Rollup merge of #133488 - Enselic:recurse-2, r=BoxyUwU

tests: Add regression test for self referential structs with cow as last field

Making compilation pass for this code was retroactively stabilized via FCP in 1.79. The code does not compile in 1.78.

See https://github.com/rust-lang/rust/issues/129541 for details.

Closes #107481

tests/ui/traits/solver-cycles/107481-self-referential-struct-cow-as-last-field.rs

@@ -0,0 +1,19 @@
+// Regression test for #107481
+
+//@ check-pass
+
+use std::{borrow::Cow, collections::HashMap};
+
+#[derive(Clone)]
+struct Foo<'a>(Cow<'a, [Self]>);
+
+#[derive(Clone)]
+struct Bar<'a>(Cow<'a, HashMap<String, Self>>);
+
+#[derive(Clone)]
+struct Baz<'a>(Cow<'a, Vec<Self>>);
+
+#[derive(Clone)]
+struct Qux<'a>(Cow<'a, Box<Self>>);
+
+fn main() {}