One neat thing here is that the compiler is aware of which types are `Copy` and not internally mutable (not contianing an `UnsafeCell`). For these types, passing `&T` and `T` are equivalent, so the compiler could just choose the faster option.

Even if it's not smart enough to do that today, it could implement this optimization in the future. This could work even without inlining, since the Rust calling convention is unstable, and an optimization based on type size could be incorporated into it.