Fix ICE in fn_delegation when child segment resolves to a trait

When a delegation path like `reuse Trait::<> as bar4` has only one
segment resolving to a trait (not a function), the child args
processing in `get_delegation_user_specified_args` called
`lower_generic_args_of_path` with `self_ty = None`. Since the trait's
generics have `has_self = true`, this triggered
`assert!(self_ty.is_some())`.

Fix by computing and providing `self_ty` when the child segment's
`def_id` has `has_self`. In valid delegation code the child segment
always resolves to a function, so this only affects error recovery.

Author: GokhanKabar

compiler/rustc_hir_analysis/src/delegation.rs

@@ -610,13 +610,25 @@ fn get_delegation_user_specified_args<'tcx>(
             }
         };
 
+        // Provide `self_ty` only when the child segment resolves to a non-function
+        // item with Self (e.g. a Trait during error recovery). For trait methods
+        // (AssocFn), `has_self` is true (inherited from parent trait) but Self
+        // comes from `parent_args`, not `self_ty`.
+        let self_ty = if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn)
+            && tcx.generics_of(def_id).has_self
+        {
+            get_delegation_self_ty(tcx, delegation_id)
+        } else {
+            None
+        };
+
         let args = lowerer
             .lower_generic_args_of_path(
                 segment.ident.span,
                 def_id,
                 parent_args,
                 segment,
-                None,
+                self_ty,
                 GenericArgPosition::Value,
             )
             .0;

tests/ui/delegation/ice-reuse-trait-path-with-empty-generics.rs

@@ -0,0 +1,13 @@
+#![feature(fn_delegation)]
+#![allow(incomplete_features)]
+
+trait Trait {
+    fn bar4();
+}
+
+impl Trait for () {
+    reuse Trait::<> as bar4;
+    //~^ ERROR expected function, found trait `Trait`
+}
+
+fn main() {}

tests/ui/delegation/ice-reuse-trait-path-with-empty-generics.stderr

@@ -0,0 +1,9 @@
+error[E0423]: expected function, found trait `Trait`
+  --> $DIR/ice-reuse-trait-path-with-empty-generics.rs:9:11
+   |
+LL |     reuse Trait::<> as bar4;
+   |           ^^^^^^^^^ not a function
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0423`.