Rollup merge of #129542 - zachs18:cow-self-test, r=compiler-errors

Add regression test for #129541

(maybe?) closes #129541 by adding a test that the code in question continues to compile.

tests/ui/traits/sized-coniductive.rs

@@ -0,0 +1,14 @@
+//@ check-pass
+// https://github.com/rust-lang/rust/issues/129541
+
+#[derive(Clone)]
+struct Test {
+    field: std::borrow::Cow<'static, [Self]>,
+}
+
+#[derive(Clone)]
+struct Hello {
+    a: <[Hello] as std::borrow::ToOwned>::Owned,
+}
+
+fn main(){}