Rollup merge of rust-lang#115210 - DogPawHat:std-agnostic-rustc_on_unimplemented-for-alloc-rc, r=WaffleLapkin

Make `rustc_on_unimplemented` std-agnostic for `alloc::rc`

See rust-lang#112923

Just a few lines related to `alloc:rc` for `Send` and `Sync`.

That seems to be all of the `... = "std::..."` issues found, but there a few notes with `std::` inside them still.

r? `@WaffleLapkin`

Author: matthiaskrgr

library/core/src/marker.rs

@@ -77,6 +77,7 @@ macro marker_impls {
 #[cfg_attr(not(test), rustc_diagnostic_item = "Send")]
 #[rustc_on_unimplemented(
     on(_Self = "std::rc::Rc<T, A>", note = "use `std::sync::Arc` instead of `std::rc::Rc`"),
+    on(_Self = "alloc::rc::Rc<T, A>", note = "use `alloc::sync::Arc` instead of `alloc::rc::Rc`"),
     message = "`{Self}` cannot be sent between threads safely",
     label = "`{Self}` cannot be sent between threads safely",
     note = "consider using `std::sync::Arc<{Self}>`; for more information visit \
@@ -632,6 +633,7 @@ impl<T: ?Sized> Copy for &T {}
         note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead",
     ),
     on(_Self = "std::rc::Rc<T, A>", note = "use `std::sync::Arc` instead of `std::rc::Rc`"),
+    on(_Self = "alloc::rc::Rc<T, A>", note = "use `alloc::sync::Arc` instead of `alloc::rc::Rc`"),
     message = "`{Self}` cannot be shared between threads safely",
     label = "`{Self}` cannot be shared between threads safely",
     note = "consider using `std::sync::Arc<{Self}>`; for more information visit \