In Rust to_lowercase returns a String, which is an owned struct with heap allocated data, so it is reasonably clear that it allocates. In contrast, trim_start for example returns a reference to a string slice with the same lifetime as the source string, so again it is reasonably obvious from the type signature what's going on.

The mistake here really is trying to do case insensitive string comparison by converting both arguments to the same case, then comparing them with case-sensitivity. It is much better to compare them directly, but case-insensitively.