Fix transmute goal

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -890,25 +890,6 @@ where
         self.infcx.well_formed_goals(param_env, arg)
     }
 
-    /*
-    pub(super) fn is_transmutable(
-        &self,
-        src_and_dst: rustc_transmute::Types<I>,
-        assume: rustc_transmute::Assume,
-    ) -> Result<Certainty, NoSolution> {
-        use rustc_transmute::Answer;
-        // FIXME(transmutability): This really should be returning nested goals for `Answer::If*`
-        match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
-            ObligationCause::dummy(),
-            src_and_dst,
-            assume,
-        ) {
-            Answer::Yes => Ok(Certainty::Yes),
-            Answer::No(_) | Answer::If(_) => Err(NoSolution),
-        }
-    }
-    */
-
     pub(super) fn trait_ref_is_knowable(
         &mut self,
         param_env: I::ParamEnv,
@@ -1016,6 +997,16 @@ where
     ) -> Option<I::Const> {
         self.infcx.try_const_eval_resolve(param_env, unevaluated)
     }
+
+    pub(super) fn is_transmutable(
+        &mut self,
+        param_env: I::ParamEnv,
+        dst: I::Ty,
+        src: I::Ty,
+        assume: I::Const,
+    ) -> Result<Certainty, NoSolution> {
+        self.infcx.is_transmutable(param_env, dst, src, assume)
+    }
 }
 
 /// Eagerly replace aliases with inference variables, emitting `AliasRelate`

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -601,12 +601,10 @@ where
     }
 
     fn consider_builtin_transmute_candidate(
-        _ecx: &mut EvalCtxt<'_, Infcx>,
-        _goal: Goal<I, Self>,
+        ecx: &mut EvalCtxt<'_, Infcx>,
+        goal: Goal<I, Self>,
     ) -> Result<Candidate<I>, NoSolution> {
-        // TODO:
-        todo!()
-        /* if goal.predicate.polarity != ty::PredicatePolarity::Positive {
+        if goal.predicate.polarity != ty::PredicatePolarity::Positive {
             return Err(NoSolution);
         }
 
@@ -615,26 +613,17 @@ where
             return Err(NoSolution);
         }
 
-        // Erase regions because we compute layouts in `rustc_transmute`,
-        // which will ICE for region vars.
-        let args = ecx.interner().erase_regions(goal.predicate.trait_ref.args);
-
-        let Some(assume) =
-            rustc_transmute::Assume::from_const(ecx.interner(), goal.param_env, args.const_at(2))
-        else {
-            return Err(NoSolution);
-        };
-
         // FIXME: This actually should destructure the `Result` we get from transmutability and
         // register candiates. We probably need to register >1 since we may have an OR of ANDs.
         ecx.probe_builtin_trait_candidate(BuiltinImplSource::Misc).enter(|ecx| {
             let certainty = ecx.is_transmutable(
-                rustc_transmute::Types { dst: args.type_at(0), src: args.type_at(1) },
-                assume,
+                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),
             )?;
             ecx.evaluate_added_goals_and_make_canonical_response(certainty)
         })
-        */
     }
 
     /// ```ignore (builtin impl example)