I wish they'd break their current backwards compat. promises to fix some accidental complexity that has been introduced. Let legacy langs provide this kind of maintainability
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.