Protocol std::builtins::Callable

protocol Callable<Args, Ret = ()> { ... }
Args: Tuple

All callable objects (named functions, closures, and function pointers)

Equivalent to a more expressive syntax Fn(Args) -> Ret or Fn(Args) when Ret is ().

Args is a tuple of the argument types of the function and Ret is the return type, for example fn foo(a: i32, b: i64) -> usize would match Callable<(i32, i64), usize>.