compiler/rustc_lexer/src/unescape.rs - toolchain/rustc - Git at Google

 //! Utilities for validating string and char literals and turning them into
 //! values they represent.

 use std::ops::Range;
 use std::str::Chars;

 use Mode::*;

 #[cfg(test)]
 mod tests;

 /// Errors and warnings that can occur during string unescaping. They mostly
 /// relate to malformed escape sequences, but there are a few that are about
 /// other problems.
 #[derive(Debug, PartialEq, Eq)]
 pub enum EscapeError {
     /// Expected 1 char, but 0 were found.
     ZeroChars,
     /// Expected 1 char, but more than 1 were found.
     MoreThanOneChar,

     /// Escaped '\' character without continuation.
     LoneSlash,
     /// Invalid escape character (e.g. '\z').
     InvalidEscape,
     /// Raw '\r' encountered.
     BareCarriageReturn,
     /// Raw '\r' encountered in raw string.
     BareCarriageReturnInRawString,
     /// Unescaped character that was expected to be escaped (e.g. raw '\t').
     EscapeOnlyChar,

     /// Numeric character escape is too short (e.g. '\x1').
     TooShortHexEscape,
     /// Invalid character in numeric escape (e.g. '\xz')
     InvalidCharInHexEscape,
     /// Character code in numeric escape is non-ascii (e.g. '\xFF').
     OutOfRangeHexEscape,

     /// '\u' not followed by '{'.
     NoBraceInUnicodeEscape,
     /// Non-hexadecimal value in '\u{..}'.
     InvalidCharInUnicodeEscape,
     /// '\u{}'
     EmptyUnicodeEscape,
     /// No closing brace in '\u{..}', e.g. '\u{12'.
     UnclosedUnicodeEscape,
     /// '\u{_12}'
     LeadingUnderscoreUnicodeEscape,
     /// More than 6 characters in '\u{..}', e.g. '\u{10FFFF_FF}'
     OverlongUnicodeEscape,
     /// Invalid in-bound unicode character code, e.g. '\u{DFFF}'.
     LoneSurrogateUnicodeEscape,
     /// Out of bounds unicode character code, e.g. '\u{FFFFFF}'.
     OutOfRangeUnicodeEscape,

     /// Unicode escape code in byte literal.
     UnicodeEscapeInByte,
     /// Non-ascii character in byte literal, byte string literal, or raw byte string literal.
     NonAsciiCharInByte,

     // `\0` in a C string literal.
     NulInCStr,

     /// After a line ending with '\', the next line contains whitespace
     /// characters that are not skipped.
     UnskippedWhitespaceWarning,

     /// After a line ending with '\', multiple lines are skipped.
     MultipleSkippedLinesWarning,
 }

 impl EscapeError {
     /// Returns true for actual errors, as opposed to warnings.
     pub fn is_fatal(&self) -> bool {
         !matches!(
             self,
             EscapeError::UnskippedWhitespaceWarning | EscapeError::MultipleSkippedLinesWarning
         )
     }
 }

 /// Takes the contents of a unicode-only (non-mixed-utf8) literal (without
 /// quotes) and produces a sequence of escaped characters or errors.
 ///
 /// Values are returned by invoking `callback`. For `Char` and `Byte` modes,
 /// the callback will be called exactly once.
 pub fn unescape_unicode<F>(src: &str, mode: Mode, callback: &mut F)
 where
     F: FnMut(Range<usize>, Result<char, EscapeError>),
 {
     match mode {
         Char | Byte => {
             let mut chars = src.chars();
             let res = unescape_char_or_byte(&mut chars, mode);
             callback(0..(src.len() - chars.as_str().len()), res);
         }
         Str | ByteStr => unescape_non_raw_common(src, mode, callback),
         RawStr | RawByteStr => check_raw_common(src, mode, callback),
         RawCStr => check_raw_common(src, mode, &mut |r, mut result| {
             if let Ok('\0') = result {
                 result = Err(EscapeError::NulInCStr);
             }
             callback(r, result)
         }),
         CStr => unreachable!(),
     }
 }

