RFC 2027: "first draft" of implementation

These are a squashed series of commits.

src/test/ui/issues/issue-19538.stderr

@@ -17,6 +17,7 @@ LL |     let test: &mut dyn Bar = &mut thing;
    |                              ^^^^^^^^^^ the trait `Bar` cannot be made into an object
    |
    = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&mut dyn Bar>` for `&mut Thing`
+   = note: required by cast to type `&mut dyn Bar`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-20692.stderr

@@ -14,6 +14,7 @@ LL |     let _ = x
    |
    = note: the trait cannot require that `Self : Sized`
    = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&dyn Array>` for `&T`
+   = note: required by cast to type `&dyn Array`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-38604.stderr

@@ -14,6 +14,7 @@ LL |         Box::new(());
    |
    = note: the trait cannot use `Self` as a type parameter in the supertraits or where-clauses
    = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<std::boxed::Box<dyn Foo>>` for `std::boxed::Box<()>`
+   = note: required by cast to type `std::boxed::Box<dyn Foo>`
 
 error: aborting due to 2 previous errors