Auto merge of #43600 - scalexm:issue-35976, r=nikomatsakis

Add a more precise error message for issue #35976

When trying to perform static dispatch on something which derefs to a trait object, and the target trait is not in scope, we had confusing error messages if the target method had a `Self: Sized` bound. We add a more precise error message in this case: "consider using trait ...".

Fixes #35976.

r? @nikomatsakis