[WIP]

Author: fmease

compiler/rustc_hir_analysis/src/delegation.rs

@@ -233,6 +233,7 @@ fn build_predicates<'tcx>(
     ty::GenericPredicates { parent, predicates: tcx.arena.alloc_from_iter(preds) }
 }
 
+// NOTE(fmease): Isn't this just a full reimplementation of `EarlyBinder::instantiate`? Investigate.
 fn build_generic_args<'tcx>(
     tcx: TyCtxt<'tcx>,
     sig_id: DefId,
@@ -276,8 +277,7 @@ fn create_generic_args<'tcx>(
                 tcx.impl_trait_header(parent).unwrap().trait_ref.instantiate_identity().args;
 
             let trait_args = ty::GenericArgs::identity_for_item(tcx, sig_id);
-            let method_args =
-                tcx.mk_args_from_iter(trait_args.iter().skip(callee_generics.parent_count));
+            let method_args = tcx.mk_args(&trait_args[callee_generics.parent_count..]);
             let method_args = build_generic_args(tcx, sig_id, def_id, method_args);
 
             tcx.mk_args_from_iter(parent_args.iter().chain(method_args))

compiler/rustc_middle/src/ty/context.rs

@@ -268,16 +268,25 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         def_id: DefId,
         args: ty::GenericArgsRef<'tcx>,
     ) -> (ty::TraitRef<'tcx>, &'tcx [ty::GenericArg<'tcx>]) {
-        assert_matches!(self.def_kind(def_id), DefKind::AssocTy | DefKind::AssocConst);
+        debug_assert_matches!(self.def_kind(def_id), DefKind::AssocTy | DefKind::AssocConst);
         let trait_def_id = self.parent(def_id);
-        assert_matches!(self.def_kind(trait_def_id), DefKind::Trait);
+        debug_assert_matches!(self.def_kind(trait_def_id), DefKind::Trait);
         let trait_generics = self.generics_of(trait_def_id);
         (
             ty::TraitRef::new_from_args(self, trait_def_id, args.truncate_to(self, trait_generics)),
             &args[trait_generics.count()..],
         )
     }
 
+    fn own_args_for_alias(
+        self,
+        def_id: DefId,
+        args: ty::GenericArgsRef<'tcx>,
+    ) -> (ty::TraitRef<'tcx>, &'tcx [ty::GenericArg<'tcx>]) {
+        debug_assert_matches!(self.def_kind(def_id), DefKind::AssocTy | DefKind::AssocConst);
+        &args[self.generics_of(def_id).parent_count..]
+    }
+
     fn mk_args(self, args: &[Self::GenericArg]) -> ty::GenericArgsRef<'tcx> {
         self.mk_args(args)
     }

compiler/rustc_middle/src/ty/generic_args.rs

@@ -588,7 +588,7 @@ impl<'tcx> GenericArgs<'tcx> {
     }
 
     pub fn truncate_to(&self, tcx: TyCtxt<'tcx>, generics: &ty::Generics) -> GenericArgsRef<'tcx> {
-        tcx.mk_args_from_iter(self.iter().take(generics.count()))
+        tcx.mk_args(&self[..generics.count()])
     }
 
     pub fn print_as_list(&self) -> String {

compiler/rustc_type_ir/src/interner.rs

@@ -182,6 +182,12 @@ pub trait Interner:
         args: Self::GenericArgs,
     ) -> (ty::TraitRef<Self>, Self::GenericArgsSlice);
 
+    fn own_args_for_alias(
+        self,
+        def_id: Self::DefId,
+        args: Self::GenericArgs,
+    ) -> (ty::TraitRef<Self>, Self::GenericArgsSlice);
+
     fn mk_args(self, args: &[Self::GenericArg]) -> Self::GenericArgs;
 
     fn mk_args_from_iter<I, T>(self, args: I) -> T::Output

compiler/rustc_type_ir/src/predicate.rs

@@ -687,7 +687,7 @@ impl<I: Interner> AliasTerm<I> {
     /// For example, if this is a projection of `<T as StreamingIterator>::Item<'a>`,
     /// then this function would return the slice `['a]` as the own args.
     pub fn own_args(self, interner: I) -> I::GenericArgsSlice {
-        self.trait_ref_and_own_args(interner).1
+        interner.own_args_for_alias(self.def_id, self.args)
     }
 }