self,
        f: &mut dyn FnMut(char) -> bool,
    ) -> Result<Option<&SourceChar>> {
        match self.peek_char().await? {
            Some(c) if f(c) => {
                let index = self.index();
                self.consume_char();
                Ok(Some(self.core.peek_char_at(index)))
            }
            _ => Ok(None),
        }
    }

    /// Extracts a string from the source code range.
    ///
    /// This function returns the source code string for the range specified by
    /// the argument. The range must specify a valid index. If the index points
    /// to a character that have not yet read, this function will panic!.
    ///
    /// # Panics
    ///
    /// If the argument index is out of bounds, i.e., pointing to an unread
    /// character.
    #[inline]
    pub fn source_string(&self, range: Range<usize>) -> String {
        self.core.source_string(range)
    }

    /// Returns a location for a given range of the source code.
    ///
    /// All the characters in the range must have been
    /// [consume](Self::consume_char)d. If the range refers to an unconsumed
    /// character, this function will panic!
    ///
    /// If the characters are from more than one [`Code`] fragment, the location
    /// will only cover the initial portion of the range sharing the same
    /// `Code`.
    ///
    /// # Panics
    ///
    /// This function will panic if the range refers to an unconsumed character.
    ///
    /// If the start index of the range is the end of input, it must have been
    /// peeked and the range must be empty, or the function will panic.
    #[must_use]
    pub fn location_range(&self, range: Range<usize>) -> Location {
        self.core.location_range(range)
    }

    /// Performs alias substitution right before the current position.
    ///
    /// This function must be called just after a [word](WordLexer::word) has been parsed that
    /// matches the name of the argument alias. No check is done in this function that there is
    /// a matching word before the current position. The characters starting from the `begin`
    /// index up to the current position are silently replaced with the alias value.
    ///
    /// The resulting part of code will be characters with a [`Source::Alias`] origin.
    ///
    /// After the substitution, the position will be set before the replaced string.
    ///
    /// # Panics
    ///
    /// If the replaced part is empty, i.e., `begin >= self.index()`.
    pub fn substitute_alias(&mut self, begin: usize, alias: &Rc<Alias>) {
        self.core.substitute_alias(begin, alias)
    }

    /// Tests if the given index is after the replacement string of alias
    /// substitution that ends with a blank.
    ///
    /// # Panics
    ///
    /// If `index` is larger than the currently read index.
    pub fn is_after_blank_ending_alias(&self, index: usize) -> bool {
        self.core.is_after_blank_ending_alias(index)
    }

    /// Parses an optional compound list that is the content of a command
    /// substitution.
    ///
    /// This function consumes characters until a token that cannot be the
    /// beginning of an and-or list is found and returns the string that was
    /// consumed.
    pub async fn inner_program(&mut self) -> Result<String> {
        let begin = self.index();

        let mut parser = super::super::Parser::new(self, &EmptyGlossary);
        parser.maybe_compound_list().await?;

        let end = parser.peek_token().await?.index;
        self.rewind(end);

        Ok(self.core.source_string(begin..end))
    }

    /// Like [`Lexer::inner_program`], but returns the future in a pinning box.
    pub fn inner_program_boxed(&mut self) -> Pin<Box<dyn Future<Output = Result<String>> + '_>> {
        Box::pin(self.inner_program())
    }
}

/// Reference to [`Lexer`] with line continuation disabled.
///
/// This struct implements the RAII pattern for temporarily disabling line
/// continuation. When you disable the line continuation of a lexer, you get an
/// instance of `PlainLexer`. You can access the original lexer via the
/// `PlainLexer` until you drop it, when the line continuation is automatically
/// re-enabled.
#[derive(Debug)]
#[must_use = "You must retain the PlainLexer to keep line continuation disabled"]
pub struct PlainLexer<'a, 'b> {
    lexer: &'a mut Lexer<'b>,
}

impl<'b> Deref for PlainLexer<'_, 'b> {
    type Target = Lexer<'b>;
    fn deref(&self) -> &Lexer<'b> {
        self.lexer
    }
}

impl<'b> DerefMut for PlainLexer<'_, 'b> {
    fn deref_mut(&mut self) -> &mut Lexer<'b> {
        self.lexer
    }
}

impl Drop for PlainLexer<'_, '_> {
    fn drop(&mut self) {
        self.lexer.line_continuation_enabled = true;
    }
}

