dd42f7a0a6
Remove a use of `StructuredDiag`, which is incompatible with automatic error tainting and error translations fixes #127219 I want to remove all of `StructuredDiag`, but it's a bit more involved as it is also used from the `ItemCtxt`, which doesn't support tainting yet.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.