Rollup merge of #95985 - jihiggins:issue-66481, r=dtolnay

Add PhantomData marker to Context to make Context !Send and !Sync

Adds `PhantomData<*mut ()>` to `Context` in order to allow for future `!Send` or `!Sync` additions to `Context`'s fields. This would allow for things like future single threaded async executor optimizations, or (re)adding `LocalWaker`, etc.

Closes #66481.

Per #66481 (comment), this is a breaking change that needs a Crater run as the next step.

(So far have tested the change locally with `cargotest` on WSL)

library/core/src/task/wake.rs

@@ -181,6 +181,9 @@ pub struct Context<'a> {
     // are contravariant while return-position lifetimes are
     // covariant).
     _marker: PhantomData<fn(&'a ()) -> &'a ()>,
+    // Ensure `Context` is `!Send` and `!Sync` in order to allow
+    // for future `!Send` and / or `!Sync` fields.
+    _marker2: PhantomData<*mut ()>,
 }
 
 impl<'a> Context<'a> {
@@ -190,7 +193,7 @@ impl<'a> Context<'a> {
     #[must_use]
     #[inline]
     pub const fn from_waker(waker: &'a Waker) -> Self {
-        Context { waker, _marker: PhantomData }
+        Context { waker, _marker: PhantomData, _marker2: PhantomData }
     }
 
     /// Returns a reference to the [`Waker`] for the current task.

library/core/tests/task.rs

@@ -1,4 +1,4 @@
-use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
+use core::task::{Poll, RawWaker, RawWakerVTable, Waker};
 
 #[test]
 fn poll_const() {
@@ -21,9 +21,5 @@ fn waker_const() {
 
     static WAKER: Waker = unsafe { Waker::from_raw(VOID_WAKER) };
 
-    static CONTEXT: Context<'static> = Context::from_waker(&WAKER);
-
-    static WAKER_REF: &'static Waker = CONTEXT.waker();
-
-    WAKER_REF.wake_by_ref();
+    WAKER.wake_by_ref();
 }