Auto merge of #100865 - compiler-errors:parent-substs-still, r=cjgillot

Don't drop parent substs when we have no generic parameters in `create_substs_for_ast_path`

This bug is being shadowed by an explicit check for `generics.params.is_empty()` in the only parent caller that could trigger it (`create_substs_for_associated_item`). I triggered it on another branch where I'm messing around with astconv stuff.

Also, the second commit simplifies `create_substs_for_associated_item`. Removing that explicit check I mentioned above^ and also the special case call to `Astconv::prohibit_generics` causes the UI test `src/test/ui/structs/struct-path-associated-type.stderr` to change, but I think that it's clearer now. The suggestion to remove the generics is actually useful.

compiler/rustc_typeck/src/astconv/mod.rs

@@ -365,7 +365,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         // here and so associated type bindings will be handled regardless of whether there are any
         // non-`Self` generic parameters.
         if generics.params.is_empty() {
-            return (tcx.intern_substs(&[]), arg_count);
+            return (tcx.intern_substs(parent_substs), arg_count);
         }
 
         struct SubstsForAstPathCtxt<'a, 'tcx> {
@@ -586,7 +586,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
     pub(crate) fn create_substs_for_associated_item(
         &self,
-        tcx: TyCtxt<'tcx>,
         span: Span,
         item_def_id: DefId,
         item_segment: &hir::PathSegment<'_>,
@@ -596,22 +595,16 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             "create_substs_for_associated_item(span: {:?}, item_def_id: {:?}, item_segment: {:?}",
             span, item_def_id, item_segment
         );
-        if tcx.generics_of(item_def_id).params.is_empty() {
-            self.prohibit_generics(slice::from_ref(item_segment).iter(), |_| {});
-
-            parent_substs
-        } else {
-            self.create_substs_for_ast_path(
-                span,
-                item_def_id,
-                parent_substs,
-                item_segment,
-                item_segment.args(),
-                item_segment.infer_args,
-                None,
-            )
-            .0
-        }
+        self.create_substs_for_ast_path(
+            span,
+            item_def_id,
+            parent_substs,
+            item_segment,
+            item_segment.args(),
+            item_segment.infer_args,
+            None,
+        )
+        .0
     }
 
     /// Instantiates the path for the given trait reference, assuming that it's
@@ -1121,7 +1114,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             };
 
             let substs_trait_ref_and_assoc_item = self.create_substs_for_associated_item(
-                tcx,
                 path_span,
                 assoc_item.def_id,
                 &item_segment,
@@ -2100,7 +2092,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             self.ast_path_to_mono_trait_ref(span, trait_def_id, self_ty, trait_segment, false);
 
         let item_substs = self.create_substs_for_associated_item(
-            tcx,
             span,
             item_def_id,
             item_segment,

compiler/rustc_typeck/src/check/fn_ctxt/mod.rs

@@ -276,7 +276,6 @@ impl<'a, 'tcx> AstConv<'tcx> for FnCtxt<'a, 'tcx> {
 
         let item_substs = <dyn AstConv<'tcx>>::create_substs_for_associated_item(
             self,
-            self.tcx,
             span,
             item_def_id,
             item_segment,

compiler/rustc_typeck/src/collect.rs

@@ -430,7 +430,6 @@ impl<'tcx> AstConv<'tcx> for ItemCtxt<'tcx> {
         if let Some(trait_ref) = poly_trait_ref.no_bound_vars() {
             let item_substs = <dyn AstConv<'tcx>>::create_substs_for_associated_item(
                 self,
-                self.tcx,
                 span,
                 item_def_id,
                 item_segment,