Redundant length check needed to optimize out panicking bounds check in loop

[okaneco]

While working on #147436, I noticed the following code has a panicking bounds check in the loop body that is removed when both slice lengths received from as_chunks are checked.

The slices are checked to be equal length at the start of the function, so the number of chunks and loop iterations is known to be equal.

I tried this code, which manually zips two instances of as_chunks array slices together with a while-loop:

#[inline(never)]
pub const fn all_less_than(s: &[u8], other: &[u8]) -> bool {
    if s.len() != other.len() {
        return false;
    }

    const N: usize = 16;
    let (a, _) = s.as_chunks::<N>();
    let (b, _) = other.as_chunks::<N>();

    let mut i = 0;
    while i < a.len() {
        let mut less_than = true;
        let mut j = 0;
        while j < N {
            less_than &= a[i][j] < b[i][j];
            j += 1;
        }

        if !less_than {
            return false;
        }

        i += 1;
    }

    true
}

I expected to not need to check the length for b since it's known to be equal.

When I change the loop condition from while i < a.len() to while i < a.len() && i < b.len(), the panic branch is removed.
https://rust.godbolt.org/z/azrbYq4Gv

while i < a.len()	while i < a.len() && i < b.len()
.LBB0_4: sub r8, 1 jb .LBB0_8 movdqu xmm0, xmmword ptr [rdi + rcx] movdqu xmm1, xmmword ptr [rdx + rcx] pmaxub xmm1, xmm0 pcmpeqb xmm1, xmm0 pmovmskb eax, xmm1 test eax, eax sete al jne .LBB0_7 add rcx, 16 cmp r9, 1 mov r9, r8 jne .LBB0_4	.LBB0_4: movdqu xmm0, xmmword ptr [rdi + rcx] movdqu xmm1, xmmword ptr [rdx + rcx] pmaxub xmm1, xmm0 pcmpeqb xmm1, xmm0 pmovmskb eax, xmm1 add rsi, -1 setb r8b test eax, eax sete al jne .LBB0_6 add rcx, 16 test r8b, r8b jne .LBB0_4

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