Add regression test for #84634

Author: GuillaumeGomez

src/test/rustdoc/anonymous-lifetime.rs

@@ -0,0 +1,28 @@
+// Regression test for https://github.com/rust-lang/rust/issues/84634
+#![crate_name = "foo"]
+
+use std::pin::Pin;
+use std::task::Poll;
+
+pub trait Stream {
+    type Item;
+
+    fn poll_next(mut self: Pin<&mut Self>) -> Poll<Option<Self::Item>>;
+    fn size_hint(&self) -> (usize, Option<usize>);
+}
+
+// @has 'foo/trait.Stream.html'
+// @has - '//*[@class="code-header in-band"]' 'impl<S: ?Sized + Stream + Unpin> Stream for &mut S'
+impl<S: ?Sized + Stream + Unpin> Stream for &mut S {
+    type Item = S::Item;
+
+    fn poll_next(
+        mut self: Pin<&mut Self>,
+    ) -> Poll<Option<Self::Item>> {
+        S::poll_next(Pin::new(&mut **self), cx)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (**self).size_hint()
+    }
+}