 /// Used for mixed utf8 string literals, i.e. those that allow both unicode
 /// chars and high bytes.
 pub enum MixedUnit {
     /// Used for ASCII chars (written directly or via `\x00`..`\x7f` escapes)
     /// and Unicode chars (written directly or via `\u` escapes).
     ///
     /// For example, if '¥' appears in a string it is represented here as
     /// `MixedUnit::Char('¥')`, and it will be appended to the relevant byte
     /// string as the two-byte UTF-8 sequence `[0xc2, 0xa5]`
     Char(char),

     /// Used for high bytes (`\x80`..`\xff`).
     ///
     /// For example, if `\xa5` appears in a string it is represented here as
     /// `MixedUnit::HighByte(0xa5)`, and it will be appended to the relevant
     /// byte string as the single byte `0xa5`.
     HighByte(u8),
 }

 impl From<char> for MixedUnit {
     fn from(c: char) -> Self {
         MixedUnit::Char(c)
     }
 }

 impl From<u8> for MixedUnit {
     fn from(n: u8) -> Self {
         if n.is_ascii() { MixedUnit::Char(n as char) } else { MixedUnit::HighByte(n) }
     }
 }

 /// Takes the contents of a mixed-utf8 literal (without quotes) and produces
 /// a sequence of escaped characters or errors.
 ///
 /// Values are returned by invoking `callback`.
 pub fn unescape_mixed<F>(src: &str, mode: Mode, callback: &mut F)
 where
     F: FnMut(Range<usize>, Result<MixedUnit, EscapeError>),
 {
     match mode {
         CStr => unescape_non_raw_common(src, mode, &mut |r, mut result| {
             if let Ok(MixedUnit::Char('\0')) = result {
                 result = Err(EscapeError::NulInCStr);
             }
             callback(r, result)
         }),
         Char | Byte | Str | RawStr | ByteStr | RawByteStr | RawCStr => unreachable!(),
     }
 }

 /// Takes a contents of a char literal (without quotes), and returns an
 /// unescaped char or an error.
 pub fn unescape_char(src: &str) -> Result<char, EscapeError> {
     unescape_char_or_byte(&mut src.chars(), Char)
 }

 /// Takes a contents of a byte literal (without quotes), and returns an
 /// unescaped byte or an error.
 pub fn unescape_byte(src: &str) -> Result<u8, EscapeError> {
     unescape_char_or_byte(&mut src.chars(), Byte).map(byte_from_char)
 }

 /// What kind of literal do we parse.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum Mode {
     Char,

     Byte,

     Str,
     RawStr,

     ByteStr,
     RawByteStr,

     CStr,
     RawCStr,
 }

 impl Mode {
     pub fn in_double_quotes(self) -> bool {
         match self {
             Str | RawStr | ByteStr | RawByteStr | CStr | RawCStr => true,
             Char | Byte => false,
         }
     }

     /// Are `\x80`..`\xff` allowed?
     fn allow_high_bytes(self) -> bool {
         match self {
             Char | Str => false,
             Byte | ByteStr | CStr => true,
             RawStr | RawByteStr | RawCStr => unreachable!(),
         }
     }

     /// Are unicode (non-ASCII) chars allowed?
     #[inline]
     fn allow_unicode_chars(self) -> bool {
         match self {
             Byte | ByteStr | RawByteStr => false,
             Char | Str | RawStr | CStr | RawCStr => true,
         }
     }

     /// Are unicode escapes (`\u`) allowed?
     fn allow_unicode_escapes(self) -> bool {
         match self {
             Byte | ByteStr => false,
             Char | Str | CStr => true,
             RawByteStr | RawStr | RawCStr => unreachable!(),
         }
     }

     pub fn prefix_noraw(self) -> &'static str {
         match self {
             Char | Str | RawStr => "",
             Byte | ByteStr | RawByteStr => "b",
             CStr | RawCStr => "c",
         }
     }
 }

 fn scan_escape<T: From<char> + From<u8>>(
     chars: &mut Chars<'_>,
     mode: Mode,
 ) -> Result<T, EscapeError> {
     // Previous character was '\\', unescape what follows.
     let res: char = match chars.next().ok_or(EscapeError::LoneSlash)? {
         '"' => '"',
         'n' => '\n',
         'r' => '\r',
         't' => '\t',
         '\\' => '\\',
         '\'' => '\'',
         '0' => '\0',
         'x' => {
             // Parse hexadecimal character code.

