Digits10 at compiletime

const_digits10.rs

#![feature(const_generics)]

struct Range {
    start: usize,
    end: usize,
}

impl Range {
    pub const fn new(start: usize, end: usize) -> Self {
        Self { start, end }
    }

    pub const fn contains(&self, other: usize) -> bool {
        (other >= self.start) & (other < self.end)
    }
}

struct Digits10<const N: usize>;

impl <const N: usize> Digits10<{N}> {
    pub const fn digits10() -> usize {
        let _1 = (Range::new(0, 10).contains(N) as u8) << 0;
        let _2 = (Range::new(10, 100).contains(N) as u8) << 1 ;
        let _3 = (Range::new(100, 1000).contains(N) as u8) << 2;
        let _4 = (Range::new(1000, 10000).contains(N) as u8) << 3;

        [1, 2, 3, 4][((_1 + _2 + _3 + _4) as usize).trailing_zeros() as usize]
        
    }
}


pub fn blah() {
    assert_eq!(Digits10::<100>::digits10(), 3);
}