Add rustc_on_unimplemented on Sync for cell types

Suggest using a lock instead.

library/core/src/marker.rs

@@ -469,6 +469,62 @@ pub macro Copy($item:item) {
 #[cfg_attr(not(test), rustc_diagnostic_item = "Sync")]
 #[lang = "sync"]
 #[rustc_on_unimplemented(
+    on(
+        _Self = "std::cell::OnceCell<T>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::OnceLock` instead"
+    ),
+    on(
+        _Self = "std::cell::Cell<u8>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU8` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<u16>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU16` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<u32>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU32` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<u64>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU64` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<usize>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicUsize` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<i8>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI8` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<i16>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI16` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<i32>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<i64>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI64` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<isize>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicIsize` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<bool>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicBool` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<T>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`",
+    ),
+    on(
+        _Self = "std::cell::RefCell<T>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead",
+    ),
     message = "`{Self}` cannot be shared between threads safely",
     label = "`{Self}` cannot be shared between threads safely"
 )]