Something could call into FFI. Or whatever. The Rust compiler tries not to talk of function kinds past the function boundary as a design principle^. So while t

Something could call into FFI. Or whatever. The Rust compiler tries not to talk of function kinds past the function boundary as a design principle^. So while the compiler can do a transitive search down all function calls and ensure there's no trickery, Rust doesn't want to do things like that. Inference should not extend past a function. Introducing a `pure` or `const` keyword to be applied on fns would work, that way the pureness of a function can be determined without recursing into its contents. We've had some proposals for the latter, but no full implementations or rfcs yet IIRC. It might happen. ^I think it's a design principle, at least.