merge builtin unsize candidates again

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -253,17 +253,6 @@ fn consider_structural_builtin_unsize_candidates(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> Vec<(CanonicalResponse<'tcx>, BuiltinImplSource)>;
-
-    /// Consider the `Unsize` candidate corresponding to coercing a sized type
-    /// into a `dyn Trait`.
-    ///
-    /// This is computed separately from the rest of the `Unsize` candidates
-    /// since it is only done once per self type, and not once per
-    /// *normalization step* (in `assemble_candidates_via_self_ty`).
-    fn consider_unsize_to_dyn_candidate(
-        ecx: &mut EvalCtxt<'_, 'tcx>,
-        goal: Goal<'tcx, Self>,
-    ) -> QueryResult<'tcx>;
 }
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
@@ -306,8 +295,6 @@ pub(super) fn assemble_and_evaluate_candidates<G: GoalKind<'tcx>>(
 
         self.assemble_object_bound_candidates(goal, &mut candidates);
 
-        self.assemble_unsize_to_dyn_candidate(goal, &mut candidates);
-
         self.assemble_blanket_impl_candidates(goal, &mut candidates);
 
         self.assemble_param_env_candidates(goal, &mut candidates);
@@ -422,24 +409,6 @@ fn assemble_non_blanket_impl_candidates<G: GoalKind<'tcx>>(
         }
     }
 
-    #[instrument(level = "debug", skip_all)]
-    fn assemble_unsize_to_dyn_candidate<G: GoalKind<'tcx>>(
-        &mut self,
-        goal: Goal<'tcx, G>,
-        candidates: &mut Vec<Candidate<'tcx>>,
-    ) {
-        let tcx = self.tcx();
-        if tcx.lang_items().unsize_trait() == Some(goal.predicate.trait_def_id(tcx)) {
-            match G::consider_unsize_to_dyn_candidate(self, goal) {
-                Ok(result) => candidates.push(Candidate {
-                    source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
-                    result,
-                }),
-                Err(NoSolution) => (),
-            }
-        }
-    }
-
     #[instrument(level = "debug", skip_all)]
     fn assemble_blanket_impl_candidates<G: GoalKind<'tcx>>(
         &mut self,

compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs

@@ -603,13 +603,6 @@ fn consider_builtin_coroutine_candidate(
         )
     }
 
-    fn consider_unsize_to_dyn_candidate(
-        _ecx: &mut EvalCtxt<'_, 'tcx>,
-        goal: Goal<'tcx, Self>,
-    ) -> QueryResult<'tcx> {
-        bug!("`Unsize` does not have an associated type: {:?}", goal)
-    }
-
     fn consider_structural_builtin_unsize_candidates(
         _ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -490,53 +490,6 @@ fn consider_builtin_transmute_candidate(
         ecx.evaluate_added_goals_and_make_canonical_response(certainty)
     }
 
-    fn consider_unsize_to_dyn_candidate(
-        ecx: &mut EvalCtxt<'_, 'tcx>,
-        goal: Goal<'tcx, Self>,
-    ) -> QueryResult<'tcx> {
-        ecx.probe(|_| ProbeKind::UnsizeAssembly).enter(|ecx| {
-            let a_ty = goal.predicate.self_ty();
-            // We need to normalize the b_ty since it's destructured as a `dyn Trait`.
-            let Some(b_ty) =
-                ecx.try_normalize_ty(goal.param_env, goal.predicate.trait_ref.args.type_at(1))
-            else {
-                return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::OVERFLOW);
-            };
-
-            let ty::Dynamic(b_data, b_region, ty::Dyn) = *b_ty.kind() else {
-                return Err(NoSolution);
-            };
-
-            let tcx = ecx.tcx();
-
-            // Can only unsize to an object-safe trait.
-            if b_data.principal_def_id().is_some_and(|def_id| !tcx.check_is_object_safe(def_id)) {
-                return Err(NoSolution);
-            }
-
-            // Check that the type implements all of the predicates of the trait object.
-            // (i.e. the principal, all of the associated types match, and any auto traits)
-            ecx.add_goals(
-                GoalSource::ImplWhereBound,
-                b_data.iter().map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty))),
-            );
-
-            // The type must be `Sized` to be unsized.
-            if let Some(sized_def_id) = tcx.lang_items().sized_trait() {
-                ecx.add_goal(
-                    GoalSource::ImplWhereBound,
-                    goal.with(tcx, ty::TraitRef::new(tcx, sized_def_id, [a_ty])),
-                );
-            } else {
-                return Err(NoSolution);
-            }
-
-            // The type must outlive the lifetime of the `dyn` we're unsizing into.
-            ecx.add_goal(GoalSource::Misc, goal.with(tcx, ty::OutlivesPredicate(a_ty, b_region)));
-            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-        })
-    }
-
     /// ```ignore (builtin impl example)
     /// trait Trait {
     ///     fn foo(&self);
@@ -588,8 +541,11 @@ fn consider_structural_builtin_unsize_candidates(
                     goal, a_data, a_region, b_data, b_region,
                 ),
 
-                // `T` -> `dyn Trait` unsizing is handled separately in `consider_unsize_to_dyn_candidate`
+                // `T` -> `dyn Trait` unsizing.
-                (_, &ty::Dynamic(..)) => vec![],
+                (_, &ty::Dynamic(b_region, b_data, ty::Dyn)) => result_to_single(
+                    ecx.consider_builtin_unsize_to_dyn_candidate(goal, b_region, b_data),
+                    BuiltinImplSource::Misc,
+                ),
 
                 // `[T; N]` -> `[T]` unsizing
                 (&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => result_to_single(
@@ -691,6 +647,42 @@ fn consider_builtin_dyn_upcast_candidates(
         responses
     }
 
+    fn consider_builtin_unsize_to_dyn_candidate(
+        &mut self,
+        goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
+        b_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+        b_region: ty::Region<'tcx>,
+    ) -> QueryResult<'tcx> {
+        let tcx = self.tcx();
+        let Goal { predicate: (a_ty, _), .. } = goal;
+
+        // Can only unsize to an object-safe trait.
+        if b_data.principal_def_id().is_some_and(|def_id| !tcx.check_is_object_safe(def_id)) {
+            return Err(NoSolution);
+        }
+
+        // Check that the type implements all of the predicates of the trait object.
+        // (i.e. the principal, all of the associated types match, and any auto traits)
+        self.add_goals(
+            GoalSource::ImplWhereBound,
+            b_data.iter().map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty))),
+        );
+
+        // The type must be `Sized` to be unsized.
+        if let Some(sized_def_id) = tcx.lang_items().sized_trait() {
+            self.add_goal(
+                GoalSource::ImplWhereBound,
+                goal.with(tcx, ty::TraitRef::new(tcx, sized_def_id, [a_ty])),
+            );
+        } else {
+            return Err(NoSolution);
+        }
+
+        // The type must outlive the lifetime of the `dyn` we're unsizing into.
+        self.add_goal(GoalSource::Misc, goal.with(tcx, ty::OutlivesPredicate(a_ty, b_region)));
+        self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+    }
+
     fn consider_builtin_upcast_to_principal(
         &mut self,
         goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,