diff --git a/compiler/rustc_trait_selection/src/solve/assembly.rs b/compiler/rustc_trait_selection/src/solve/assembly.rs
index 1c4e498c57e..5690b6536bb 100644
--- a/compiler/rustc_trait_selection/src/solve/assembly.rs
+++ b/compiler/rustc_trait_selection/src/solve/assembly.rs
@@ -184,7 +184,7 @@ pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy + Eq {
 
     // `dyn Trait1` can be unsized to `dyn Trait2` if they are the same trait, or
     // if `Trait2` is a (transitive) supertrait of `Trait2`.
-    fn consider_builtin_dyn_unsize_candidates(
+    fn consider_builtin_dyn_upcast_candidates(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> Vec<CanonicalResponse<'tcx>>;
@@ -334,7 +334,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         // There may be multiple unsize candidates for a trait with several supertraits:
         // `trait Foo: Bar<A> + Bar<B>` and `dyn Foo: Unsize<dyn Bar<_>>`
         if lang_items.unsize_trait() == Some(trait_def_id) {
-            for result in G::consider_builtin_dyn_unsize_candidates(self, goal) {
+            for result in G::consider_builtin_dyn_upcast_candidates(self, goal) {
                 candidates.push(Candidate { source: CandidateSource::BuiltinImpl, result });
             }
         }
diff --git a/compiler/rustc_trait_selection/src/solve/project_goals.rs b/compiler/rustc_trait_selection/src/solve/project_goals.rs
index f9506446aba..879f18843c9 100644
--- a/compiler/rustc_trait_selection/src/solve/project_goals.rs
+++ b/compiler/rustc_trait_selection/src/solve/project_goals.rs
@@ -562,7 +562,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
         bug!("`Unsize` does not have an associated type: {:?}", goal);
     }
 
-    fn consider_builtin_dyn_unsize_candidates(
+    fn consider_builtin_dyn_upcast_candidates(
         _ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> Vec<super::CanonicalResponse<'tcx>> {
diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
index c79f1f05512..29ee9da38e0 100644
--- a/compiler/rustc_trait_selection/src/solve/trait_goals.rs
+++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
@@ -262,11 +262,9 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                 // `T` -> `dyn Trait` unsizing
                 (_, &ty::Dynamic(data, region, ty::Dyn)) => {
                     // Can only unsize to an object-safe type
-                    // FIXME: Can auto traits be *not* object safe?
                     if data
-                        .auto_traits()
-                        .chain(data.principal_def_id())
-                        .any(|def_id| !tcx.is_object_safe(def_id))
+                        .principal_def_id()
+                        .map_or(false, |def_id| !tcx.check_is_object_safe(def_id))
                     {
                         return Err(NoSolution);
                     }
@@ -365,7 +363,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         })
     }
 
-    fn consider_builtin_dyn_unsize_candidates(
+    fn consider_builtin_dyn_upcast_candidates(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> Vec<CanonicalResponse<'tcx>> {
@@ -387,9 +385,8 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             return vec![];
         }
 
-        let mut responses = vec![];
         let mut unsize_dyn_to_principal = |principal: Option<ty::PolyExistentialTraitRef<'tcx>>| {
-            let _ = ecx.infcx.probe(|_| -> Result<(), NoSolution> {
+            ecx.infcx.probe(|_| -> Result<_, NoSolution> {
                 // Require that all of the trait predicates from A match B, except for
                 // the auto traits. We do this by constructing a new A type with B's
                 // auto traits, and equating these types.
@@ -414,16 +411,17 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                     goal.with(tcx, ty::Binder::dummy(ty::OutlivesPredicate(a_region, b_region))),
                 );
 
-                responses.push(ecx.evaluate_all_and_make_canonical_response(nested_obligations)?);
-
-                Ok(())
-            });
+                ecx.evaluate_all_and_make_canonical_response(nested_obligations)
+            })
         };
 
+        let mut responses = vec![];
         // If the principal def ids match (or are both none), then we're not doing
         // trait upcasting. We're just removing auto traits (or shortening the lifetime).
         if a_data.principal_def_id() == b_data.principal_def_id() {
-            unsize_dyn_to_principal(a_data.principal());
+            if let Ok(response) = unsize_dyn_to_principal(a_data.principal()) {
+                responses.push(response);
+            }
         } else if let Some(a_principal) = a_data.principal()
             && let Some(b_principal) = b_data.principal()
         {
@@ -433,7 +431,9 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                 }
                 let erased_trait_ref = super_trait_ref
                     .map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
-                unsize_dyn_to_principal(Some(erased_trait_ref));
+                if let Ok(response) = unsize_dyn_to_principal(Some(erased_trait_ref)) {
+                    responses.push(response);
+                }
             }
         }
 
diff --git a/compiler/rustc_ty_utils/src/ty.rs b/compiler/rustc_ty_utils/src/ty.rs
index b5005c1d8d8..41e837e8b75 100644
--- a/compiler/rustc_ty_utils/src/ty.rs
+++ b/compiler/rustc_ty_utils/src/ty.rs
@@ -426,10 +426,6 @@ fn unsizing_params_for_adt<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> BitSet<u32
         },
     };
 
-    // FIXME(eddyb) cache this (including computing `unsizing_params`)
-    // by putting it in a query; it would only need the `DefId` as it
-    // looks at declared field types, not anything substituted.
-
     // The last field of the structure has to exist and contain type/const parameters.
     let Some((tail_field, prefix_fields)) =
         def.non_enum_variant().fields.split_last() else