             let hi = chars.next().ok_or(EscapeError::TooShortHexEscape)?;
             let hi = hi.to_digit(16).ok_or(EscapeError::InvalidCharInHexEscape)?;

             let lo = chars.next().ok_or(EscapeError::TooShortHexEscape)?;
             let lo = lo.to_digit(16).ok_or(EscapeError::InvalidCharInHexEscape)?;

             let value = (hi * 16 + lo) as u8;

             return if !mode.allow_high_bytes() && !value.is_ascii() {
                 Err(EscapeError::OutOfRangeHexEscape)
             } else {
                 // This may be a high byte, but that will only happen if `T` is
                 // `MixedUnit`, because of the `allow_high_bytes` check above.
                 Ok(T::from(value as u8))
             };
         }
         'u' => return scan_unicode(chars, mode.allow_unicode_escapes()).map(T::from),
         _ => return Err(EscapeError::InvalidEscape),
     };
     Ok(T::from(res))
 }

 fn scan_unicode(chars: &mut Chars<'_>, allow_unicode_escapes: bool) -> Result<char, EscapeError> {
     // We've parsed '\u', now we have to parse '{..}'.

     if chars.next() != Some('{') {
         return Err(EscapeError::NoBraceInUnicodeEscape);
     }

     // First character must be a hexadecimal digit.
     let mut n_digits = 1;
     let mut value: u32 = match chars.next().ok_or(EscapeError::UnclosedUnicodeEscape)? {
         '_' => return Err(EscapeError::LeadingUnderscoreUnicodeEscape),
         '}' => return Err(EscapeError::EmptyUnicodeEscape),
         c => c.to_digit(16).ok_or(EscapeError::InvalidCharInUnicodeEscape)?,
     };

     // First character is valid, now parse the rest of the number
     // and closing brace.
     loop {
         match chars.next() {
             None => return Err(EscapeError::UnclosedUnicodeEscape),
             Some('_') => continue,
             Some('}') => {
                 if n_digits > 6 {
                     return Err(EscapeError::OverlongUnicodeEscape);
                 }

                 // Incorrect syntax has higher priority for error reporting
                 // than unallowed value for a literal.
                 if !allow_unicode_escapes {
                     return Err(EscapeError::UnicodeEscapeInByte);
                 }

                 break std::char::from_u32(value).ok_or_else(|| {
                     if value > 0x10FFFF {
                         EscapeError::OutOfRangeUnicodeEscape
                     } else {
                         EscapeError::LoneSurrogateUnicodeEscape
                     }
                 });
             }
             Some(c) => {
                 let digit: u32 = c.to_digit(16).ok_or(EscapeError::InvalidCharInUnicodeEscape)?;
                 n_digits += 1;
                 if n_digits > 6 {
                     // Stop updating value since we're sure that it's incorrect already.
                     continue;
                 }
                 value = value * 16 + digit;
             }
         };
     }
 }

 #[inline]
 fn ascii_check(c: char, allow_unicode_chars: bool) -> Result<char, EscapeError> {
     if allow_unicode_chars || c.is_ascii() { Ok(c) } else { Err(EscapeError::NonAsciiCharInByte) }
 }

 fn unescape_char_or_byte(chars: &mut Chars<'_>, mode: Mode) -> Result<char, EscapeError> {
     let c = chars.next().ok_or(EscapeError::ZeroChars)?;
     let res = match c {
         '\\' => scan_escape(chars, mode),
         '\n' | '\t' | '\'' => Err(EscapeError::EscapeOnlyChar),
         '\r' => Err(EscapeError::BareCarriageReturn),
         _ => ascii_check(c, mode.allow_unicode_chars()),
     }?;
     if chars.next().is_some() {
         return Err(EscapeError::MoreThanOneChar);
     }
     Ok(res)
 }

