Разбор ASCII-строк с игнорированием регистра

main.rs

#[allow(dead_code)]
const fn is_ascii_lowercase(s: &str) -> bool {
    let s = s.as_bytes();
    let len = s.len();
    let mut i = 0;
    while i < len {
        if !s[i].is_ascii() || s[i].is_ascii_uppercase() {
            return false;
        }
        i += 1;
    }
    true
}

#[allow(dead_code)]
const fn are_all_ascii_lowercase(ss: &[&str]) -> bool {
    let len = ss.len();
    let mut i = 0;
    while i < len {
        if !is_ascii_lowercase(&ss[i]) {
            return false;
        }
        i += 1;
    }
    true
}

macro_rules! ascii_case_insensitive {
    (match $value:ident {
        $($(|)? $($pattern:literal)|+ $(if $condition:expr)? => $arm:expr,)*
        _ => $catch_all:expr $(,)?
    }) => {{
        #[deny(const_err)]
        const _ARE_ALL_ASCII_LOWERCASE: [(); 1] = [(); are_all_ascii_lowercase(&[$($($pattern,)+)*]) as _];
        #[allow(dead_code)]
        fn non_repeating(s: &str) {
            #[deny(unreachable_patterns)]
            match s {
                $($(| $pattern)+ => (),)*
                _ => (),
            }
        }
        match $value {
            $(x if ($(x.eq_ignore_ascii_case($pattern))||+) $(&& $condition)? => $arm,)*
            _ => $catch_all,
        }
    }}
}

#[derive(Debug)]
enum Example {
    Foo,
    Bar,
    FourtyTwo,
}

impl std::str::FromStr for Example {
    type Err = String;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        Ok(ascii_case_insensitive!(match s {
            "foo" => Self::Foo,
            "bar" if s.as_bytes()[0].is_ascii_lowercase() => Self::Bar,
            "fourtytwo" | "fourty_two" | "42" => Self::FourtyTwo,
            _ => return Err(s.into()),
        }))
    }
}

fn main() {
    let inputs = [
        "foo",
        "Foo",
        "FOO",
        "bar",
        "bAr",
        "BAR",
        "fourtytwo",
        "Fourtytwo",
        "FOURTYTWO",
        "fourty_two",
        "fOuRtY_tWo",
        "42",
        "bogus",
    ];
    for &input in &inputs[..] {
        println!("{:?}", input.parse::<Example>());
    }
}