Fix generics_of for impl's RPITIT synthesized associated type

compiler/rustc_ty_utils/src/assoc.rs

@@ -383,6 +383,8 @@ fn impl_associated_item_for_impl_trait_in_trait(
     impl_assoc_ty.impl_defaultness(tcx.impl_defaultness(impl_fn_def_id));
 
     // Copy generics_of the trait's associated item but the impl as the parent.
+    // FIXME(-Zlower-impl-trait-in-trait-to-assoc-ty) resolves to the trait instead of the impl
+    // generics.
     impl_assoc_ty.generics_of({
         let trait_assoc_generics = tcx.generics_of(trait_assoc_def_id);
         let trait_assoc_parent_count = trait_assoc_generics.parent_count;
@@ -391,16 +393,10 @@ fn impl_associated_item_for_impl_trait_in_trait(
         let parent_generics = tcx.generics_of(impl_def_id);
         let parent_count = parent_generics.parent_count + parent_generics.params.len();
 
-        let mut impl_fn_params = tcx.generics_of(impl_fn_def_id).params.clone();
-
         for param in &mut params {
-            param.index = param.index + parent_count as u32 + impl_fn_params.len() as u32
+            param.index = param.index + parent_count as u32 - trait_assoc_parent_count as u32;
-                - trait_assoc_parent_count as u32;
         }
 
-        impl_fn_params.extend(params);
-        params = impl_fn_params;
-
         let param_def_id_to_index =
             params.iter().map(|param| (param.def_id, param.index)).collect();
 

tests/ui/async-await/in-trait/issue-102310.rs

@@ -1,5 +1,7 @@
 // check-pass
 // edition:2021
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(async_fn_in_trait)]
 #![allow(incomplete_features)]

tests/ui/impl-trait/in-trait/early.rs

@@ -1,5 +1,7 @@
 // check-pass
 // edition:2021
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(async_fn_in_trait, return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]

tests/ui/impl-trait/in-trait/issue-102301.rs

@@ -1,4 +1,6 @@
 // check-pass
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]

tests/ui/impl-trait/in-trait/opaque-in-impl.rs

@@ -1,4 +1,6 @@
 // check-pass
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]

tests/ui/impl-trait/in-trait/where-clause.rs

@@ -1,5 +1,7 @@
 // check-pass
 // edition: 2021
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 #![feature(return_position_impl_trait_in_trait)]
 #![allow(incomplete_features)]