/// Context in which a [word](crate::syntax::Word) is parsed.
///
/// The parse of the word of a [switch](crate::syntax::Switch) depends on
/// whether the parameter expansion containing the switch is part of a text or a
/// word. A `WordContext` value is used to decide the behavior of the lexer.
///
/// Parser functions that depend on the context are implemented in
/// [`WordLexer`].
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum WordContext {
    /// The text unit being parsed is part of a [text](crate::syntax::Text).
    Text,
    /// The text unit being parsed is part of a [word](crate::syntax::Word).
    Word,
}

/// Lexer with additional information for parsing [texts](crate::syntax::Text)
/// and [words](crate::syntax::Word).
#[derive(Debug)]
pub struct WordLexer<'a, 'b> {
    pub lexer: &'a mut Lexer<'b>,
    pub context: WordContext,
}

impl<'b> Deref for WordLexer<'_, 'b> {
    type Target = Lexer<'b>;
    fn deref(&self) -> &Lexer<'b> {
        self.lexer
    }
}

impl<'b> DerefMut for WordLexer<'_, 'b> {
    fn deref_mut(&mut self) -> &mut Lexer<'b> {
        self.lexer
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::input::Input;
    use crate::parser::error::ErrorCause;
    use crate::parser::error::SyntaxError;
    use assert_matches::assert_matches;
    use futures_util::FutureExt;

    #[test]
    fn lexer_core_peek_char_empty_source() {
        let input = Memory::new("");
        let line = NonZeroU64::new(32).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        let result = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(result, Ok(PeekChar::EndOfInput(location)) => {
            assert_eq!(*location.code.value.borrow(), "");
            assert_eq!(location.code.start_line_number, line);
            assert_eq!(*location.code.source, Source::Unknown);
            assert_eq!(location.range, 0..0);
        });
    }

    #[test]
    fn lexer_core_peek_char_io_error() {
        #[derive(Debug)]
        struct Failing;
        impl fmt::Display for Failing {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                write!(f, "Failing")
            }
        }
        impl std::error::Error for Failing {}
        impl Input for Failing {
            async fn next_line(&mut self, _: &Context) -> crate::input::Result {
                Err(std::io::Error::new(std::io::ErrorKind::Other, Failing))
            }
        }
        let line = NonZeroU64::new(42).unwrap();
        let mut lexer = LexerCore::new(Box::new(Failing), line, Rc::new(Source::Unknown));

        let e = lexer.peek_char().now_or_never().unwrap().unwrap_err();
        assert_matches!(e.cause, ErrorCause::Io(io_error) => {
            assert_eq!(io_error.kind(), std::io::ErrorKind::Other);
        });
        assert_eq!(*e.location.code.value.borrow(), "");
        assert_eq!(e.location.code.start_line_number, line);
        assert_eq!(*e.location.code.source, Source::Unknown);
        assert_eq!(e.location.range, 0..0);
    }

    #[test]
    fn lexer_core_peek_char_context_is_first_line() {
        // In this test case, this mock input function will be called twice.
        struct InputMock {
            first: bool,
        }
        impl Input for InputMock {
            async fn next_line(&mut self, context: &Context) -> crate::input::Result {
                assert_eq!(context.is_first_line(), self.first);
                self.first = false;
                Ok("\n".to_owned())
            }
        }

        let input = InputMock { first: true };
        let line = NonZeroU64::new(42).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));

        let peek = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(peek, Ok(PeekChar::Char(_)));
        lexer.consume_char();

        let peek = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(peek, Ok(PeekChar::Char(_)));
        lexer.consume_char();
    }

