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.
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.