 /// Takes a contents of a string literal (without quotes) and produces a
 /// sequence of escaped characters or errors.
 fn unescape_non_raw_common<F, T: From<char> + From<u8>>(src: &str, mode: Mode, callback: &mut F)
 where
     F: FnMut(Range<usize>, Result<T, EscapeError>),
 {
     let mut chars = src.chars();
     let allow_unicode_chars = mode.allow_unicode_chars(); // get this outside the loop

     // The `start` and `end` computation here is complicated because
     // `skip_ascii_whitespace` makes us to skip over chars without counting
     // them in the range computation.
     while let Some(c) = chars.next() {
         let start = src.len() - chars.as_str().len() - c.len_utf8();
         let res = match c {
             '\\' => {
                 match chars.clone().next() {
                     Some('\n') => {
                         // Rust language specification requires us to skip whitespaces
                         // if unescaped '\' character is followed by '\n'.
                         // For details see [Rust language reference]
                         // (https://doc.rust-lang.org/reference/tokens.html#string-literals).
                         skip_ascii_whitespace(&mut chars, start, &mut |range, err| {
                             callback(range, Err(err))
                         });
                         continue;
                     }
                     _ => scan_escape::<T>(&mut chars, mode),
                 }
             }
             '"' => Err(EscapeError::EscapeOnlyChar),
             '\r' => Err(EscapeError::BareCarriageReturn),
             _ => ascii_check(c, allow_unicode_chars).map(T::from),
         };
         let end = src.len() - chars.as_str().len();
         callback(start..end, res);
     }
 }

 fn skip_ascii_whitespace<F>(chars: &mut Chars<'_>, start: usize, callback: &mut F)
 where
     F: FnMut(Range<usize>, EscapeError),
 {
     let tail = chars.as_str();
     let first_non_space = tail
         .bytes()
         .position(|b| b != b' ' && b != b'\t' && b != b'\n' && b != b'\r')
         .unwrap_or(tail.len());
     if tail[1..first_non_space].contains('\n') {
         // The +1 accounts for the escaping slash.
         let end = start + first_non_space + 1;
         callback(start..end, EscapeError::MultipleSkippedLinesWarning);
     }
     let tail = &tail[first_non_space..];
     if let Some(c) = tail.chars().next() {
         if c.is_whitespace() {
             // For error reporting, we would like the span to contain the character that was not
             // skipped. The +1 is necessary to account for the leading \ that started the escape.
             let end = start + first_non_space + c.len_utf8() + 1;
             callback(start..end, EscapeError::UnskippedWhitespaceWarning);
         }
     }
     *chars = tail.chars();
 }

 /// Takes a contents of a string literal (without quotes) and produces a
 /// sequence of characters or errors.
 /// NOTE: Raw strings do not perform any explicit character escaping, here we
 /// only produce errors on bare CR.
 fn check_raw_common<F>(src: &str, mode: Mode, callback: &mut F)
 where
     F: FnMut(Range<usize>, Result<char, EscapeError>),
 {
     let mut chars = src.chars();
     let allow_unicode_chars = mode.allow_unicode_chars(); // get this outside the loop

     // The `start` and `end` computation here matches the one in
     // `unescape_non_raw_common` for consistency, even though this function
     // doesn't have to worry about skipping any chars.
     while let Some(c) = chars.next() {
         let start = src.len() - chars.as_str().len() - c.len_utf8();
         let res = match c {
             '\r' => Err(EscapeError::BareCarriageReturnInRawString),
             _ => ascii_check(c, allow_unicode_chars),
         };
         let end = src.len() - chars.as_str().len();
         callback(start..end, res);
     }
 }

 #[inline]
 pub fn byte_from_char(c: char) -> u8 {
     let res = c as u32;
     debug_assert!(res <= u8::MAX as u32, "guaranteed because of ByteStr");
     res as u8
 }
	//! Utilities for validating string and char literals and turning them into
	//! values they represent.

