interpret: adjust vtable validity check for higher-ranked types

3 files changed, 14 insertions, 56 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
index e110c155da0..86fdfae1ffb 100644
--- a/compiler/rustc_const_eval/src/interpret/cast.rs
+++ b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -430,10 +430,12 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                     };
                     let erased_trait_ref =
                         ty::ExistentialTraitRef::erase_self_ty(*self.tcx, upcast_trait_ref);
-                    assert!(data_b.principal().is_some_and(|b| self.eq_in_param_env(
-                        erased_trait_ref,
-                        self.tcx.instantiate_bound_regions_with_erased(b)
-                    )));
+                    assert_eq!(
+                        data_b.principal().map(|b| {
+                            self.tcx.normalize_erasing_late_bound_regions(self.typing_env, b)
+                        }),
+                        Some(erased_trait_ref),
+                    );
                 } else {
                     // In this case codegen would keep using the old vtable. We don't want to do
                     // that as it has the wrong trait. The reason codegen can do this is that
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index 95a72d3cbc1..242cf6484dd 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -4,9 +4,6 @@ use either::{Left, Right};
 use rustc_abi::{Align, HasDataLayout, Size, TargetDataLayout};
 use rustc_errors::DiagCtxtHandle;
 use rustc_hir::def_id::DefId;
-use rustc_infer::infer::TyCtxtInferExt;
-use rustc_infer::infer::at::ToTrace;
-use rustc_infer::traits::ObligationCause;
 use rustc_middle::mir::interpret::{ErrorHandled, InvalidMetaKind, ReportedErrorInfo};
 use rustc_middle::query::TyCtxtAt;
 use rustc_middle::ty::layout::{
@@ -17,8 +14,7 @@ use rustc_middle::{mir, span_bug};
 use rustc_session::Limit;
 use rustc_span::Span;
 use rustc_target::callconv::FnAbi;
-use rustc_trait_selection::traits::ObligationCtxt;
-use tracing::{debug, instrument, trace};
+use tracing::{debug, trace};
 
 use super::{
     Frame, FrameInfo, GlobalId, InterpErrorInfo, InterpErrorKind, InterpResult, MPlaceTy, Machine,
@@ -323,40 +319,6 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         }
     }
 
-    /// Check if the two things are equal in the current param_env, using an infcx to get proper
-    /// equality checks.
-    #[instrument(level = "trace", skip(self), ret)]
-    pub(super) fn eq_in_param_env<T>(&self, a: T, b: T) -> bool
-    where
-        T: PartialEq + TypeFoldable<TyCtxt<'tcx>> + ToTrace<'tcx>,
-    {
-        // Fast path: compare directly.
-        if a == b {
-            return true;
-        }
-        // Slow path: spin up an inference context to check if these traits are sufficiently equal.
-        let (infcx, param_env) = self.tcx.infer_ctxt().build_with_typing_env(self.typing_env);
-        let ocx = ObligationCtxt::new(&infcx);
-        let cause = ObligationCause::dummy_with_span(self.cur_span());
-        // equate the two trait refs after normalization
-        let a = ocx.normalize(&cause, param_env, a);
-        let b = ocx.normalize(&cause, param_env, b);
-
-        if let Err(terr) = ocx.eq(&cause, param_env, a, b) {
-            trace!(?terr);
-            return false;
-        }
-
-        let errors = ocx.select_all_or_error();
-        if !errors.is_empty() {
-            trace!(?errors);
-            return false;
-        }
-
-        // All good.
-        true
-    }
-
     /// Walks up the callstack from the intrinsic's callsite, searching for the first callsite in a
     /// frame which is not `#[track_caller]`. This matches the `caller_location` intrinsic,
     /// and is primarily intended for the panic machinery.
diff --git a/compiler/rustc_const_eval/src/interpret/traits.rs b/compiler/rustc_const_eval/src/interpret/traits.rs
index 4cfaacebfcd..a5029eea5a7 100644
--- a/compiler/rustc_const_eval/src/interpret/traits.rs
+++ b/compiler/rustc_const_eval/src/interpret/traits.rs
@@ -86,21 +86,15 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             throw_ub!(InvalidVTableTrait { vtable_dyn_type, expected_dyn_type });
         }
 
+        // This checks whether there is a subtyping relation between the predicates in either direction.
+        // For example:
+        // - casting between `dyn for<'a> Trait<fn(&'a u8)>` and `dyn Trait<fn(&'static u8)>` is OK
+        // - casting between `dyn Trait<for<'a> fn(&'a u8)>` and either of the above is UB
         for (a_pred, b_pred) in std::iter::zip(sorted_vtable, sorted_expected) {
-            let is_eq = match (a_pred.skip_binder(), b_pred.skip_binder()) {
-                (
-                    ty::ExistentialPredicate::Trait(a_data),
-                    ty::ExistentialPredicate::Trait(b_data),
-                ) => self.eq_in_param_env(a_pred.rebind(a_data), b_pred.rebind(b_data)),
+            let a_pred = self.tcx.normalize_erasing_late_bound_regions(self.typing_env, a_pred);
+            let b_pred = self.tcx.normalize_erasing_late_bound_regions(self.typing_env, b_pred);
 
-                (
-                    ty::ExistentialPredicate::Projection(a_data),
-                    ty::ExistentialPredicate::Projection(b_data),
-                ) => self.eq_in_param_env(a_pred.rebind(a_data), b_pred.rebind(b_data)),
-
-                _ => false,
-            };
-            if !is_eq {
+            if a_pred != b_pred {
                 throw_ub!(InvalidVTableTrait { vtable_dyn_type, expected_dyn_type });
             }
         }