    #[test]
    fn lexer_core_consume_char_success() {
        let input = Memory::new("a\nb");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));

        let result = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(result, Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'a');
            assert_eq!(*c.location.code.value.borrow(), "a\n");
            assert_eq!(c.location.code.start_line_number, line);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 0..1);
        });
        assert_matches!(result, Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'a');
            assert_eq!(*c.location.code.value.borrow(), "a\n");
            assert_eq!(c.location.code.start_line_number, line);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 0..1);
        });
        lexer.consume_char();

        let result = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(result, Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, '\n');
            assert_eq!(*c.location.code.value.borrow(), "a\n");
            assert_eq!(c.location.code.start_line_number, line);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 1..2);
        });
        lexer.consume_char();

        let result = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(result, Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'b');
            assert_eq!(*c.location.code.value.borrow(), "a\nb");
            assert_eq!(c.location.code.start_line_number.get(), 1);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 2..3);
        });
        lexer.consume_char();

        let result = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(result, Ok(PeekChar::EndOfInput(location)) => {
            assert_eq!(*location.code.value.borrow(), "a\nb");
            assert_eq!(location.code.start_line_number.get(), 1);
            assert_eq!(*location.code.source, Source::Unknown);
            assert_eq!(location.range, 3..3);
        });
    }

    #[test]
    #[should_panic(expected = "A character must have been peeked before being consumed: index=0")]
    fn lexer_core_consume_char_panic() {
        let input = Memory::new("a");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        lexer.consume_char();
    }

    #[test]
    fn lexer_core_peek_char_at() {
        let input = Memory::new("a\nb");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));

        let c0 = assert_matches!(
            lexer.peek_char().now_or_never().unwrap(),
            Ok(PeekChar::Char(c)) => c.clone()
        );
        lexer.consume_char();

        let c1 = assert_matches!(
            lexer.peek_char().now_or_never().unwrap(),
            Ok(PeekChar::Char(c)) => c.clone()
        );
        lexer.consume_char();

        let c2 = assert_matches!(
            lexer.peek_char().now_or_never().unwrap(),
            Ok(PeekChar::Char(c)) => c.clone()
        );

        assert_eq!(lexer.peek_char_at(0), &c0);
        assert_eq!(lexer.peek_char_at(1), &c1);
        assert_eq!(lexer.peek_char_at(2), &c2);
    }

    #[test]
    fn lexer_core_index() {
        let input = Memory::new("a\nb");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));

        assert_eq!(lexer.index(), 0);
        lexer.peek_char().now_or_never().unwrap().unwrap();
        assert_eq!(lexer.index(), 0);
        lexer.consume_char();

        assert_eq!(lexer.index(), 1);
        lexer.peek_char().now_or_never().unwrap().unwrap();
        lexer.consume_char();

        assert_eq!(lexer.index(), 2);
        lexer.peek_char().now_or_never().unwrap().unwrap();
        lexer.consume_char();

        assert_eq!(lexer.index(), 3);
    }

    #[test]
    fn lexer_core_rewind_success() {
        let input = Memory::new("abc");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        lexer.rewind(0);
        assert_eq!(lexer.index(), 0);

        let _ = lexer.peek_char().now_or_never().unwrap();
        lexer.consume_char();
        let _ = lexer.peek_char().now_or_never().unwrap();
        lexer.consume_char();
        lexer.rewind(0);

        let result = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(result, Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'a');
            assert_eq!(*c.location.code.value.borrow(), "abc");
            assert_eq!(c.location.code.start_line_number, line);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 0..1);
        });
    }

    #[test]
    #[should_panic(expected = "The new index 1 must not be larger than the current index 0")]
    fn lexer_core_rewind_invalid_index() {
        let input = Memory::new("abc");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        lexer.rewind(1);
    }

    #[test]
    fn lexer_core_source_string() {
        let input = Memory::new("ab\ncd");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        for _ in 0..4 {
            let _ = lexer.peek_char().now_or_never().unwrap();
            lexer.consume_char();
        }

        let result = lexer.source_string(1..4);
        assert_eq!(result, "b\nc");
    }

    #[test]
    #[should_panic(expected = "begin index 0 should be less than end index 0")]
    fn lexer_core_substitute_alias_with_invalid_index() {
        let input = Memory::new("a b");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        let alias = Rc::new(Alias {
            name: "a".to_string(),
            replacement: "".to_string(),
            global: false,
            origin: Location::dummy("dummy"),
        });
        lexer.substitute_alias(0, &alias);
    }