	use std::ops::Range;
	use std::str::Chars;

	use Mode::*;

	#[cfg(test)]
	mod tests;

	/// Errors and warnings that can occur during string unescaping. They mostly
	/// relate to malformed escape sequences, but there are a few that are about
	/// other problems.
	#[derive(Debug, PartialEq, Eq)]
	pub enum EscapeError {
	/// Expected 1 char, but 0 were found.
	ZeroChars,
	/// Expected 1 char, but more than 1 were found.
	MoreThanOneChar,

	/// Escaped '\' character without continuation.
	LoneSlash,
	/// Invalid escape character (e.g. '\z').
	InvalidEscape,
	/// Raw '\r' encountered.
	BareCarriageReturn,
	/// Raw '\r' encountered in raw string.
	BareCarriageReturnInRawString,
	/// Unescaped character that was expected to be escaped (e.g. raw '\t').
	EscapeOnlyChar,

	/// Numeric character escape is too short (e.g. '\x1').
	TooShortHexEscape,
	/// Invalid character in numeric escape (e.g. '\xz')
	InvalidCharInHexEscape,
	/// Character code in numeric escape is non-ascii (e.g. '\xFF').
	OutOfRangeHexEscape,

	/// '\u' not followed by '{'.
	NoBraceInUnicodeEscape,
	/// Non-hexadecimal value in '\u{..}'.
	InvalidCharInUnicodeEscape,
	/// '\u{}'
	EmptyUnicodeEscape,
	/// No closing brace in '\u{..}', e.g. '\u{12'.
	UnclosedUnicodeEscape,
	/// '\u{_12}'
	LeadingUnderscoreUnicodeEscape,
	/// More than 6 characters in '\u{..}', e.g. '\u{10FFFF_FF}'
	OverlongUnicodeEscape,
	/// Invalid in-bound unicode character code, e.g. '\u{DFFF}'.
	LoneSurrogateUnicodeEscape,
	/// Out of bounds unicode character code, e.g. '\u{FFFFFF}'.
	OutOfRangeUnicodeEscape,

	/// Unicode escape code in byte literal.
	UnicodeEscapeInByte,
	/// Non-ascii character in byte literal, byte string literal, or raw byte string literal.
	NonAsciiCharInByte,

	// `\0` in a C string literal.
	NulInCStr,

	/// After a line ending with '\', the next line contains whitespace
	/// characters that are not skipped.
	UnskippedWhitespaceWarning,

	/// After a line ending with '\', multiple lines are skipped.
	MultipleSkippedLinesWarning,
	}

	impl EscapeError {
	/// Returns true for actual errors, as opposed to warnings.
	pub fn is_fatal(&self) -> bool {
	!matches!(
	self,
	EscapeError::UnskippedWhitespaceWarning \| EscapeError::MultipleSkippedLinesWarning
	)
	}
	}

	/// Takes the contents of a unicode-only (non-mixed-utf8) literal (without
	/// quotes) and produces a sequence of escaped characters or errors.
	///
	/// Values are returned by invoking `callback`. For `Char` and `Byte` modes,
	/// the callback will be called exactly once.
	pub fn unescape_unicode<F>(src: &str, mode: Mode, callback: &mut F)
	where
	F: FnMut(Range<usize>, Result<char, EscapeError>),
	{
	match mode {
	Char \| Byte => {
	let mut chars = src.chars();
	let res = unescape_char_or_byte(&mut chars, mode);
	callback(0..(src.len() - chars.as_str().len()), res);
	}
	Str \| ByteStr => unescape_non_raw_common(src, mode, callback),
	RawStr \| RawByteStr => check_raw_common(src, mode, callback),
	RawCStr => check_raw_common(src, mode, &mut \|r, mut result\| {
	if let Ok('\0') = result {
	result = Err(EscapeError::NulInCStr);
	}
	callback(r, result)
	}),
	CStr => unreachable!(),
	}
	}

