[nrc]

To be clear, there are not use cases in the std lib. The use case is as an intermediate representation for the compiler, and that is no motivation for inclusion in the language (in the same way that we don't have vtables or register allocation in the surface syntax)

[wtetzner]

Being able to have a generic "Mappable" trait seems like a good use case for the standard library. It's something I've wanted in the past, and was disappointed that it didn't exist.