Revert "Rollup merge of #97346 - JohnTitor:remove-back-compat-hacks, r=oli-obk"

This reverts commit c703d11dcc, reversing
changes made to 64eb9ab869.

compiler/rustc_trait_selection/src/traits/project.rs

@@ -252,10 +252,20 @@ fn project_and_unify_type<'cx, 'tcx>(
         Err(InProgress) => return ProjectAndUnifyResult::Recursive,
     };
     debug!(?normalized, ?obligations, "project_and_unify_type result");
-    match infcx
-        .at(&obligation.cause, obligation.param_env)
-        .eq(normalized, obligation.predicate.term)
-    {
+    let actual = obligation.predicate.term;
+    // For an example where this is neccessary see src/test/ui/impl-trait/nested-return-type2.rs
+    // This allows users to omit re-mentioning all bounds on an associated type and just use an
+    // `impl Trait` for the assoc type to add more bounds.
+    let InferOk { value: actual, obligations: new } =
+        selcx.infcx().replace_opaque_types_with_inference_vars(
+            actual,
+            obligation.cause.body_id,
+            obligation.cause.span,
+            obligation.param_env,
+        );
+    obligations.extend(new);
+
+    match infcx.at(&obligation.cause, obligation.param_env).eq(normalized, actual) {
         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
             obligations.extend(inferred_obligations);
             ProjectAndUnifyResult::Holds(obligations)