    #[test]
    fn lexer_core_substitute_alias_single_line_replacement() {
        let input = Memory::new("a b");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        let alias = Rc::new(Alias {
            name: "a".to_string(),
            replacement: "lex".to_string(),
            global: false,
            origin: Location::dummy("dummy"),
        });

        let _ = lexer.peek_char().now_or_never().unwrap();
        lexer.consume_char();

        lexer.substitute_alias(0, &alias);

        assert_matches!(lexer.peek_char().now_or_never().unwrap(), Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'l');
            assert_eq!(*c.location.code.value.borrow(), "lex");
            assert_eq!(c.location.code.start_line_number.get(), 1);
            assert_matches!(&*c.location.code.source,
                Source::Alias { original, alias: alias2 } => {
                assert_eq!(*original.code.value.borrow(), "a b");
                assert_eq!(original.code.start_line_number, line);
                assert_eq!(*original.code.source, Source::Unknown);
                assert_eq!(original.range, 0..1);
                assert_eq!(alias2, &alias);
            });
            assert_eq!(c.location.range, 0..1);
        });
        lexer.consume_char();

        assert_matches!(lexer.peek_char().now_or_never().unwrap(), Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'e');
            assert_eq!(*c.location.code.value.borrow(), "lex");
            assert_eq!(c.location.code.start_line_number, line);
            assert_matches!(&*c.location.code.source,
                Source::Alias { original, alias: alias2 } => {
                assert_eq!(*original.code.value.borrow(), "a b");
                assert_eq!(original.code.start_line_number, line);
                assert_eq!(*original.code.source, Source::Unknown);
                assert_eq!(original.range, 0..1);
                assert_eq!(alias2, &alias);
            });
            assert_eq!(c.location.range, 1..2);
        });
        lexer.consume_char();

        assert_matches!(lexer.peek_char().now_or_never().unwrap(), Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'x');
            assert_eq!(*c.location.code.value.borrow(), "lex");
            assert_eq!(c.location.code.start_line_number, line);
            assert_matches!(&*c.location.code.source,
                Source::Alias { original, alias: alias2 } => {
                assert_eq!(*original.code.value.borrow(), "a b");
                assert_eq!(original.code.start_line_number, line);
                assert_eq!(*original.code.source, Source::Unknown);
                assert_eq!(original.range, 0..1);
                assert_eq!(alias2, &alias);
            });
            assert_eq!(c.location.range, 2..3);
        });
        lexer.consume_char();

        assert_matches!(lexer.peek_char().now_or_never().unwrap(), Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, ' ');
            assert_eq!(*c.location.code.value.borrow(), "a b");
            assert_eq!(c.location.code.start_line_number, line);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 1..2);
        });
        lexer.consume_char();
    }

    #[test]
    fn lexer_core_substitute_alias_multi_line_replacement() {
        let input = Memory::new(" foo b");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        let alias = Rc::new(Alias {
            name: "foo".to_string(),
            replacement: "x\ny".to_string(),
            global: true,
            origin: Location::dummy("loc"),
        });

        for _ in 0..4 {
            let _ = lexer.peek_char().now_or_never().unwrap();
            lexer.consume_char();
        }

        lexer.substitute_alias(1, &alias);

        assert_matches!(lexer.peek_char().now_or_never().unwrap(), Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'x');
            assert_eq!(*c.location.code.value.borrow(), "x\ny");
            assert_eq!(c.location.code.start_line_number, line);
            assert_matches!(&*c.location.code.source,
                Source::Alias { original, alias: alias2 } => {
                assert_eq!(*original.code.value.borrow(), " foo b");
                assert_eq!(original.code.start_line_number, line);
                assert_eq!(*original.code.source, Source::Unknown);
                assert_eq!(original.range, 1..4);
                assert_eq!(alias2, &alias);
            });
            assert_eq!(c.location.range, 0..1);
        });
        lexer.consume_char();

