vendor/syn-1.0.85/src/whitespace.rs

pub fn skip(mut s: &str) -> &str {
    'skip: while !s.is_empty() {
        let byte = s.as_bytes()[0];
        if byte == b'/' {
            if s.starts_with("//")
                && (!s.starts_with("///") || s.starts_with("////"))
                && !s.starts_with("//!")
            {
                if let Some(i) = s.find('\n') {
                    s = &s[i + 1..];
                    continue;
                } else {
                    return "";
                }
            } else if s.starts_with("/**/") {
                s = &s[4..];
                continue;
            } else if s.starts_with("/*")
                && (!s.starts_with("/**") || s.starts_with("/***"))
                && !s.starts_with("/*!")
            {
                let mut depth = 0;
                let bytes = s.as_bytes();
                let mut i = 0;
                let upper = bytes.len() - 1;
                while i < upper {
                    if bytes[i] == b'/' && bytes[i + 1] == b'*' {
                        depth += 1;
                        i += 1; // eat '*'
                    } else if bytes[i] == b'*' && bytes[i + 1] == b'/' {
                        depth -= 1;
                        if depth == 0 {
                            s = &s[i + 2..];
                            continue 'skip;
                        }
                        i += 1; // eat '/'
                    }
                    i += 1;
                }
                return s;
            }
        }
        match byte {
            b' ' | 0x09..=0x0d => {
                s = &s[1..];
                continue;
            }
            b if b <= 0x7f => {}
            _ => {
                let ch = s.chars().next().unwrap();
                if is_whitespace(ch) {
                    s = &s[ch.len_utf8()..];
                    continue;
                }
            }
        }
        return s;
    }
    s
}

fn is_whitespace(ch: char) -> bool {
    // Rust treats left-to-right mark and right-to-left mark as whitespace
    ch.is_whitespace() || ch == '\u{200e}' || ch == '\u{200f}'
}