Add all RPITITs when augmenting param-env with GAT bounds in check_type_bounds

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -2162,7 +2162,7 @@ pub(super) fn check_type_bounds<'tcx>(
     impl_ty: ty::AssocItem,
     impl_trait_ref: ty::TraitRef<'tcx>,
 ) -> Result<(), ErrorGuaranteed> {
-    let param_env = param_env_with_gat_bounds(tcx, trait_ty, impl_ty, impl_trait_ref);
+    let param_env = param_env_with_gat_bounds(tcx, impl_ty, impl_trait_ref);
     debug!(?param_env);
 
     let container_id = impl_ty.container_id(tcx);
@@ -2288,7 +2288,6 @@ pub(super) fn check_type_bounds<'tcx>(
 /// the trait (notably, that `X: Eq` and `T: Family`).
 fn param_env_with_gat_bounds<'tcx>(
     tcx: TyCtxt<'tcx>,
-    trait_ty: ty::AssocItem,
     impl_ty: ty::AssocItem,
     impl_trait_ref: ty::TraitRef<'tcx>,
 ) -> ty::ParamEnv<'tcx> {
@@ -2296,13 +2295,35 @@ fn param_env_with_gat_bounds<'tcx>(
     let container_id = impl_ty.container_id(tcx);
     let mut predicates = param_env.caller_bounds().to_vec();
 
+    // for RPITITs, we should install predicates that allow us to project all
+    // of the RPITITs associated with the same body. This is because checking
+    // the item bounds of RPITITs often involves nested RPITITs having to prove
+    // bounds about themselves.
+    let impl_tys_to_install = match impl_ty.opt_rpitit_info {
+        None => vec![impl_ty],
+        Some(
+            ty::ImplTraitInTraitData::Impl { fn_def_id }
+            | ty::ImplTraitInTraitData::Trait { fn_def_id, .. },
+        ) => tcx
+            .associated_types_for_impl_traits_in_associated_fn(fn_def_id)
+            .iter()
+            .map(|def_id| tcx.associated_item(*def_id))
+            .collect(),
+    };
+
+    for impl_ty in impl_tys_to_install {
+        let trait_ty = match impl_ty.container {
+            ty::AssocItemContainer::TraitContainer => impl_ty,
+            ty::AssocItemContainer::ImplContainer => {
+                tcx.associated_item(impl_ty.trait_item_def_id.unwrap())
+            }
+        };
+
         let mut bound_vars: smallvec::SmallVec<[ty::BoundVariableKind; 8]> =
             smallvec::SmallVec::with_capacity(tcx.generics_of(impl_ty.def_id).params.len());
         // Extend the impl's identity args with late-bound GAT vars
-    let normalize_impl_ty_args = ty::GenericArgs::identity_for_item(tcx, container_id).extend_to(
+        let normalize_impl_ty_args = ty::GenericArgs::identity_for_item(tcx, container_id)
-        tcx,
+            .extend_to(tcx, impl_ty.def_id, |param, _| match param.kind {
-        impl_ty.def_id,
-        |param, _| match param.kind {
                 GenericParamDefKind::Type { .. } => {
                     let kind = ty::BoundTyKind::Param(param.def_id, param.name);
                     let bound_var = ty::BoundVariableKind::Ty(kind);
@@ -2321,7 +2342,10 @@ fn param_env_with_gat_bounds<'tcx>(
                     ty::Region::new_late_bound(
                         tcx,
                         ty::INNERMOST,
-                    ty::BoundRegion { var: ty::BoundVar::from_usize(bound_vars.len() - 1), kind },
+                        ty::BoundRegion {
+                            var: ty::BoundVar::from_usize(bound_vars.len() - 1),
+                            kind,
+                        },
                     )
                     .into()
                 }
@@ -2338,8 +2362,7 @@ fn param_env_with_gat_bounds<'tcx>(
                     )
                     .into()
                 }
-        },
+            });
-    );
         // When checking something like
         //
         // trait X { type Y: PartialEq<<Self as X>::Y> }
@@ -2349,8 +2372,10 @@ fn param_env_with_gat_bounds<'tcx>(
         // we want <T as X>::Y to normalize to S. This is valid because we are
         // checking the default value specifically here. Add this equality to the
         // ParamEnv for normalization specifically.
-    let normalize_impl_ty = tcx.type_of(impl_ty.def_id).instantiate(tcx, normalize_impl_ty_args);
+        let normalize_impl_ty =
-    let rebased_args = normalize_impl_ty_args.rebase_onto(tcx, container_id, impl_trait_ref.args);
+            tcx.type_of(impl_ty.def_id).instantiate(tcx, normalize_impl_ty_args);
+        let rebased_args =
+            normalize_impl_ty_args.rebase_onto(tcx, container_id, impl_trait_ref.args);
         let bound_vars = tcx.mk_bound_variable_kinds(&bound_vars);
 
         match normalize_impl_ty.kind() {
@@ -2374,6 +2399,7 @@ fn param_env_with_gat_bounds<'tcx>(
                 .to_predicate(tcx),
             ),
         };
+    }
 
     ty::ParamEnv::new(tcx.mk_clauses(&predicates), Reveal::UserFacing)
 }

tests/ui/impl-trait/in-trait/nested-rpitit-bounds.rs

@@ -0,0 +1,11 @@
+// check-pass
+
+use std::ops::Deref;
+
+trait Foo {
+    fn foo() -> impl Deref<Target = impl Deref<Target = impl Sized>> {
+        &&()
+    }
+}
+
+fn main() {}