Deny RPITIT for object safety

compiler/rustc_middle/src/traits/mod.rs

@@ -908,6 +908,12 @@ impl ObjectSafetyViolation {
             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfOutput, _) => {
                 format!("method `{}` references the `Self` type in its return type", name).into()
             }
+            ObjectSafetyViolation::Method(
+                name,
+                MethodViolationCode::ReferencesImplTraitInTrait,
+                _,
+            ) => format!("method `{}` references an `impl Trait` type in its return type", name)
+                .into(),
             ObjectSafetyViolation::Method(
                 name,
                 MethodViolationCode::WhereClauseReferencesSelf,
@@ -1014,6 +1020,9 @@ pub enum MethodViolationCode {
     /// e.g., `fn foo(&self) -> Self`
     ReferencesSelfOutput,
 
+    /// e.g., `fn foo(&self) -> impl Sized`
+    ReferencesImplTraitInTrait,
+
     /// e.g., `fn foo(&self) where Self: Clone`
     WhereClauseReferencesSelf,