Comment by yoshuaw

Return Type Notation (RTN) I believe is the big one blocked by the new solver. This is the primitive that makes it possible to add bounds to futures returned by async functions. This should look something like: `where <T as Trait>::method(..): Send`.

This is one of two remaining features needed to make Async Functions in Traits (AFITs) practical. Dyn dispatch support being the other missing feature, though that isn't blocked by the new solver.