	/// Used for mixed utf8 string literals, i.e. those that allow both unicode
	/// chars and high bytes.
	pub enum MixedUnit {
	/// Used for ASCII chars (written directly or via `\x00`..`\x7f` escapes)
	/// and Unicode chars (written directly or via `\u` escapes).
	///
	/// For example, if '¥' appears in a string it is represented here as
	/// `MixedUnit::Char('¥')`, and it will be appended to the relevant byte
	/// string as the two-byte UTF-8 sequence `[0xc2, 0xa5]`
	Char(char),

	/// Used for high bytes (`\x80`..`\xff`).
	///
	/// For example, if `\xa5` appears in a string it is represented here as
	/// `MixedUnit::HighByte(0xa5)`, and it will be appended to the relevant
	/// byte string as the single byte `0xa5`.
	HighByte(u8),
	}

	impl From<char> for MixedUnit {
	fn from(c: char) -> Self {
	MixedUnit::Char(c)
	}
	}

	impl From<u8> for MixedUnit {
	fn from(n: u8) -> Self {
	if n.is_ascii() { MixedUnit::Char(n as char) } else { MixedUnit::HighByte(n) }
	}
	}

	/// Takes the contents of a mixed-utf8 literal (without quotes) and produces
	/// a sequence of escaped characters or errors.
	///
	/// Values are returned by invoking `callback`.
	pub fn unescape_mixed<F>(src: &str, mode: Mode, callback: &mut F)
	where
	F: FnMut(Range<usize>, Result<MixedUnit, EscapeError>),
	{
	match mode {
	CStr => unescape_non_raw_common(src, mode, &mut \|r, mut result\| {
	if let Ok(MixedUnit::Char('\0')) = result {
	result = Err(EscapeError::NulInCStr);
	}
	callback(r, result)
	}),
	Char \| Byte \| Str \| RawStr \| ByteStr \| RawByteStr \| RawCStr => unreachable!(),
	}
	}

	/// Takes a contents of a char literal (without quotes), and returns an
	/// unescaped char or an error.
	pub fn unescape_char(src: &str) -> Result<char, EscapeError> {
	unescape_char_or_byte(&mut src.chars(), Char)
	}

	/// Takes a contents of a byte literal (without quotes), and returns an
	/// unescaped byte or an error.
	pub fn unescape_byte(src: &str) -> Result<u8, EscapeError> {
	unescape_char_or_byte(&mut src.chars(), Byte).map(byte_from_char)
	}

	/// What kind of literal do we parse.
	#[derive(Debug, Clone, Copy, PartialEq)]
	pub enum Mode {
	Char,

	Byte,

	Str,
	RawStr,

	ByteStr,
	RawByteStr,

	CStr,
	RawCStr,
	}

	impl Mode {
	pub fn in_double_quotes(self) -> bool {
	match self {
	Str \| RawStr \| ByteStr \| RawByteStr \| CStr \| RawCStr => true,
	Char \| Byte => false,
	}
	}

	/// Are `\x80`..`\xff` allowed?
	fn allow_high_bytes(self) -> bool {
	match self {
	Char \| Str => false,
	Byte \| ByteStr \| CStr => true,
	RawStr \| RawByteStr \| RawCStr => unreachable!(),
	}
	}

	/// Are unicode (non-ASCII) chars allowed?
	#[inline]
	fn allow_unicode_chars(self) -> bool {
	match self {
	Byte \| ByteStr \| RawByteStr => false,
	Char \| Str \| RawStr \| CStr \| RawCStr => true,
	}
	}

	/// Are unicode escapes (`\u`) allowed?
	fn allow_unicode_escapes(self) -> bool {
	match self {
	Byte \| ByteStr => false,
	Char \| Str \| CStr => true,
	RawByteStr \| RawStr \| RawCStr => unreachable!(),
	}
	}

	pub fn prefix_noraw(self) -> &'static str {
	match self {
	Char \| Str \| RawStr => "",
	Byte \| ByteStr \| RawByteStr => "b",
	CStr \| RawCStr => "c",
	}
	}
	}

	fn scan_escape<T: From<char> + From<u8>>(
	chars: &mut Chars<'_>,
	mode: Mode,
	) -> Result<T, EscapeError> {
	// Previous character was '\\', unescape what follows.
	let res: char = match chars.next().ok_or(EscapeError::LoneSlash)? {
	'"' => '"',
	'n' => '\n',
	'r' => '\r',
	't' => '\t',
	'\\' => '\\',
	'\'' => '\'',
	'0' => '\0',
	'x' => {
	// Parse hexadecimal character code.

