Avoid once_cell in static wakers (rust-lang#2332)

futures-task/Cargo.toml

@@ -13,7 +13,7 @@ Tools for working with tasks.
 
 [features]
 default = ["std"]
-std = ["alloc", "once_cell"]
+std = ["alloc"]
 alloc = []
 
 # Unstable features
@@ -23,7 +23,6 @@ unstable = []
 cfg-target-has-atomic = []
 
 [dependencies]
-once_cell = { version = "1.3.1", default-features = false, features = ["std"], optional = true }
 
 [dev-dependencies]
 futures = { path = "../futures" }

futures-task/src/lib.rs

@@ -46,7 +46,6 @@ pub use crate::future_obj::{FutureObj, LocalFutureObj, UnsafeFutureObj};
 
 mod noop_waker;
 pub use crate::noop_waker::noop_waker;
-#[cfg(feature = "std")]
 pub use crate::noop_waker::noop_waker_ref;
 
 #[doc(no_inline)]

futures-task/src/noop_waker.rs

@@ -2,8 +2,6 @@
 
 use core::ptr::null;
 use core::task::{RawWaker, RawWakerVTable, Waker};
-#[cfg(feature = "std")]
-use once_cell::sync::Lazy;
 
 unsafe fn noop_clone(_data: *const ()) -> RawWaker {
     noop_raw_waker()
@@ -13,7 +11,7 @@ unsafe fn noop(_data: *const ()) {}
 
 const NOOP_WAKER_VTABLE: RawWakerVTable = RawWakerVTable::new(noop_clone, noop, noop, noop);
 
-fn noop_raw_waker() -> RawWaker {
+const fn noop_raw_waker() -> RawWaker {
     RawWaker::new(null(), &NOOP_WAKER_VTABLE)
 }
 
@@ -29,6 +27,7 @@ fn noop_raw_waker() -> RawWaker {
 /// ```
 #[inline]
 pub fn noop_waker() -> Waker {
+    // FIXME: Since 1.46.0 we can use transmute in consts, allowing this function to be const.
     unsafe { Waker::from_raw(noop_raw_waker()) }
 }
 
@@ -43,10 +42,14 @@ pub fn noop_waker() -> Waker {
 /// waker.wake_by_ref();
 /// ```
 #[inline]
-#[cfg(feature = "std")]
 pub fn noop_waker_ref() -> &'static Waker {
-    static NOOP_WAKER_INSTANCE: Lazy<Waker> = Lazy::new(noop_waker);
-    &*NOOP_WAKER_INSTANCE
+    struct SyncRawWaker(RawWaker);
+    unsafe impl Sync for SyncRawWaker {}
+
+    static NOOP_WAKER_INSTANCE: SyncRawWaker = SyncRawWaker(noop_raw_waker());
+
+    // SAFETY: `Waker` is #[repr(transparent)] over its `RawWaker`.
+    unsafe { &*(&NOOP_WAKER_INSTANCE.0 as *const RawWaker as *const Waker) }
 }
 
 #[cfg(test)]

futures-test/Cargo.toml

@@ -19,15 +19,14 @@ futures-util = { version = "0.3.12", path = "../futures-util", default-features
 futures-executor = { version = "0.3.12", path = "../futures-executor", default-features = false }
 futures-sink = { version = "0.3.12", path = "../futures-sink", default-features = false }
 pin-utils = { version = "0.1.0", default-features = false }
-once_cell = { version = "1.3.1", default-features = false, features = ["std"], optional = true }
 pin-project = "1.0.1"
 
 [dev-dependencies]
 futures = { path = "../futures", default-features = false, features = ["std", "executor"] }
 
 [features]
 default = ["std"]
-std = ["futures-core/std", "futures-task/std", "futures-io/std", "futures-util/std", "futures-util/io", "futures-executor/std", "once_cell"]
+std = ["futures-core/std", "futures-task/std", "futures-io/std", "futures-util/std", "futures-util/io", "futures-executor/std"]
 
 [package.metadata.docs.rs]
 all-features = true

futures-test/src/task/panic_waker.rs

@@ -1,6 +1,5 @@
 use core::ptr::null;
 use futures_core::task::{RawWaker, RawWakerVTable, Waker};
-use once_cell::sync::Lazy;
 
 unsafe fn clone_panic_waker(_data: *const ()) -> RawWaker {
     raw_panic_waker()
@@ -17,7 +16,7 @@ unsafe fn wake_panic(_data: *const ()) {
 const PANIC_WAKER_VTABLE: RawWakerVTable =
     RawWakerVTable::new(clone_panic_waker, wake_panic, wake_panic, noop);
 
-fn raw_panic_waker() -> RawWaker {
+const fn raw_panic_waker() -> RawWaker {
     RawWaker::new(null(), &PANIC_WAKER_VTABLE)
 }
 
@@ -34,6 +33,7 @@ fn raw_panic_waker() -> RawWaker {
 /// waker.wake(); // Will panic
 /// ```
 pub fn panic_waker() -> Waker {
+    // FIXME: Since 1.46.0 we can use transmute in consts, allowing this function to be const.
     unsafe { Waker::from_raw(raw_panic_waker()) }
 }
 
@@ -50,8 +50,13 @@ pub fn panic_waker() -> Waker {
 /// waker.wake_by_ref(); // Will panic
 /// ```
 pub fn panic_waker_ref() -> &'static Waker {
-    static PANIC_WAKER_INSTANCE: Lazy<Waker> = Lazy::new(panic_waker);
-    &*PANIC_WAKER_INSTANCE
+    struct SyncRawWaker(RawWaker);
+    unsafe impl Sync for SyncRawWaker {}
+
+    static PANIC_WAKER_INSTANCE: SyncRawWaker = SyncRawWaker(raw_panic_waker());
+
+    // SAFETY: `Waker` is #[repr(transparent)] over its `RawWaker`.
+    unsafe { &*(&PANIC_WAKER_INSTANCE.0 as *const RawWaker as *const Waker) }
 }
 
 #[cfg(test)]