Bounds check eliminated only when array length is a constant

I tried this code (godbolt):

#[no_mangle]
pub fn shift_left(arr: &mut [u64], n: usize) {
    let len = arr.len();
    if n >= len {
        arr.fill(0);
    } else {
        let c = len - n;
        for i in 0..c {
            arr[i] = arr[i + n];
        }
        arr[c..].fill(0);
    }
}

#[no_mangle]
pub fn shift_left_4(arr: &mut [u64; 4], n: usize) {
    shift_left(arr, n);
}

I expected to see this happen: no bound checks in both functions

Instead, this happened: bounds check present in the generic shift_left (the arr[i + n])

Meta

rustc --version --verbose:

rustc 1.89.0-nightly (5e16c6620 2025-05-24)
binary: rustc
commit-hash: 5e16c662062fd6dee91f0fe2a1580483488d80cf
commit-date: 2025-05-24
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

<backtrace>

Reproduce the difference between shift_left and shift_left_4 using the linked Godbolt example and the reported rustc nightly version. Compare the generated code for the generic slice and fixed-size array cases, then trace the compiler optimization stage responsible for the remaining arr[i + n] bounds check. Done means the generic function no longer emits that check while preserving the demonstrated behavior.