rustc fails to detect variable bounds

[dragostis]

This is probably an LLVM issue, but I decided to start my journey here. rustc/LLVM seems to be unable to detect bounds of a variable inside of this simple loop:

pub fn walk(slice: &mut [u64]) -> bool {
    let mut cursor = 0;

    for i in 0..slice.len() {
        cursor += (slice[i] != 0) as usize;
    }
    
    cursor <= slice.len()
}

The code generated for the function above is not a simple constant return, but it actually processes the whole slice.

The reason why this is an interesting use case is because you cannot seem to write a fully optimized compact function without unsafe code:

pub fn compact_slow(slice: &mut [u64]) {
    let mut cursor = 0;

    for i in 0..slice.len() {
        let val = slice[i];

        slice[cursor] = val;

        cursor += (val != 0) as usize;
    }
}

godbolt (with bounds check)

pub fn compact_fast(slice: &mut [u64]) {
    let mut cursor = 0;

    for i in 0..slice.len() {
        let val = unsafe {
            let val = *slice.get_unchecked(i);

            *slice.get_unchecked_mut(cursor) = val;

            val
        };

        cursor += (val != 0) as usize;
    }
}

godbolt (without bounds check)