        assert_matches!(lexer.peek_char().now_or_never().unwrap(), Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, '\n');
            assert_eq!(*c.location.code.value.borrow(), "x\ny");
            assert_eq!(c.location.code.start_line_number, line);
            assert_matches!(&*c.location.code.source,
                Source::Alias { original, alias: alias2 } => {
                assert_eq!(*original.code.value.borrow(), " foo b");
                assert_eq!(original.code.start_line_number, line);
                assert_eq!(*original.code.source, Source::Unknown);
                assert_eq!(original.range, 1..4);
                assert_eq!(alias2, &alias);
            });
            assert_eq!(c.location.range, 1..2);
        });
        lexer.consume_char();

        assert_matches!(lexer.peek_char().now_or_never().unwrap(), Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'y');
            assert_eq!(*c.location.code.value.borrow(), "x\ny");
            assert_eq!(c.location.code.start_line_number, line);
            assert_matches!(&*c.location.code.source, Source::Alias { original, alias: alias2 } => {
                assert_eq!(*original.code.value.borrow(), " foo b");
                assert_eq!(original.code.start_line_number, line);
                assert_eq!(*original.code.source, Source::Unknown);
                assert_eq!(original.range, 1..4);
                assert_eq!(alias2, &alias);
            });
            assert_eq!(c.location.range, 2..3);
        });
        lexer.consume_char();

        assert_matches!(lexer.peek_char().now_or_never().unwrap(), Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, ' ');
            assert_eq!(*c.location.code.value.borrow(), " foo b");
            assert_eq!(c.location.code.start_line_number, line);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 4..5);
        });
        lexer.consume_char();
    }

    #[test]
    fn lexer_core_substitute_alias_empty_replacement() {
        let input = Memory::new("x ");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        let alias = Rc::new(Alias {
            name: "x".to_string(),
            replacement: "".to_string(),
            global: false,
            origin: Location::dummy("dummy"),
        });

        let _ = lexer.peek_char().now_or_never().unwrap();
        lexer.consume_char();

        lexer.substitute_alias(0, &alias);

        assert_matches!(lexer.peek_char().now_or_never().unwrap(), Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, ' ');
            assert_eq!(*c.location.code.value.borrow(), "x ");
            assert_eq!(c.location.code.start_line_number, line);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 1..2);
        });
    }

    #[test]
    fn lexer_core_peek_char_after_alias_substitution() {
        let input = Memory::new("a\nb");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));

        lexer.peek_char().now_or_never().unwrap().unwrap();
        lexer.consume_char();

        let alias = Rc::new(Alias {
            name: "a".to_string(),
            replacement: "".to_string(),
            global: false,
            origin: Location::dummy("dummy"),
        });
        lexer.substitute_alias(0, &alias);

        let result = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(result, Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, '\n');
            assert_eq!(*c.location.code.value.borrow(), "a\n");
            assert_eq!(c.location.code.start_line_number, line);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 1..2);
        });
        lexer.consume_char();

        let result = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(result, Ok(PeekChar::Char(c)) => {
            assert_eq!(c.value, 'b');
            assert_eq!(*c.location.code.value.borrow(), "a\nb");
            assert_eq!(c.location.code.start_line_number.get(), 1);
            assert_eq!(*c.location.code.source, Source::Unknown);
            assert_eq!(c.location.range, 2..3);
        });
        lexer.consume_char();

        let result = lexer.peek_char().now_or_never().unwrap();
        assert_matches!(result, Ok(PeekChar::EndOfInput(location)) => {
            assert_eq!(*location.code.value.borrow(), "a\nb");
            assert_eq!(location.code.start_line_number.get(), 1);
            assert_eq!(*location.code.source, Source::Unknown);
            assert_eq!(location.range, 3..3);
        });
    }

    #[test]
    fn lexer_core_is_after_blank_ending_alias_index_0() {
        let original = Location::dummy("original");
        let alias = Rc::new(Alias {
            name: "a".to_string(),
            replacement: " ".to_string(),
            global: false,
            origin: Location::dummy("origin"),
        });
        let source = Source::Alias { original, alias };
        let input = Memory::new("a");
        let line = NonZeroU64::new(1).unwrap();
        let lexer = LexerCore::new(Box::new(input), line, Rc::new(source));
        assert!(!lexer.is_after_blank_ending_alias(0));
    }

