Can't you just provide a default implementation of the method on the trait itself? Unless there are more requirements I'm not parsing from what you wrote, that seems to do exactly what they wanted.
Yeah, I'm not sure I fully understand as well. The only thing I can think of is that the trait was supposed to be from outside the crate (i.e. not modifyable), but then then, I'm not sure why writing a function that was generic over T: ThatTrait that took an instance as a parameter (or even `impl ThatTrait`) wouldn't work