Implement assumed_wf_types for RPITITs' implementations

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

@@ -246,6 +246,7 @@ provide! { tcx, def_id, other, cdata,
         debug_assert_eq!(tcx.def_kind(def_id), DefKind::OpaqueTy);
         cdata.root.tables.is_type_alias_impl_trait.get(cdata, def_id.index)
     }
+    assumed_wf_types_for_rpitit => { table }
     collect_return_position_impl_trait_in_trait_tys => {
         Ok(cdata
             .root

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1560,6 +1560,12 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         }
         if let Some(rpitit_info) = item.opt_rpitit_info {
             record!(self.tables.opt_rpitit_info[def_id] <- rpitit_info);
+            if matches!(rpitit_info, ty::ImplTraitInTraitData::Trait { .. }) {
+                record_array!(
+                    self.tables.assumed_wf_types_for_rpitit[def_id]
+                        <- self.tcx.assumed_wf_types_for_rpitit(def_id)
+                );
+            }
         }
     }
 

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -457,6 +457,7 @@ define_tables! {
     trait_impl_trait_tys: Table<DefIndex, LazyValue<FxHashMap<DefId, ty::EarlyBinder<Ty<'static>>>>>,
     doc_link_resolutions: Table<DefIndex, LazyValue<DocLinkResMap>>,
     doc_link_traits_in_scope: Table<DefIndex, LazyArray<DefId>>,
+    assumed_wf_types_for_rpitit: Table<DefIndex, LazyArray<(Ty<'static>, Span)>>,
 }
 
 #[derive(TyEncodable, TyDecodable)]

compiler/rustc_middle/src/query/mod.rs

@@ -885,6 +885,13 @@ rustc_queries! {
         desc { |tcx| "computing the implied bounds of `{}`", tcx.def_path_str(key) }
     }
 
+    /// We need to store the assumed_wf_types for an RPITIT so that impls of foreign
+    /// traits with return-position impl trait in traits can inherit the right wf types.
+    query assumed_wf_types_for_rpitit(key: DefId) -> &'tcx [(Ty<'tcx>, Span)] {
+        desc { |tcx| "computing the implied bounds of `{}`", tcx.def_path_str(key) }
+        separate_provide_extern
+    }
+
     /// Computes the signature of the function.
     query fn_sig(key: DefId) -> ty::EarlyBinder<ty::PolyFnSig<'tcx>> {
         desc { |tcx| "computing function signature of `{}`", tcx.def_path_str(key) }

compiler/rustc_ty_utils/src/implied_bounds.rs

@@ -9,7 +9,11 @@ use rustc_span::Span;
 use std::iter;
 
 pub fn provide(providers: &mut Providers) {
-    *providers = Providers { assumed_wf_types, ..*providers };
+    *providers = Providers {
+        assumed_wf_types,
+        assumed_wf_types_for_rpitit: |tcx, def_id| tcx.assumed_wf_types(def_id),
+        ..*providers
+    };
 }
 
 fn assumed_wf_types<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> &'tcx [(Ty<'tcx>, Span)] {
@@ -44,12 +48,15 @@ fn assumed_wf_types<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> &'tcx [(Ty<'
             let mut impl_spans = impl_spans(tcx, def_id);
             tcx.arena.alloc_from_iter(tys.into_iter().map(|ty| (ty, impl_spans.next().unwrap())))
         }
-        DefKind::AssocTy  if let Some(ty::ImplTraitInTraitData::Trait { fn_def_id, opaque_def_id }) = tcx.opt_rpitit_info(def_id.to_def_id()) => {
+        DefKind::AssocTy if let Some(data) = tcx.opt_rpitit_info(def_id.to_def_id()) => match data {
+            ty::ImplTraitInTraitData::Trait { fn_def_id, opaque_def_id } => {
                 let hir::OpaqueTy { lifetime_mapping, .. } =
                     *tcx.hir().expect_item(opaque_def_id.expect_local()).expect_opaque_ty();
                 let mut mapping = FxHashMap::default();
                 let generics = tcx.generics_of(def_id);
-            for &(lifetime, new_early_bound_def_id) in lifetime_mapping {
+                for &(lifetime, new_early_bound_def_id) in
+                    lifetime_mapping.expect("expected lifetime mapping for RPITIT")
+                {
                     if let Some(rbv::ResolvedArg::LateBound(_, _, def_id)) =
                         tcx.named_bound_var(lifetime.hir_id)
                     {
@@ -74,11 +81,29 @@ fn assumed_wf_types<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> &'tcx [(Ty<'
                         );
                     }
                 }
-            let a = tcx.fold_regions(tcx.assumed_wf_types(fn_def_id.expect_local()).to_vec(), |re, _| {
+                let a = tcx.fold_regions(
+                    tcx.assumed_wf_types(fn_def_id.expect_local()).to_vec(),
+                    |re, _| {
                         if let Some(re) = mapping.get(&re) { *re } else { re }
-            });
+                    },
+                );
                 tcx.arena.alloc_from_iter(a)
             }
+            ty::ImplTraitInTraitData::Impl { .. } => {
+                let impl_def_id = tcx.local_parent(def_id);
+                let rpitit_def_id = tcx.associated_item(def_id).trait_item_def_id.unwrap();
+                let args = ty::GenericArgs::identity_for_item(tcx, def_id).rebase_onto(
+                    tcx,
+                    impl_def_id.to_def_id(),
+                    tcx.impl_trait_ref(impl_def_id).unwrap().instantiate_identity().args,
+                );
+                tcx.arena.alloc_from_iter(
+                    ty::EarlyBinder::bind(tcx.assumed_wf_types_for_rpitit(rpitit_def_id))
+                        .iter_instantiated_copied(tcx, args)
+                        .chain(tcx.assumed_wf_types(impl_def_id).into_iter().copied()),
+                )
+            }
+        },
         DefKind::AssocConst | DefKind::AssocTy => tcx.assumed_wf_types(tcx.local_parent(def_id)),
         DefKind::OpaqueTy => match tcx.def_kind(tcx.local_parent(def_id)) {
             DefKind::TyAlias => ty::List::empty(),

tests/ui/impl-trait/in-trait/assumed-wf-bounds-in-impl.rs

@@ -0,0 +1,29 @@
+// check-pass
+// edition: 2021
+// issue: 113796
+
+#![feature(async_fn_in_trait)]
+
+trait AsyncLendingIterator {
+    type Item<'a>
+    where
+        Self: 'a;
+
+    async fn next(&mut self) -> Option<Self::Item<'_>>;
+}
+
+struct Lend<I>(I);
+impl<I> AsyncLendingIterator for Lend<I> {
+    type Item<'a> = &'a I
+    where
+        Self: 'a;
+
+    // Checking that the synthetic `<Self as AsyncLendingIterator>::next()` GAT
+    // is well-formed requires being able to assume the WF types of `next`.
+
+    async fn next(&mut self) -> Option<Self::Item<'_>> {
+        todo!()
+    }
+}
+
+fn main() {}