`FIXME(-Znext-solver)` triage

Co-authored-by: Michael Goulet <michael@errs.io>

2 files changed, 13 insertions, 9 deletions

diff --git a/compiler/rustc_hir_analysis/src/autoderef.rs b/compiler/rustc_hir_analysis/src/autoderef.rs
index 99e495d9266..c88c534e135 100644
--- a/compiler/rustc_hir_analysis/src/autoderef.rs
+++ b/compiler/rustc_hir_analysis/src/autoderef.rs
@@ -160,7 +160,7 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
             self.param_env,
             ty::Binder::dummy(trait_ref),
         );
-        if !self.infcx.predicate_may_hold(&obligation) {
+        if !self.infcx.next_trait_solver() && !self.infcx.predicate_may_hold(&obligation) {
             debug!("overloaded_deref_ty: cannot match obligation");
             return None;
         }
@@ -184,17 +184,17 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
             self.param_env,
             ty,
         ) else {
-            // We shouldn't have errors here, except for evaluate/fulfill mismatches,
-            // but that's not a reason for an ICE (`predicate_may_hold` is conservative
-            // by design).
-            // FIXME(-Znext-solver): This *actually* shouldn't happen then.
+            // We shouldn't have errors here in the old solver, except for
+            // evaluate/fulfill mismatches, but that's not a reason for an ICE.
             return None;
         };
         let errors = ocx.select_where_possible();
         if !errors.is_empty() {
-            // This shouldn't happen, except for evaluate/fulfill mismatches,
-            // but that's not a reason for an ICE (`predicate_may_hold` is conservative
-            // by design).
+            if self.infcx.next_trait_solver() {
+                unreachable!();
+            }
+            // We shouldn't have errors here in the old solver, except for
+            // evaluate/fulfill mismatches, but that's not a reason for an ICE.
             debug!(?errors, "encountered errors while fulfilling");
             return None;
         }
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
index 2a37a8bdbd4..b22590e0a91 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -805,7 +805,11 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     ty::ClauseKind::Trait(ty::TraitPredicate { trait_ref, polarity })
                 });
                 let bound = (bound.upcast(tcx), span);
-                // FIXME(-Znext-solver): We can likely remove this hack once the new trait solver lands.
+                // FIXME(-Znext-solver): We can likely remove this hack once the
+                // new trait solver lands. This fixed an overflow in the old solver.
+                // This may have performance implications, so please check perf when
+                // removing it.
+                // This was added in <https://github.com/rust-lang/rust/pull/123302>.
                 if tcx.is_lang_item(trait_def_id, rustc_hir::LangItem::Sized) {
                     bounds.insert(0, bound);
                 } else {