Remove unnecessary FulfillmentContext from need_migrate_deref_output_trait_object

Author: compiler-errors

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -8,7 +8,7 @@
 use hir::LangItem;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
-use rustc_infer::traits::TraitEngine;
+use rustc_infer::traits::ObligationCause;
 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
 use rustc_lint_defs::builtin::DEREF_INTO_DYN_SUPERTRAIT;
 use rustc_middle::ty::print::with_no_trimmed_paths;
@@ -706,7 +706,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn need_migrate_deref_output_trait_object(
         &mut self,
         ty: Ty<'tcx>,
-        cause: &traits::ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> Option<(Ty<'tcx>, DefId)> {
         let tcx = self.tcx();
@@ -721,32 +720,35 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
 
         let obligation = traits::Obligation::new(
-            cause.clone(),
+            ObligationCause::dummy(),
             param_env,
             ty::Binder::dummy(trait_ref).without_const().to_predicate(tcx),
         );
         if !self.infcx.predicate_may_hold(&obligation) {
             return None;
         }
 
-        let mut fulfillcx = traits::FulfillmentContext::new_in_snapshot();
-        let normalized_ty = fulfillcx.normalize_projection_type(
-            &self.infcx,
+        let ty = traits::normalize_projection_type(
+            self,
             param_env,
             ty::ProjectionTy {
                 item_def_id: tcx.lang_items().deref_target()?,
                 substs: trait_ref.substs,
             },
-            cause.clone(),
-        );
-
-        let ty::Dynamic(data, ..) = normalized_ty.kind() else {
-            return None;
-        };
-
-        let def_id = data.principal_def_id()?;
-
-        return Some((normalized_ty, def_id));
+            ObligationCause::dummy(),
+            0,
+            // We're *intentionally* throwing these away,
+            // since we don't actually use them.
+            &mut vec![],
+        )
+        .ty()
+        .unwrap();
+
+        if let ty::Dynamic(data, ..) = ty.kind() {
+            Some((ty, data.principal_def_id()?))
+        } else {
+            None
+        }
     }
 
     /// Searches for unsizing that might apply to `obligation`.
@@ -807,11 +809,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         let target_trait_did = principal_def_id_b.unwrap();
                         let source_trait_ref = principal_a.with_self_ty(self.tcx(), source);
                         if let Some((deref_output_ty, deref_output_trait_did)) = self
-                            .need_migrate_deref_output_trait_object(
-                                source,
-                                &obligation.cause,
-                                obligation.param_env,
-                            )
+                            .need_migrate_deref_output_trait_object(source, obligation.param_env)
                         {
                             if deref_output_trait_did == target_trait_did {
                                 self.tcx().struct_span_lint_hir(