rustdoc: Add regression test for Iterator as notable trait on &mut T

src/test/rustdoc/doc-notable_trait-mut_t_is_not_an_iterator.rs

@@ -0,0 +1,23 @@
+//! Test case for [#80737].
+//!
+//! A SomeTrait that is implemented for `&mut T where T: SomeTrait`
+//! should not be marked as "notable" for return values that do not
+//! have bounds on the trait itself.
+//!
+//! [#80737]: https://github.com/rust-lang/rust/issues/80737
+
+#![feature(rustdoc_internals)]
+#![no_std]
+
+#[doc(primitive = "reference")]
+/// Some useless docs, wouhou!
+///
+/// We need to put this in here, because notable traits
+/// that are implemented on foreign types don't show up.
+mod reference {}
+
+// @has doc_notable_trait_mut_t_is_not_an_iterator/fn.fn_no_matches.html
+// @!has - '//code[@class="content"]' 'Iterator'
+pub fn fn_no_matches<'a, T: 'a>() -> &'a mut T {
+    panic!()
+}