Encode whether foreign opaques are TAITs or not

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -2602,7 +2602,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         match path.res {
             Res::Def(DefKind::OpaqueTy | DefKind::ImplTraitPlaceholder, did) => {
                 // Check for desugared `impl Trait`.
-                assert!(ty::is_impl_trait_defn(tcx, did).is_none());
+                assert!(tcx.is_type_alias_impl_trait(did));
                 let item_segment = path.segments.split_last().unwrap();
                 self.prohibit_generics(item_segment.1.iter(), |err| {
                     err.note("`impl Trait` types can't have type parameters");

compiler/rustc_hir_analysis/src/collect.rs

@@ -76,6 +76,7 @@ pub fn provide(providers: &mut Providers) {
         is_foreign_item,
         generator_kind,
         collect_mod_item_types,
+        is_type_alias_impl_trait,
         ..*providers
     };
 }
@@ -1537,3 +1538,13 @@ fn generator_kind(tcx: TyCtxt<'_>, def_id: DefId) -> Option<hir::GeneratorKind>
         _ => bug!("generator_kind applied to non-local def-id {:?}", def_id),
     }
 }
+
+fn is_type_alias_impl_trait<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
+    match tcx.hir().get_if_local(def_id) {
+        Some(Node::Item(hir::Item { kind: hir::ItemKind::OpaqueTy(opaque), .. })) => {
+            matches!(opaque.origin, hir::OpaqueTyOrigin::TyAlias)
+        }
+        Some(_) => bug!("tried getting opaque_ty_origin for non-opaque: {:?}", def_id),
+        _ => bug!("tried getting opaque_ty_origin for non-local def-id {:?}", def_id),
+    }
+}

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -223,6 +223,7 @@ provide! { tcx, def_id, other, cdata,
     generator_kind => { table }
     trait_def => { table }
     deduced_param_attrs => { table }
+    is_type_alias_impl_trait => { table }
     collect_return_position_impl_trait_in_trait_tys => {
         Ok(cdata
             .root

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1514,6 +1514,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             }
             hir::ItemKind::OpaqueTy(..) => {
                 self.encode_explicit_item_bounds(def_id);
+                record!(self.tables.is_type_alias_impl_trait[def_id] <- self.tcx.is_type_alias_impl_trait(def_id));
             }
             hir::ItemKind::Enum(..) => {
                 let adt_def = self.tcx.adt_def(def_id);

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -404,6 +404,7 @@ define_tables! {
     proc_macro: Table<DefIndex, MacroKind>,
     module_reexports: Table<DefIndex, LazyArray<ModChild>>,
     deduced_param_attrs: Table<DefIndex, LazyArray<DeducedParamAttrs>>,
+    is_type_alias_impl_trait: Table<DefIndex, LazyValue<bool>>,
 
     trait_impl_trait_tys: Table<DefIndex, LazyValue<FxHashMap<DefId, Ty<'static>>>>,
 }

compiler/rustc_middle/src/query/mod.rs

@@ -177,6 +177,12 @@ rustc_queries! {
         separate_provide_extern
     }
 
+    query is_type_alias_impl_trait(key: DefId) -> bool
+    {
+        desc { "determine whether the opaque is a type-alias impl trait" }
+        separate_provide_extern
+    }
+
     query analysis(key: ()) -> Result<(), ErrorGuaranteed> {
         eval_always
         desc { "running analysis passes on this crate" }

compiler/rustc_middle/src/ty/parameterized.rs

@@ -52,6 +52,7 @@ trivially_parameterized_over_tcx! {
     usize,
     (),
     u32,
+    bool,
     std::string::String,
     crate::metadata::ModChild,
     crate::middle::codegen_fn_attrs::CodegenFnAttrs,

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -654,7 +654,7 @@ impl<'tcx> WfPredicates<'tcx> {
                     // All of the requirements on type parameters
                     // have already been checked for `impl Trait` in
                     // return position. We do need to check type-alias-impl-trait though.
-                    if ty::is_impl_trait_defn(self.tcx, def_id).is_none() {
+                    if self.tcx.is_type_alias_impl_trait(def_id) {
                         let obligations = self.nominal_obligations(def_id, substs);
                         self.out.extend(obligations);
                     }