Add UI test for Pin PartialEq unsoundness

Author: LunaBorowska

src/test/ui/issues/issue-67039-unsound-pin-partialeq.rs

@@ -0,0 +1,27 @@
+// Pin's PartialEq implementation allowed to access the pointer allowing for
+// unsoundness by using Rc::get_mut to move value within Rc.
+// See https://internals.rust-lang.org/t/unsoundness-in-pin/11311/73 for more details.
+
+use std::ops::Deref;
+use std::pin::Pin;
+use std::rc::Rc;
+
+struct Apple;
+
+impl Deref for Apple {
+    type Target = Apple;
+    fn deref(&self) -> &Apple {
+        &Apple
+    }
+}
+
+impl PartialEq<Rc<Apple>> for Apple {
+    fn eq(&self, _rc: &Rc<Apple>) -> bool {
+        unreachable!()
+    }
+}
+
+fn main() {
+    let _ = Pin::new(Apple) == Rc::pin(Apple);
+    //~^ ERROR type mismatch resolving
+}

src/test/ui/issues/issue-67039-unsound-pin-partialeq.stderr

@@ -0,0 +1,13 @@
+error[E0271]: type mismatch resolving `<std::rc::Rc<Apple> as std::ops::Deref>::Target == std::rc::Rc<Apple>`
+  --> $DIR/issue-67039-unsound-pin-partialeq.rs:25:29
+   |
+LL |     let _ = Pin::new(Apple) == Rc::pin(Apple);
+   |                             ^^ expected struct `Apple`, found struct `std::rc::Rc`
+   |
+   = note: expected type `Apple`
+            found struct `std::rc::Rc<Apple>`
+   = note: required because of the requirements on the impl of `std::cmp::PartialEq<std::pin::Pin<std::rc::Rc<Apple>>>` for `std::pin::Pin<Apple>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0271`.