    #[test]
    fn lexer_core_is_after_blank_ending_alias_not_blank_ending() {
        let input = Memory::new("a x");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        let alias = Rc::new(Alias {
            name: "a".to_string(),
            replacement: " b".to_string(),
            global: false,
            origin: Location::dummy("dummy"),
        });

        lexer.peek_char().now_or_never().unwrap().unwrap();
        lexer.consume_char();

        lexer.substitute_alias(0, &alias);

        assert!(!lexer.is_after_blank_ending_alias(0));
        assert!(!lexer.is_after_blank_ending_alias(1));
        assert!(!lexer.is_after_blank_ending_alias(2));
        assert!(!lexer.is_after_blank_ending_alias(3));
    }

    #[test]
    fn lexer_core_is_after_blank_ending_alias_blank_ending() {
        let input = Memory::new("a x");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        let alias = Rc::new(Alias {
            name: "a".to_string(),
            replacement: " b ".to_string(),
            global: false,
            origin: Location::dummy("dummy"),
        });

        lexer.peek_char().now_or_never().unwrap().unwrap();
        lexer.consume_char();

        lexer.substitute_alias(0, &alias);

        assert!(!lexer.is_after_blank_ending_alias(0));
        assert!(!lexer.is_after_blank_ending_alias(1));
        assert!(!lexer.is_after_blank_ending_alias(2));
        assert!(lexer.is_after_blank_ending_alias(3));
        assert!(lexer.is_after_blank_ending_alias(4));
    }

    #[test]
    fn lexer_core_is_after_blank_ending_alias_after_line_continuation() {
        let input = Memory::new("a\\\n x");
        let line = NonZeroU64::new(1).unwrap();
        let mut lexer = LexerCore::new(Box::new(input), line, Rc::new(Source::Unknown));
        let alias = Rc::new(Alias {
            name: "a".to_string(),
            replacement: " b ".to_string(),
            global: false,
            origin: Location::dummy("dummy"),
        });

        lexer.peek_char().now_or_never().unwrap().unwrap();
        lexer.consume_char();
        lexer.substitute_alias(0, &alias);

        while let Ok(PeekChar::Char(_)) = lexer.peek_char().now_or_never().unwrap() {
            lexer.consume_char();
        }
        lexer.mark_line_continuation(3..5);

        assert!(!lexer.is_after_blank_ending_alias(0));
        assert!(!lexer.is_after_blank_ending_alias(1));
        assert!(!lexer.is_after_blank_ending_alias(2));
        assert!(lexer.is_after_blank_ending_alias(5));
        assert!(lexer.is_after_blank_ending_alias(6));
    }

    #[test]
    fn lexer_with_empty_source() {
        let mut lexer = Lexer::from_memory("", Source::Unknown);
        assert_eq!(lexer.peek_char().now_or_never().unwrap(), Ok(None));
    }

    #[test]
    fn lexer_peek_char_with_line_continuation_enabled_stopping_on_non_backslash() {
        let mut lexer = Lexer::from_memory("\\\n\n\\", Source::Unknown);
        assert_eq!(lexer.peek_char().now_or_never().unwrap(), Ok(Some('\n')));
        assert_eq!(lexer.index(), 2);
    }

    #[test]
    fn lexer_peek_char_with_line_continuation_enabled_stopping_on_non_newline() {
        let mut lexer = Lexer::from_memory("\\\n\\\n\\\n\\\\", Source::Unknown);
        assert_eq!(lexer.peek_char().now_or_never().unwrap(), Ok(Some('\\')));
        assert_eq!(lexer.index(), 6);
    }

    #[test]
    fn lexer_peek_char_with_line_continuation_disabled() {
        let mut lexer = Lexer::from_memory("\\\n\\\n\\\\", Source::Unknown);
        let mut lexer = lexer.disable_line_continuation();
        assert_eq!(lexer.peek_char().now_or_never().unwrap(), Ok(Some('\\')));
        assert_eq!(lexer.index(), 0);
    }

