[DougBTX]

There's a principle in Rust that only the function signature is needed to know if a function call is valid, without any need to look at the function body.

It makes things much easier to think about: at a minimum, if you change the body of a function it doesn't affect whether other code compiles or not. It's also one of the big differences between generic parameters (all in the function signature) and template metaprogramming (templates can do duck typing).