If the array length is known to be strictly less than 255 then there is definitely an out-of-bounds access inside the loop, but since this is a panic rather than undefined behavior it could matter how many loop iterations are executed before the out of bounds access occurs, so the check can't be omitted.

If the array size is definitely greater than or equal to 255 then all the array accesses in the loop will be in bounds and no further bounds check is required.

But the loop goes up to 255. So if len(arr) == 10, then assert!(len(arr) <= 255) woulds succeed, but you'd get an out-of-bounds access if you tried to access arr at 11.

len(arr) == 255 should be okay, 0..255 doesn't include the end index[0]. In the Rust playground[1] I see it only print up to 254.

[0] https://doc.rust-lang.org/stable/rust-by-example/flow_contro... [1] https://play.rust-lang.org/?version=stable&mode=debug&editio...