Pure functions are functions where the return value only depends on the function arguments. If the function arguments are not known at compile time, obviously you can't evaluate it at compile time. It would only be possible to do that when all the arguments are also known at compile time (constants).
But a const fn can also do that: if given non-constant parameters it will be evaluated at run-time. So I (having never used Rust before) still haven't see the distinction between pure and const. What's an example of a function that is pure but cannot be evaluated at compile time with constant parameters?
The parent didn't specify calling with constant parameters, which makes a huge difference. To answer your question, basically anything the compiler doesn't know how to evaluate - which has been expanded in this release, but does not include everything still.
Well, that is not the definition. Output of a pure functions can depend on input arguments, and those arguments can definitely depend on runtime properties.
