Don't ICE on anonymous struct in enum variant

compiler/rustc_hir_analysis/src/collect.rs

@@ -1025,7 +1025,13 @@ fn adt_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::AdtDef<'_> {
 
     let is_anonymous = item.ident.name == kw::Empty;
     let repr = if is_anonymous {
-        tcx.adt_def(tcx.local_parent(def_id)).repr()
+        let parent = tcx.local_parent(def_id);
+        if matches!(tcx.hir_node_by_def_id(parent), Node::Item(_)) {
+            tcx.adt_def(tcx.local_parent(def_id)).repr()
+        } else {
+            tcx.dcx().span_delayed_bug(item.span, "anonymous field inside non struct/union");
+            tcx.repr_options_of_def(def_id.to_def_id())
+        }
     } else {
         tcx.repr_options_of_def(def_id.to_def_id())
     };

tests/ui/union/unnamed-fields/anon-struct-in-enum-issue-121446.rs

@@ -0,0 +1,11 @@
+#![crate_type = "lib"]
+#![feature(unnamed_fields)]
+#![allow(unused, incomplete_features)]
+
+enum K {
+    M {
+        _ : struct { field: u8 },
+        //~^ error: unnamed fields are not allowed outside of structs or unions
+        //~| error: anonymous structs are not allowed outside of unnamed struct or union fields
+    }
+}

tests/ui/union/unnamed-fields/anon-struct-in-enum-issue-121446.stderr

@@ -0,0 +1,16 @@
+error: unnamed fields are not allowed outside of structs or unions
+  --> $DIR/anon-struct-in-enum-issue-121446.rs:7:9
+   |
+LL |         _ : struct { field: u8 },
+   |         -^^^^^^^^^^^^^^^^^^^^^^^
+   |         |
+   |         unnamed field declared here
+
+error: anonymous structs are not allowed outside of unnamed struct or union fields
+  --> $DIR/anon-struct-in-enum-issue-121446.rs:7:13
+   |
+LL |         _ : struct { field: u8 },
+   |             ^^^^^^^^^^^^^^^^^^^^ anonymous struct declared here
+
+error: aborting due to 2 previous errors
+