In this code:

fn binary_search_repro(needle: &str) -> Option<&str> { let haystack = &["a", "b"]; haystack.binary_search(&needle).ok().map(|i| haystack[i]) }

the compiler should be able to use the hint from slice::binary_search_by in order to elide the length check in the index operation (haystack[i]).

Seems the knowledge from this assert_unchecked is lost somehow:

Godbolt: https://rust.godbolt.org/z/8zqq8vrrv