Fix transmute goal

compiler/rustc_next_trait_solver/src/solve/mod.rs

@@ -295,6 +295,37 @@ where
             Ok(ty)
         }
     }
+
+    /// Normalize a const for when it is structurally matched on, or more likely
+    /// when it needs `.try_to_*` called on it (e.g. to turn it into a usize).
+    ///
+    /// This function is necessary in nearly all cases before matching on a const.
+    /// Not doing so is likely to be incomplete and therefore unsound during
+    /// coherence.
+    #[instrument(level = "trace", skip(self, param_env), ret)]
+    fn structurally_normalize_const(
+        &mut self,
+        param_env: I::ParamEnv,
+        ct: I::Const,
+    ) -> Result<I::Const, NoSolution> {
+        if let ty::ConstKind::Unevaluated(..) = ct.kind() {
+            let normalized_ct = self.next_const_infer();
+            let alias_relate_goal = Goal::new(
+                self.cx(),
+                param_env,
+                ty::PredicateKind::AliasRelate(
+                    ct.into(),
+                    normalized_ct.into(),
+                    ty::AliasRelationDirection::Equate,
+                ),
+            );
+            self.add_goal(GoalSource::Misc, alias_relate_goal);
+            self.try_evaluate_added_goals()?;
+            Ok(self.resolve_vars_if_possible(normalized_ct))
+        } else {
+            Ok(ct)
+        }
+    }
 }
 
 fn response_no_constraints_raw<I: Interner>(

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -627,11 +627,16 @@ where
         }
 
         ecx.probe_builtin_trait_candidate(BuiltinImplSource::Misc).enter(|ecx| {
+            let assume = ecx.structurally_normalize_const(
+                goal.param_env,
+                goal.predicate.trait_ref.args.const_at(2),
+            )?;
+
             let certainty = ecx.is_transmutable(
                 goal.param_env,
                 goal.predicate.trait_ref.args.type_at(0),
                 goal.predicate.trait_ref.args.type_at(1),
-                goal.predicate.trait_ref.args.const_at(2),
+                assume,
             )?;
             ecx.evaluate_added_goals_and_make_canonical_response(certainty)
         })