Rollup merge of rust-lang#110259 - ndrewxie:issue-109964-fix-gitstuff, r=cjgillot

Added diagnostic for pin! macro in addition to Box::pin if Unpin isn't implemented

I made a PR earlier, but accidentally renamed a branch and that deleted the PR... sorry for the duplicate

Currently, if an operation on `Pin<T>` is performed that requires `T` to implement `Unpin`, the diagnostic suggestion is to use `Box::pin` ("note: consider using `Box::pin`").

This PR suggests pin! as well, as that's another valid way of pinning a value, and avoids a heap allocation. Appropriate diagnostic suggestions were included to highlight the difference in semantics (local pinning for pin! vs non-local for Box::pin).

Fixes rust-lang#109964

Author: matthiaskrgr

core/src/marker.rs

@@ -823,7 +823,7 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
 /// [`pin` module]: crate::pin
 #[stable(feature = "pin", since = "1.33.0")]
 #[rustc_on_unimplemented(
-    note = "consider using `Box::pin`",
+    note = "consider using the `pin!` macro\nconsider using `Box::pin` if you need to access the pinned value outside of the current scope",
     message = "`{Self}` cannot be unpinned"
 )]
 #[lang = "unpin"]