    #[test]
    fn lexer_flush() {
        let mut lexer = Lexer::from_memory(" \n\n\t\n", Source::Unknown);
        let location_1 = lexer.location().now_or_never().unwrap().unwrap().clone();
        assert_eq!(*location_1.code.value.borrow(), " \n");

        lexer.consume_char();
        lexer.peek_char().now_or_never().unwrap().unwrap();
        lexer.consume_char();
        lexer.peek_char().now_or_never().unwrap().unwrap();
        lexer.consume_char();
        lexer.flush();
        lexer.peek_char().now_or_never().unwrap().unwrap();
        lexer.consume_char();

        let location_2 = lexer.location().now_or_never().unwrap().unwrap().clone();

        assert_eq!(*location_1.code.value.borrow(), " \n\n");
        assert_eq!(location_1.code.start_line_number.get(), 1);
        assert_eq!(*location_1.code.source, Source::Unknown);
        assert_eq!(location_1.range, 0..1);
        assert_eq!(*location_2.code.value.borrow(), "\t\n");
        assert_eq!(location_2.code.start_line_number.get(), 3);
        assert_eq!(*location_2.code.source, Source::Unknown);
        assert_eq!(location_2.range, 1..2);
    }

    #[test]
    fn lexer_consume_char_if() {
        let mut lexer = Lexer::from_memory("word\n", Source::Unknown);

        let mut called = 0;
        let c = lexer
            .consume_char_if(|c| {
                assert_eq!(c, 'w');
                called += 1;
                true
            })
            .now_or_never()
            .unwrap()
            .unwrap()
            .unwrap();
        assert_eq!(called, 1);
        assert_eq!(c.value, 'w');
        assert_eq!(*c.location.code.value.borrow(), "word\n");
        assert_eq!(c.location.code.start_line_number.get(), 1);
        assert_eq!(*c.location.code.source, Source::Unknown);
        assert_eq!(c.location.range, 0..1);

        let mut called = 0;
        let r = lexer
            .consume_char_if(|c| {
                assert_eq!(c, 'o');
                called += 1;
                false
            })
            .now_or_never()
            .unwrap();
        assert_eq!(called, 1);
        assert_eq!(r, Ok(None));

        let mut called = 0;
        let r = lexer
            .consume_char_if(|c| {
                assert_eq!(c, 'o');
                called += 1;
                false
            })
            .now_or_never()
            .unwrap();
        assert_eq!(called, 1);
        assert_eq!(r, Ok(None));

        let mut called = 0;
        let c = lexer
            .consume_char_if(|c| {
                assert_eq!(c, 'o');
                called += 1;
                true
            })
            .now_or_never()
            .unwrap()
            .unwrap()
            .unwrap();
        assert_eq!(called, 1);
        assert_eq!(c.value, 'o');
        assert_eq!(*c.location.code.value.borrow(), "word\n");
        assert_eq!(c.location.code.start_line_number.get(), 1);
        assert_eq!(*c.location.code.source, Source::Unknown);
        assert_eq!(c.location.range, 1..2);

        lexer
            .consume_char_if(|c| {
                assert_eq!(c, 'r');
                true
            })
            .now_or_never()
            .unwrap()
            .unwrap()
            .unwrap();
        lexer
            .consume_char_if(|c| {
                assert_eq!(c, 'd');
                true
            })
            .now_or_never()
            .unwrap()
            .unwrap()
            .unwrap();
        lexer
            .consume_char_if(|c| {
                assert_eq!(c, '\n');
                true
            })
            .now_or_never()
            .unwrap()
            .unwrap()
            .unwrap();

        // end of input
        let r = lexer
            .consume_char_if(|c| {
                unreachable!("unexpected call to the decider function: argument={}", c)
            })
            .now_or_never()
            .unwrap();
        assert_eq!(r, Ok(None));
    }

    #[test]
    fn lexer_location_range_with_empty_range() {
        let mut lexer = Lexer::from_memory("", Source::Unknown);
        lexer.peek_char().now_or_never().unwrap().unwrap();
        let location = lexer.location_range(0..0);
        assert_eq!(*location.code.value.borrow(), "");
        assert_eq!(location.code.start_line_number.get(), 1);
        assert_eq!(*location.code.source, Source::Unknown);
        assert_eq!(location.range, 0..0);
    }

