Rustc may still need to do runtime bounds check if it can’t conclude that a dynamically computed index is in-bounds.
The length is known statically, but whether the index is in-bounds may not be.
The binary search GP talks about is exactly one such case, go on Godbolt, write up a simple binary search (with a static size so you get code), and you’ll see that the compiler checks against the literal array size on every iteration.
