Counter in slice iter fails to eliminate bounds check

[clubby789]

The following code (somewhat extracted from #126425) includes an unreachable bounds check panic

pub fn getnonzero(buf: &[u8; 128]) -> &[u8] {
    let mut curr = 0;
    for n in buf.iter() {
        if *n == 0 { break; }
        curr += 1;
    }
    &buf[..curr]
}

https://godbolt.org/z/vzooGexnj

This bounds check can be eliminated: Alive2

This also occurs with reverse iteration (like in the original) and iterating 0..buf.len() instead of direct slice iteration. Lowering the slice length to the point where the loop is fully unrolled does eliminate this, but the exact number depends on the specific code.

[clubby789]

Another similar example: Godbolt
With N < 75, the assert is optimized out