Rollup merge of #101798 - y86-dev:const_waker, r=lcnr

Make `from_waker`, `waker` and `from_raw` unstably `const`

Make
- `Context::from_waker`
- `Context::waker`
- `Waker::from_raw`

`const`.

Also added a small test.

library/alloc/src/lib.rs

@@ -109,6 +109,7 @@
 #![feature(core_intrinsics)]
 #![feature(const_eval_select)]
 #![feature(const_pin)]
+#![feature(const_waker)]
 #![feature(cstr_from_bytes_until_nul)]
 #![feature(dispatch_from_dyn)]
 #![cfg_attr(not(bootstrap), feature(error_generic_member_access))]

library/core/src/lib.rs

@@ -145,6 +145,7 @@
 #![feature(const_default_impls)]
 #![feature(const_unicode_case_lookup)]
 #![feature(const_unsafecell_get_mut)]
+#![feature(const_waker)]
 #![feature(core_panic)]
 #![feature(duration_consts_float)]
 #![feature(maybe_uninit_uninit_array)]

library/core/src/task/wake.rs

@@ -186,17 +186,19 @@ pub struct Context<'a> {
 impl<'a> Context<'a> {
     /// Create a new `Context` from a [`&Waker`](Waker).
     #[stable(feature = "futures_api", since = "1.36.0")]
+    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
     #[must_use]
     #[inline]
-    pub fn from_waker(waker: &'a Waker) -> Self {
+    pub const fn from_waker(waker: &'a Waker) -> Self {
         Context { waker, _marker: PhantomData }
     }
 
     /// Returns a reference to the [`Waker`] for the current task.
     #[stable(feature = "futures_api", since = "1.36.0")]
+    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
     #[must_use]
     #[inline]
-    pub fn waker(&self) -> &'a Waker {
+    pub const fn waker(&self) -> &'a Waker {
         &self.waker
     }
 }
@@ -311,7 +313,8 @@ impl Waker {
     #[inline]
     #[must_use]
     #[stable(feature = "futures_api", since = "1.36.0")]
-    pub unsafe fn from_raw(waker: RawWaker) -> Waker {
+    #[rustc_const_unstable(feature = "const_waker", issue = "102012")]
+    pub const unsafe fn from_raw(waker: RawWaker) -> Waker {
         Waker { waker }
     }
 

library/core/tests/lib.rs

@@ -74,6 +74,7 @@
 #![feature(iterator_try_reduce)]
 #![feature(const_mut_refs)]
 #![feature(const_pin)]
+#![feature(const_waker)]
 #![feature(never_type)]
 #![feature(unwrap_infallible)]
 #![feature(pointer_byte_offsets)]

library/core/tests/task.rs

@@ -1,4 +1,4 @@
-use core::task::Poll;
+use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 
 #[test]
 fn poll_const() {
@@ -12,3 +12,18 @@ fn poll_const() {
     const IS_PENDING: bool = POLL.is_pending();
     assert!(IS_PENDING);
 }
+
+#[test]
+fn waker_const() {
+    const VOID_TABLE: RawWakerVTable = RawWakerVTable::new(|_| VOID_WAKER, |_| {}, |_| {}, |_| {});
+
+    const VOID_WAKER: RawWaker = RawWaker::new(&(), &VOID_TABLE);
+
+    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();
+}

library/std/src/lib.rs

@@ -315,6 +315,7 @@
 #![feature(strict_provenance)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(const_maybe_uninit_uninit_array)]
+#![feature(const_waker)]
 //
 // Library features (alloc):
 #![feature(alloc_layout_extra)]