    #[test]
    fn lexer_location_range_with_nonempty_range() {
        let mut lexer = Lexer::from_memory("cat foo", Source::Stdin);
        for _ in 0..4 {
            lexer.peek_char().now_or_never().unwrap().unwrap();
            lexer.consume_char();
        }
        lexer.peek_char().now_or_never().unwrap().unwrap();

        let location = lexer.location_range(1..4);
        assert_eq!(*location.code.value.borrow(), "cat foo");
        assert_eq!(location.code.start_line_number.get(), 1);
        assert_eq!(*location.code.source, Source::Stdin);
        assert_eq!(location.range, 1..4);
    }

    #[test]
    fn lexer_location_range_with_range_starting_at_end() {
        let mut lexer = Lexer::from_memory("cat", Source::Stdin);
        for _ in 0..3 {
            lexer.peek_char().now_or_never().unwrap().unwrap();
            lexer.consume_char();
        }
        lexer.peek_char().now_or_never().unwrap().unwrap();

        let location = lexer.location_range(3..3);
        assert_eq!(*location.code.value.borrow(), "cat");
        assert_eq!(location.code.start_line_number.get(), 1);
        assert_eq!(*location.code.source, Source::Stdin);
        assert_eq!(location.range, 3..3);
    }

    #[test]
    #[should_panic]
    fn lexer_location_range_with_unconsumed_code() {
        let lexer = Lexer::from_memory("echo ok", Source::Unknown);
        let _ = lexer.location_range(0..0);
    }

    #[test]
    #[should_panic(expected = "The index 1 must not be larger than the current index 0")]
    fn lexer_location_range_with_range_out_of_bounds() {
        let lexer = Lexer::from_memory("", Source::Unknown);
        let _ = lexer.location_range(1..2);
    }

    #[test]
    fn lexer_location_range_with_alias_substitution() {
        let mut lexer = Lexer::from_memory(" a;", Source::Unknown);
        let alias_def = Rc::new(Alias {
            name: "a".to_string(),
            replacement: "abc".to_string(),
            global: false,
            origin: Location::dummy("dummy"),
        });
        for _ in 0..2 {
            lexer.peek_char().now_or_never().unwrap().unwrap();
            lexer.consume_char();
        }
        lexer.substitute_alias(1, &alias_def);
        for _ in 1..5 {
            lexer.peek_char().now_or_never().unwrap().unwrap();
            lexer.consume_char();
        }

        let location = lexer.location_range(2..5);
        assert_eq!(*location.code.value.borrow(), "abc");
        assert_eq!(location.code.start_line_number.get(), 1);
        assert_matches!(&*location.code.source, Source::Alias { original, alias } => {
            assert_eq!(*original.code.value.borrow(), " a;");
            assert_eq!(original.code.start_line_number.get(), 1);
            assert_eq!(*original.code.source, Source::Unknown);
            assert_eq!(original.range, 1..2);
            assert_eq!(alias, &alias_def);
        });
        assert_eq!(location.range, 1..3);
    }

    #[test]
    fn lexer_inner_program_success() {
        let mut lexer = Lexer::from_memory("x y )", Source::Unknown);
        let source = lexer.inner_program().now_or_never().unwrap().unwrap();
        assert_eq!(source, "x y ");
    }

    #[test]
    fn lexer_inner_program_failure() {
        let mut lexer = Lexer::from_memory("<< )", Source::Unknown);
        let e = lexer.inner_program().now_or_never().unwrap().unwrap_err();
        assert_eq!(
            e.cause,
            ErrorCause::Syntax(SyntaxError::MissingHereDocDelimiter)
        );
        assert_eq!(*e.location.code.value.borrow(), "<< )");
        assert_eq!(e.location.code.start_line_number.get(), 1);
        assert_eq!(*e.location.code.source, Source::Unknown);
        assert_eq!(e.location.range, 3..4);
    }
}