Rust has compile time environment variables and const fns can parse them. It's one very nice and easy way to experiment with or configure rust code at compile time, and should be explored more.
The functions are pure but they take an input from a built-in (looks like a macro) that reads the environment variable at compile time.
Also, you only compile once, so how could you tell the difference? You could say - if it was using const fn that it's a "templated" function that depends on compile time settings.
There's two lookups that can occur, distinguishing them makes it clearer.
Looking up the value of an environment variable at runtime is not a const operation, and produces an error if you try to do it in a const fn.
Looking up the value of an environment variable during compile time _can_ be done in a const context, but it'll only happen once. The environment should be considered an input to a const fn, and that makes it "pure".
EDIT: These two operations can both be done in non-const functions too, they're different functions (well, one's a macro).
