Make hidden type registration opt-in, so that each site can be reviewed on its own and we have the right defaults for trait solvers

compiler/rustc_trait_selection/src/traits/project.rs

@@ -286,7 +286,12 @@ fn project_and_unify_type<'cx, 'tcx>(
         );
     obligations.extend(new);
 
-    match infcx.at(&obligation.cause, obligation.param_env).eq(normalized, actual) {
+    match infcx
+        .at(&obligation.cause, obligation.param_env)
+        // This is needed to support nested opaque types like `impl Fn() -> impl Trait`
+        .define_opaque_types(true)
+        .eq(normalized, actual)
+    {
         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
             obligations.extend(inferred_obligations);
             ProjectAndUnifyResult::Holds(obligations)