Ah, I see where the misunderstanding is. You can make it so you change only the function signature and behaviour or you can make it so you have to also change the function call site.

Ever since https://github.com/rust-lang/rust/pull/34828 you can transform any `f` that takes a `T` into an `f` that takes an `Option<T>` without any of the call sites changing.

For instance, look at this playground https://play.rust-lang.org/?version=stable&mode=debug&editio...

Your function `get_string_raw` which just handles `i32` can be transformed into a function `get_string` which handles `Option<i32>` without the thing calling changing how it calls the function. And the new `get_string` can accept `Some(i32)` or `None` or just `i32`.

Of course, this is slightly broad for brevity: you can now pass in anything that can become an `Option<i32>` but you can just define a trait to restrict that if you wanted.

You can get that sort of covariant effort that you wanted.