2 files changed, 61 insertions, 15 deletions

diff --git a/compiler/rustc_hir_analysis/src/collect/generics_of.rs b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
index 9cc6c16c126..c86788db988 100644
--- a/compiler/rustc_hir_analysis/src/collect/generics_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
@@ -14,6 +14,43 @@ use rustc_span::Span;
 pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
     use rustc_hir::*;
 
+    // For an RPITIT, synthesize generics which are equal to the opaque's generics
+    // and parent fn's generics compressed into one list.
+    if let Some(ty::ImplTraitInTraitData::Trait { fn_def_id, opaque_def_id }) =
+        tcx.opt_rpitit_info(def_id.to_def_id())
+    {
+        let trait_def_id = tcx.parent(fn_def_id);
+        let opaque_ty_generics = tcx.generics_of(opaque_def_id);
+        let opaque_ty_parent_count = opaque_ty_generics.parent_count;
+        let mut params = opaque_ty_generics.params.clone();
+
+        let parent_generics = tcx.generics_of(trait_def_id);
+        let parent_count = parent_generics.parent_count + parent_generics.params.len();
+
+        let mut trait_fn_params = tcx.generics_of(fn_def_id).params.clone();
+
+        for param in &mut params {
+            param.index = param.index + parent_count as u32 + trait_fn_params.len() as u32
+                - opaque_ty_parent_count as u32;
+        }
+
+        trait_fn_params.extend(params);
+        params = trait_fn_params;
+
+        let param_def_id_to_index =
+            params.iter().map(|param| (param.def_id, param.index)).collect();
+
+        return ty::Generics {
+            parent: Some(trait_def_id),
+            parent_count,
+            params,
+            param_def_id_to_index,
+            has_self: opaque_ty_generics.has_self,
+            has_late_bound_regions: opaque_ty_generics.has_late_bound_regions,
+            host_effect_index: parent_generics.host_effect_index,
+        };
+    }
+
     let hir_id = tcx.local_def_id_to_hir_id(def_id);
 
     let node = tcx.hir_node(hir_id);
diff --git a/compiler/rustc_hir_analysis/src/collect/type_of.rs b/compiler/rustc_hir_analysis/src/collect/type_of.rs
index 417f0fceaa8..2217e5280a7 100644
--- a/compiler/rustc_hir_analysis/src/collect/type_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/type_of.rs
@@ -5,7 +5,7 @@ use rustc_hir::HirId;
 use rustc_middle::query::plumbing::CyclePlaceholder;
 use rustc_middle::ty::print::with_forced_trimmed_paths;
 use rustc_middle::ty::util::IntTypeExt;
-use rustc_middle::ty::{self, ImplTraitInTraitData, IsSuggestable, Ty, TyCtxt, TypeVisitableExt};
+use rustc_middle::ty::{self, IsSuggestable, Ty, TyCtxt, TypeVisitableExt};
 use rustc_span::symbol::Ident;
 use rustc_span::{Span, DUMMY_SP};
 
@@ -350,22 +350,31 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::EarlyBinder<Ty
     // If we are computing `type_of` the synthesized associated type for an RPITIT in the impl
     // side, use `collect_return_position_impl_trait_in_trait_tys` to infer the value of the
     // associated type in the impl.
-    if let Some(ImplTraitInTraitData::Impl { fn_def_id, .. }) =
-        tcx.opt_rpitit_info(def_id.to_def_id())
-    {
-        match tcx.collect_return_position_impl_trait_in_trait_tys(fn_def_id) {
-            Ok(map) => {
-                let assoc_item = tcx.associated_item(def_id);
-                return map[&assoc_item.trait_item_def_id.unwrap()];
-            }
-            Err(_) => {
-                return ty::EarlyBinder::bind(Ty::new_error_with_message(
-                    tcx,
-                    DUMMY_SP,
-                    "Could not collect return position impl trait in trait tys",
-                ));
+    match tcx.opt_rpitit_info(def_id.to_def_id()) {
+        Some(ty::ImplTraitInTraitData::Impl { fn_def_id }) => {
+            match tcx.collect_return_position_impl_trait_in_trait_tys(fn_def_id) {
+                Ok(map) => {
+                    let assoc_item = tcx.associated_item(def_id);
+                    return map[&assoc_item.trait_item_def_id.unwrap()];
+                }
+                Err(_) => {
+                    return ty::EarlyBinder::bind(Ty::new_error_with_message(
+                        tcx,
+                        DUMMY_SP,
+                        "Could not collect return position impl trait in trait tys",
+                    ));
+                }
             }
         }
+        // For an RPITIT in a trait, just return the corresponding opaque.
+        Some(ty::ImplTraitInTraitData::Trait { opaque_def_id, .. }) => {
+            return ty::EarlyBinder::bind(Ty::new_opaque(
+                tcx,
+                opaque_def_id,
+                ty::GenericArgs::identity_for_item(tcx, opaque_def_id),
+            ));
+        }
+        None => {}
     }
 
     let hir_id = tcx.local_def_id_to_hir_id(def_id);