The compiler would already evaluate most of this stuff at compile time, you just weren't allowed to rely on it until now.

e.g. char::is_ascii_alphabetic() was made const in 1.47, but 1.46 would already run it at compile time: https://godbolt.org/z/hWf65f

The difference is in 1.47 you can use the result to assign something that must be known at compile time.