Move trait_ref_to_existential to a closure

review comment

Author: estebank

src/librustc_typeck/astconv.rs

@@ -1224,25 +1224,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         )
     }
 
-    /// Transform a `PolyTraitRef` into a `PolyExistentialTraitRef` by
-    /// removing the dummy `Self` type (`trait_object_dummy_self`).
-    fn trait_ref_to_existential(
-        &self,
-        trait_ref: ty::TraitRef<'tcx>,
-    ) -> ty::ExistentialTraitRef<'tcx> {
-        if trait_ref.self_ty() != self.tcx().types.trait_object_dummy_self {
-            // FIXME: There appears to be a missing filter on top of `expand_trait_aliases`, which
-            // picks up non-supertraits where clauses - but also, the object safety completely
-            // ignores trait aliases, which could be object safety hazards. We `delay_span_bug`
-            // here to avoid an ICE in stable even when the feature is disabled. (#66420)
-            self.tcx().sess.delay_span_bug(DUMMY_SP, &format!(
-                "trait_ref_to_existential called on {:?} with non-dummy Self",
-                trait_ref,
-            ));
-        }
-        ty::ExistentialTraitRef::erase_self_ty(self.tcx(), trait_ref)
-    }
-
     fn conv_object_ty_poly_trait_ref(&self,
         span: Span,
         trait_bounds: &[hir::PolyTraitRef],
@@ -1424,13 +1405,30 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         debug!("regular_traits: {:?}", regular_traits);
         debug!("auto_traits: {:?}", auto_traits);
 
+        // Transform a `PolyTraitRef` into a `PolyExistentialTraitRef` by
+        // removing the dummy `Self` type (`trait_object_dummy_self`).
+        let trait_ref_to_existential = |trait_ref: ty::TraitRef<'tcx>| {
+            if trait_ref.self_ty() != dummy_self {
+                // FIXME: There appears to be a missing filter on top of `expand_trait_aliases`,
+                // which picks up non-supertraits where clauses - but also, the object safety
+                // completely ignores trait aliases, which could be object safety hazards. We
+                // `delay_span_bug` here to avoid an ICE in stable even when the feature is
+                // disabled. (#66420)
+                tcx.sess.delay_span_bug(DUMMY_SP, &format!(
+                    "trait_ref_to_existential called on {:?} with non-dummy Self",
+                    trait_ref,
+                ));
+            }
+            ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
+        };
+
         // Erase the `dummy_self` (`trait_object_dummy_self`) used above.
         let existential_trait_refs = regular_traits.iter().map(|i| {
-            i.trait_ref().map_bound(|trait_ref| self.trait_ref_to_existential(trait_ref))
+            i.trait_ref().map_bound(|trait_ref| trait_ref_to_existential(trait_ref))
         });
         let existential_projections = bounds.projection_bounds.iter().map(|(bound, _)| {
             bound.map_bound(|b| {
-                let trait_ref = self.trait_ref_to_existential(b.projection_ty.trait_ref(tcx));
+                let trait_ref = trait_ref_to_existential(b.projection_ty.trait_ref(tcx));
                 ty::ExistentialProjection {
                     ty: b.ty,
                     item_def_id: b.projection_ty.item_def_id,