review

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -2192,6 +2192,15 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         &self,
         trait_predicate: Option<ty::Predicate<'tcx>>,
     ) -> bool {
+        // This function is what hacky and doesn't perfectly do what we want it to.
+        // It's not soundness critical and we should be able to freely improve this
+        // in the future.
+        //
+        // Some concrete edge cases include the fact that `goal_may_hold_opaque_types_jank`
+        // also fails if there are any constraints opaques which are never used as a self
+        // type. We also allow where-bounds which are currently ambiguous but end up
+        // constraining an opaque later on.
+
         // Check whether the trait candidate would not be applicable if the
         // opaque type were rigid.
         if let Some(predicate) = trait_predicate {

compiler/rustc_infer/src/infer/canonical/query_response.rs

@@ -87,14 +87,11 @@ impl<'tcx> InferCtxt<'tcx> {
     {
         // While we ignore region constraints and pending obligations,
         // we do return constrained opaque types to avoid unconstrained
-        // inference variables in the response. This is still slightly
-        // insufficient as ambiguous `Projection` obligations have the
-        // same issue.
+        // inference variables in the response. This is important as we want
+        // to check that opaques in deref steps stay unconstrained.
         //
-        // FIXME(-Znext-solver): We could alternatively extend the `var_values`
-        // each time we call `make_query_response_ignoring_pending_obligations`
-        // and equate inference variables created inside of the query this way.
-        // This is what we do for `CanonicalState` and is probably a bit nicer.
+        // This doesn't handle the more general case for non-opaques as
+        // ambiguous `Projection` obligations have same the issue.
         let opaque_types = if self.next_trait_solver() {
             self.inner
                 .borrow_mut()