Erase regions when confirming transmutability candidate

3 files changed, 37 insertions, 35 deletions

diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt.rs
index c29b5b04e00..bb574954587 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt.rs
+++ b/compiler/rustc_trait_selection/src/solve/eval_ctxt.rs
@@ -649,7 +649,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         // FIXME(transmutability): This really should be returning nested goals for `Answer::If*`
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
             ObligationCause::dummy(),
-            ty::Binder::dummy(src_and_dst),
+            src_and_dst,
             scope,
             assume,
         ) {
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index cef982fcb41..0352f0f380d 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -742,7 +742,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         {
                             // Recompute the safe transmute reason and use that for the error reporting
                             self.get_safe_transmute_error_and_reason(
-                                trait_predicate,
                                 obligation.clone(),
                                 trait_ref,
                                 span,
@@ -1629,7 +1628,6 @@ trait InferCtxtPrivExt<'tcx> {
 
     fn get_safe_transmute_error_and_reason(
         &self,
-        trait_predicate: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
         obligation: Obligation<'tcx, ty::Predicate<'tcx>>,
         trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
         span: Span,
@@ -2921,18 +2919,20 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 
     fn get_safe_transmute_error_and_reason(
         &self,
-        trait_predicate: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
         obligation: Obligation<'tcx, ty::Predicate<'tcx>>,
         trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
         span: Span,
     ) -> (String, Option<String>) {
-        let src_and_dst = trait_predicate.map_bound(|p| rustc_transmute::Types {
-            dst: p.trait_ref.substs.type_at(0),
-            src: p.trait_ref.substs.type_at(1),
-        });
-        let scope = trait_ref.skip_binder().substs.type_at(2);
+        // Erase regions because layout code doesn't particularly care about regions.
+        let trait_ref = self.tcx.erase_regions(self.tcx.erase_late_bound_regions(trait_ref));
+
+        let src_and_dst = rustc_transmute::Types {
+            dst: trait_ref.substs.type_at(0),
+            src: trait_ref.substs.type_at(1),
+        };
+        let scope = trait_ref.substs.type_at(2);
         let Some(assume) =
-            rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, trait_ref.skip_binder().substs.const_at(3)) else {
+            rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, trait_ref.substs.const_at(3)) else {
                 span_bug!(span, "Unable to construct rustc_transmute::Assume where it was previously possible");
             };
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
@@ -2942,8 +2942,8 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             assume,
         ) {
             rustc_transmute::Answer::No(reason) => {
-                let dst = trait_ref.skip_binder().substs.type_at(0);
-                let src = trait_ref.skip_binder().substs.type_at(1);
+                let dst = trait_ref.substs.type_at(0);
+                let src = trait_ref.substs.type_at(1);
                 let custom_err_msg = format!(
                     "`{src}` cannot be safely transmuted into `{dst}` in the defining scope of `{scope}`"
                 );
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 5e60bc01bb6..3bba11262f5 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -275,33 +275,35 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
         debug!(?obligation, "confirm_transmutability_candidate");
 
-        let predicate = obligation.predicate;
-
-        let type_at = |i| predicate.map_bound(|p| p.trait_ref.substs.type_at(i));
-        let const_at = |i| predicate.skip_binder().trait_ref.substs.const_at(i);
-
-        let src_and_dst = predicate.map_bound(|p| rustc_transmute::Types {
-            dst: p.trait_ref.substs.type_at(0),
-            src: p.trait_ref.substs.type_at(1),
-        });
-
-        let scope = type_at(2).skip_binder();
-
-        let Some(assume) =
-            rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
-                return Err(Unimplemented);
-            };
-
-        let cause = obligation.cause.clone();
+        // We erase regions here because transmutability calls layout queries,
+        // which does not handle inference regions and doesn't particularly
+        // care about other regions. Erasing late-bound regions is equivalent
+        // to instantiating the binder with placeholders then erasing those
+        // placeholder regions.
+        let predicate =
+            self.tcx().erase_regions(self.tcx().erase_late_bound_regions(obligation.predicate));
+
+        let Some(assume) = rustc_transmute::Assume::from_const(
+            self.infcx.tcx,
+            obligation.param_env,
+            predicate.trait_ref.substs.const_at(3)
+        ) else {
+            return Err(Unimplemented);
+        };
 
         let mut transmute_env = rustc_transmute::TransmuteTypeEnv::new(self.infcx);
-
-        let maybe_transmutable = transmute_env.is_transmutable(cause, src_and_dst, scope, assume);
-
-        use rustc_transmute::Answer;
+        let maybe_transmutable = transmute_env.is_transmutable(
+            obligation.cause.clone(),
+            rustc_transmute::Types {
+                dst: predicate.trait_ref.substs.type_at(0),
+                src: predicate.trait_ref.substs.type_at(1),
+            },
+            predicate.trait_ref.substs.type_at(2),
+            assume,
+        );
 
         match maybe_transmutable {
-            Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
+            rustc_transmute::Answer::Yes => Ok(ImplSourceBuiltinData { nested: vec![] }),
             _ => Err(Unimplemented),
         }
     }