regex_syntax/
debug.rs

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/// A type that wraps a single byte with a convenient fmt::Debug impl that
/// escapes the byte.
pub(crate) struct Byte(pub(crate) u8);

impl core::fmt::Debug for Byte {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        // Special case ASCII space. It's too hard to read otherwise, so
        // put quotes around it. I sometimes wonder whether just '\x20' would
        // be better...
        if self.0 == b' ' {
            return write!(f, "' '");
        }
        // 10 bytes is enough to cover any output from ascii::escape_default.
        let mut bytes = [0u8; 10];
        let mut len = 0;
        for (i, mut b) in core::ascii::escape_default(self.0).enumerate() {
            // capitalize \xab to \xAB
            if i >= 2 && b'a' <= b && b <= b'f' {
                b -= 32;
            }
            bytes[len] = b;
            len += 1;
        }
        write!(f, "{}", core::str::from_utf8(&bytes[..len]).unwrap())
    }
}

/// A type that provides a human readable debug impl for arbitrary bytes.
///
/// This generally works best when the bytes are presumed to be mostly UTF-8,
/// but will work for anything.
///
/// N.B. This is copied nearly verbatim from regex-automata. Sigh.
pub(crate) struct Bytes<'a>(pub(crate) &'a [u8]);

impl<'a> core::fmt::Debug for Bytes<'a> {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        write!(f, "\"")?;
        // This is a sad re-implementation of a similar impl found in bstr.
        let mut bytes = self.0;
        while let Some(result) = utf8_decode(bytes) {
            let ch = match result {
                Ok(ch) => ch,
                Err(byte) => {
                    write!(f, r"\x{:02x}", byte)?;
                    bytes = &bytes[1..];
                    continue;
                }
            };
            bytes = &bytes[ch.len_utf8()..];
            match ch {
                '\0' => write!(f, "\\0")?,
                // ASCII control characters except \0, \n, \r, \t
                '\x01'..='\x08'
                | '\x0b'
                | '\x0c'
                | '\x0e'..='\x19'
                | '\x7f' => {
                    write!(f, "\\x{:02x}", u32::from(ch))?;
                }
                '\n' | '\r' | '\t' | _ => {
                    write!(f, "{}", ch.escape_debug())?;
                }
            }
        }
        write!(f, "\"")?;
        Ok(())
    }
}

/// Decodes the next UTF-8 encoded codepoint from the given byte slice.
///
/// If no valid encoding of a codepoint exists at the beginning of the given
/// byte slice, then the first byte is returned instead.
///
/// This returns `None` if and only if `bytes` is empty.
pub(crate) fn utf8_decode(bytes: &[u8]) -> Option<Result<char, u8>> {
    fn len(byte: u8) -> Option<usize> {
        if byte <= 0x7F {
            return Some(1);
        } else if byte & 0b1100_0000 == 0b1000_0000 {
            return None;
        } else if byte <= 0b1101_1111 {
            Some(2)
        } else if byte <= 0b1110_1111 {
            Some(3)
        } else if byte <= 0b1111_0111 {
            Some(4)
        } else {
            None
        }
    }

    if bytes.is_empty() {
        return None;
    }
    let len = match len(bytes[0]) {
        None => return Some(Err(bytes[0])),
        Some(len) if len > bytes.len() => return Some(Err(bytes[0])),
        Some(1) => return Some(Ok(char::from(bytes[0]))),
        Some(len) => len,
    };
    match core::str::from_utf8(&bytes[..len]) {
        Ok(s) => Some(Ok(s.chars().next().unwrap())),
        Err(_) => Some(Err(bytes[0])),
    }
}