The parameter `In` of `call_inner` is completely unconstrained by its
arguments and return type. We are only able to infer it by assuming that
the only associated type equal to `In::Param<'_>` is `In::Param<'_>`
itself. It could just as well be some other associated type which only
normalizes to `In::Param<'_>`. This will change with the next-generation
trait solver and was encountered by a crater run
https://github.com/rust-lang/rust/pull/133502-
cc
https://github.com/rust-lang/trait-system-refactor-initiative/issues/168
I couldn't think of a cleaner alternative here. I first tried to just
provide `In` as an explicit type parameter. This is also kinda ugly as I
need to provide a variable number of them and `${ignore(..)}` is
currently still unstable https://github.com/rust-lang/rust/issues/83527.
Sorry for the inconvenience. Also fun that this function exists to avoid
a separate solver bug in the first place 😅
2a2ce6e0ba