	let hi = chars.next().ok_or(EscapeError::TooShortHexEscape)?;
	let hi = hi.to_digit(16).ok_or(EscapeError::InvalidCharInHexEscape)?;

	let lo = chars.next().ok_or(EscapeError::TooShortHexEscape)?;
	let lo = lo.to_digit(16).ok_or(EscapeError::InvalidCharInHexEscape)?;

	let value = (hi * 16 + lo) as u8;

	return if !mode.allow_high_bytes() && !value.is_ascii() {
	Err(EscapeError::OutOfRangeHexEscape)
	} else {
	// This may be a high byte, but that will only happen if `T` is
	// `MixedUnit`, because of the `allow_high_bytes` check above.
	Ok(T::from(value as u8))
	};
	}
	'u' => return scan_unicode(chars, mode.allow_unicode_escapes()).map(T::from),
	_ => return Err(EscapeError::InvalidEscape),
	};
	Ok(T::from(res))
	}

	fn scan_unicode(chars: &mut Chars<'_>, allow_unicode_escapes: bool) -> Result<char, EscapeError> {
	// We've parsed '\u', now we have to parse '{..}'.

	if chars.next() != Some('{') {
	return Err(EscapeError::NoBraceInUnicodeEscape);
	}

	// First character must be a hexadecimal digit.
	let mut n_digits = 1;
	let mut value: u32 = match chars.next().ok_or(EscapeError::UnclosedUnicodeEscape)? {
	'_' => return Err(EscapeError::LeadingUnderscoreUnicodeEscape),
	'}' => return Err(EscapeError::EmptyUnicodeEscape),
	c => c.to_digit(16).ok_or(EscapeError::InvalidCharInUnicodeEscape)?,
	};

	// First character is valid, now parse the rest of the number
	// and closing brace.
	loop {
	match chars.next() {
	None => return Err(EscapeError::UnclosedUnicodeEscape),
	Some('_') => continue,
	Some('}') => {
	if n_digits > 6 {
	return Err(EscapeError::OverlongUnicodeEscape);
	}

	// Incorrect syntax has higher priority for error reporting
	// than unallowed value for a literal.
	if !allow_unicode_escapes {
	return Err(EscapeError::UnicodeEscapeInByte);
	}

	break std::char::from_u32(value).ok_or_else(\|\| {
	if value > 0x10FFFF {
	EscapeError::OutOfRangeUnicodeEscape
	} else {
	EscapeError::LoneSurrogateUnicodeEscape
	}
	});
	}
	Some(c) => {
	let digit: u32 = c.to_digit(16).ok_or(EscapeError::InvalidCharInUnicodeEscape)?;
	n_digits += 1;
	if n_digits > 6 {
	// Stop updating value since we're sure that it's incorrect already.
	continue;
	}
	value = value * 16 + digit;
	}
	};
	}
	}

	#[inline]
	fn ascii_check(c: char, allow_unicode_chars: bool) -> Result<char, EscapeError> {
	if allow_unicode_chars \|\| c.is_ascii() { Ok(c) } else { Err(EscapeError::NonAsciiCharInByte) }
	}

	fn unescape_char_or_byte(chars: &mut Chars<'_>, mode: Mode) -> Result<char, EscapeError> {
	let c = chars.next().ok_or(EscapeError::ZeroChars)?;
	let res = match c {
	'\\' => scan_escape(chars, mode),
	'\n' \| '\t' \| '\'' => Err(EscapeError::EscapeOnlyChar),
	'\r' => Err(EscapeError::BareCarriageReturn),
	_ => ascii_check(c, mode.allow_unicode_chars()),
	}?;
	if chars.next().is_some() {
	return Err(EscapeError::MoreThanOneChar);
	}
	Ok(res)
	}

