It's not worth it.

For example, it seems as though "Word".contains(char::is_ascii_lowercase) should work, because after all "Word".contains(char::is_lowercase) works and char::is_ascii_lowercase exists and does what you expect, the same but for ASCII only.

However, for compatibility reasons char::is_lowercase takes a char, while char::is_ascii_lowercase takes a reference to a char, and so you can't just pass it to contains() and will need to write a closure to pass to contains to do ASCII instead.

Writing the closure is ugly, but it's nowhere close to how awful a compatibility break would be so even a million things like this (and so far maybe I can think of a dozen) aren't worth it.

They have the Editions mechanism for unavoidable breaking changes, but their target market is people who care a whole lot about stability and maintainability, so it wouldn't make any sense for them to jettison that design goal.

Unfortunately the Editions system (as nice as it is) only works for the language, not for the standard or core libraries. It's possible to create such a system (similar to glibc symbol versioning), but AFIAK Rust doesn't currently have one.

It's a systems language so they're not gonna break BC. Although if there's a really great feature that comes out that requires BC breaking, then they should do so.