da98ef9a5d
As a workaround for the full `#[refine]` semantics not being implemented yet, forbit returning a concrete future type like `Box<dyn Future>` or a manually implemented Future. `-> impl Future` is still permitted; while that can also cause accidental refinement, that's behind a different feature gate (`return_position_impl_trait_in_trait`) and that problem exists regardless of whether the trait method is async, so will have to be solved more generally. Fixes #102745
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.