Serialize return-position impl Trait in trait hidden values in foreign libraries

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

@@ -223,6 +223,15 @@ provide! { tcx, def_id, other, cdata,
     fn_arg_names => { table }
     generator_kind => { table }
     trait_def => { table }
+    collect_trait_impl_trait_tys => {
+        Ok(cdata
+            .root
+            .tables
+            .trait_impl_trait_tys
+            .get(cdata, def_id.index)
+            .map(|lazy| lazy.decode((cdata, tcx)))
+            .process_decoded(tcx, || panic!("{:?} does not have trait_impl_trait_tys", def_id)))
+     }
 
     visibility => { cdata.get_visibility(def_id.index) }
     adt_def => { cdata.get_adt_def(def_id.index, tcx) }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1059,6 +1059,34 @@ fn should_encode_const(def_kind: DefKind) -> bool {
     }
 }
 
+fn should_encode_trait_impl_trait_tys<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
+    if tcx.def_kind(def_id) != DefKind::AssocFn {
+        return false;
+    }
+
+    let Some(item) = tcx.opt_associated_item(def_id) else { return false; };
+    if item.container != ty::AssocItemContainer::ImplContainer {
+        return false;
+    }
+
+    let Some(trait_item_def_id) = item.trait_item_def_id else { return false; };
+
+    // FIXME(RPITIT): This does a somewhat manual walk through the signature
+    // of the trait fn to look for any RPITITs, but that's kinda doing a lot
+    // of work. We can probably remove this when we refactor RPITITs to be
+    // associated types.
+    tcx.fn_sig(trait_item_def_id).skip_binder().output().walk().any(|arg| {
+        if let ty::GenericArgKind::Type(ty) = arg.unpack()
+            && let ty::Projection(data) = ty.kind()
+            && tcx.def_kind(data.item_def_id) == DefKind::ImplTraitPlaceholder
+        {
+            true
+        } else {
+            false
+        }
+    })
+}
+
 impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
     fn encode_attrs(&mut self, def_id: LocalDefId) {
         let mut attrs = self
@@ -1128,6 +1156,11 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             if let DefKind::Trait | DefKind::TraitAlias = def_kind {
                 record!(self.tables.super_predicates_of[def_id] <- self.tcx.super_predicates_of(def_id));
             }
+            if should_encode_trait_impl_trait_tys(tcx, def_id)
+                && let Ok(table) = self.tcx.collect_trait_impl_trait_tys(def_id)
+            {
+                record!(self.tables.trait_impl_trait_tys[def_id] <- table);
+            }
         }
         let inherent_impls = tcx.crate_inherent_impls(());
         for (def_id, implementations) in inherent_impls.inherent_impls.iter() {

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -1,6 +1,7 @@
 use crate::creader::CrateMetadataRef;
 use decoder::Metadata;
 use def_path_hash_map::DefPathHashMapRef;
+use rustc_data_structures::fx::FxHashMap;
 use table::TableBuilder;
 
 use rustc_ast as ast;
@@ -399,6 +400,8 @@ define_tables! {
     macro_definition: Table<DefIndex, LazyValue<ast::MacArgs>>,
     proc_macro: Table<DefIndex, MacroKind>,
     module_reexports: Table<DefIndex, LazyArray<ModChild>>,
+
+    trait_impl_trait_tys: Table<DefIndex, LazyValue<FxHashMap<DefId, Ty<'static>>>>,
 }
 
 #[derive(TyEncodable, TyDecodable)]

compiler/rustc_middle/src/ty/parameterized.rs

@@ -1,3 +1,4 @@
+use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def_id::{DefId, DefIndex};
 use rustc_index::vec::{Idx, IndexVec};
 
@@ -29,6 +30,10 @@ impl<I: Idx + 'static, T: ParameterizedOverTcx> ParameterizedOverTcx for IndexVe
     type Value<'tcx> = IndexVec<I, T::Value<'tcx>>;
 }
 
+impl<I: 'static, T: ParameterizedOverTcx> ParameterizedOverTcx for FxHashMap<I, T> {
+    type Value<'tcx> = FxHashMap<I, T::Value<'tcx>>;
+}
+
 impl<T: ParameterizedOverTcx> ParameterizedOverTcx for ty::Binder<'static, T> {
     type Value<'tcx> = ty::Binder<'tcx, T::Value<'tcx>>;
 }

src/test/ui/impl-trait/in-trait/auxiliary/rpitit.rs

@@ -0,0 +1,11 @@
+#![feature(return_position_impl_trait_in_trait)]
+
+pub trait Foo {
+    fn bar() -> impl Sized;
+}
+
+pub struct Foreign;
+
+impl Foo for Foreign {
+    fn bar() {}
+}

src/test/ui/impl-trait/in-trait/foreign.rs

@@ -0,0 +1,9 @@
+// check-pass
+// aux-build: rpitit.rs
+
+extern crate rpitit;
+
+fn main() {
+    // Witness an RPITIT from another crate
+    let () = <rpitit::Foreign as rpitit::Foo>::bar();
+}