https://github.com/rust-lang/rust/blob/master/library/core/src/marker.rs
#[stable(feature = "rust1", since = "1.0.0")] #[cfg_attr(not(test), rustc_diagnostic_item = "sync_trait")] #[lang = "sync"] #[rustc_on_unimplemented( message = "`{Self}` cannot be shared between threads safely", label = "`{Self}` cannot be shared between threads safely" )] pub unsafe auto trait Sync { // Empty } #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> !Sync for *const T {} #[stable(feature = "rust1", since = "1.0.0")] impl<T: ?Sized> !Sync for *mut T {}