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#![allow(internal_features)]
#![feature(rustc_attrs)]
impl char {
    #[inline]
    #[rustc_allow_incoherent_impl]
    fn my_to_digit(self, radix: u32) -> Option<u32> {
        assert!(
            radix >= 2 && radix <= 36,
            "to_digit: invalid radix -- radix must be in the range 2 to 36 inclusive"
        );
        // check radix to remove letter handling code when radix is a known constant
        let value = if self > '9' && radix > 10 {
            // convert ASCII letters to lowercase
            let lower = self as u32 | 0x20;
            // convert an ASCII letter to the corresponding value,
            // non-letters convert to values > 36
            lower.wrapping_sub('a' as u32) as u64 + 10
        } else {
            // convert digit to value, non-digits wrap to values > 36
            (self as u32).wrapping_sub('0' as u32) as u64
        };
        // FIXME(const-hack): once then_some is const fn, use it here
        if value < radix as u64 {
            Some(value as u32)
        } else {
            None
        }
    }
}

pub fn to_digit_loop(digits: &[u8], radix: u32) -> Option<u64> {
    let mut retval = 0u64;
    for &digit in digits {
        retval = retval.wrapping_mul(radix as _);
        retval = retval.wrapping_add((digit as char).my_to_digit(radix)? as _);
    }
    Some(retval)
}

pub fn to_digit_loop_constant(v: &[u8]) -> Option<u64> {
    to_digit_loop(v, 36)
}