	/// Takes a contents of a string literal (without quotes) and produces a
	/// sequence of escaped characters or errors.
	fn unescape_non_raw_common<F, T: From<char> + From<u8>>(src: &str, mode: Mode, callback: &mut F)
	where
	F: FnMut(Range<usize>, Result<T, EscapeError>),
	{
	let mut chars = src.chars();
	let allow_unicode_chars = mode.allow_unicode_chars(); // get this outside the loop

	// The `start` and `end` computation here is complicated because
	// `skip_ascii_whitespace` makes us to skip over chars without counting
	// them in the range computation.
	while let Some(c) = chars.next() {
	let start = src.len() - chars.as_str().len() - c.len_utf8();
	let res = match c {
	'\\' => {
	match chars.clone().next() {
	Some('\n') => {
	// Rust language specification requires us to skip whitespaces
	// if unescaped '\' character is followed by '\n'.
	// For details see [Rust language reference]
	// (https://doc.rust-lang.org/reference/tokens.html#string-literals).
	skip_ascii_whitespace(&mut chars, start, &mut \|range, err\| {
	callback(range, Err(err))
	});
	continue;
	}
	_ => scan_escape::<T>(&mut chars, mode),
	}
	}
	'"' => Err(EscapeError::EscapeOnlyChar),
	'\r' => Err(EscapeError::BareCarriageReturn),
	_ => ascii_check(c, allow_unicode_chars).map(T::from),
	};
	let end = src.len() - chars.as_str().len();
	callback(start..end, res);
	}
	}

	fn skip_ascii_whitespace<F>(chars: &mut Chars<'_>, start: usize, callback: &mut F)
	where
	F: FnMut(Range<usize>, EscapeError),
	{
	let tail = chars.as_str();
	let first_non_space = tail
	.bytes()
	.position(\|b\| b != b' ' && b != b'\t' && b != b'\n' && b != b'\r')
	.unwrap_or(tail.len());
	if tail[1..first_non_space].contains('\n') {
	// The +1 accounts for the escaping slash.
	let end = start + first_non_space + 1;
	callback(start..end, EscapeError::MultipleSkippedLinesWarning);
	}
	let tail = &tail[first_non_space..];
	if let Some(c) = tail.chars().next() {
	if c.is_whitespace() {
	// For error reporting, we would like the span to contain the character that was not
	// skipped. The +1 is necessary to account for the leading \ that started the escape.
	let end = start + first_non_space + c.len_utf8() + 1;
	callback(start..end, EscapeError::UnskippedWhitespaceWarning);
	}
	}
	*chars = tail.chars();
	}

	/// Takes a contents of a string literal (without quotes) and produces a
	/// sequence of characters or errors.
	/// NOTE: Raw strings do not perform any explicit character escaping, here we
	/// only produce errors on bare CR.
	fn check_raw_common<F>(src: &str, mode: Mode, callback: &mut F)
	where
	F: FnMut(Range<usize>, Result<char, EscapeError>),
	{
	let mut chars = src.chars();
	let allow_unicode_chars = mode.allow_unicode_chars(); // get this outside the loop

	// The `start` and `end` computation here matches the one in
	// `unescape_non_raw_common` for consistency, even though this function
	// doesn't have to worry about skipping any chars.
	while let Some(c) = chars.next() {
	let start = src.len() - chars.as_str().len() - c.len_utf8();
	let res = match c {
	'\r' => Err(EscapeError::BareCarriageReturnInRawString),
	_ => ascii_check(c, allow_unicode_chars),
	};
	let end = src.len() - chars.as_str().len();
	callback(start..end, res);
	}
	}

	#[inline]
	pub fn byte_from_char(c: char) -> u8 {
	let res = c as u32;
	debug_assert!(res <= u8::MAX as u32, "guaranteed because of ByteStr");
	res as u8
	}