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

Author: Enselic

Diff for: 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() {}