Rollup merge of rust-lang#91828 - oxalica:feat/waker-getters, r=dtolnay

Implement `RawWaker` and `Waker` getters for underlying pointers

implement rust-lang#87021

New APIs:
- `RawWaker::data(&self) -> *const ()`
- `RawWaker::vtable(&self) -> &'static RawWakerVTable`
- ~`Waker::as_raw_waker(&self) -> &RawWaker`~ `Waker::as_raw(&self) -> &RawWaker`

This third one is an auxiliary function to make the two APIs above more useful. Since we can only get `&Waker` in `Future::poll`, without this, we need to `transmute` it into `&RawWaker` (relying on `repr(transparent)`) in order to access its data/vtable pointers.

~Not sure if it should be named `as_raw` or `as_raw_waker`. Seems we always use `as_<something-raw>` instead of just `as_raw`. But `as_raw_waker` seems not quite consistent with `Waker::from_raw`.~ As suggested in rust-lang#91828 (comment), use `as_raw`.

library/core/src/task/wake.rs

@@ -43,6 +43,22 @@ impl RawWaker {
     pub const fn new(data: *const (), vtable: &'static RawWakerVTable) -> RawWaker {
         RawWaker { data, vtable }
     }
+
+    /// Get the `data` pointer used to create this `RawWaker`.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "waker_getters", issue = "87021")]
+    pub fn data(&self) -> *const () {
+        self.data
+    }
+
+    /// Get the `vtable` pointer used to create this `RawWaker`.
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "waker_getters", issue = "87021")]
+    pub fn vtable(&self) -> &'static RawWakerVTable {
+        self.vtable
+    }
 }
 
 /// A virtual function pointer table (vtable) that specifies the behavior
@@ -260,6 +276,14 @@ impl Waker {
     pub unsafe fn from_raw(waker: RawWaker) -> Waker {
         Waker { waker }
     }
+
+    /// Get a reference to the underlying [`RawWaker`].
+    #[inline]
+    #[must_use]
+    #[unstable(feature = "waker_getters", issue = "87021")]
+    pub fn as_raw(&self) -> &RawWaker {
+        &self.waker
+    }
 }
 
 #[stable(feature = "futures_api", since = "1.36.0")]

library/core/tests/lib.rs

@@ -90,6 +90,7 @@
 #![feature(unzip_option)]
 #![feature(const_array_from_ref)]
 #![feature(const_slice_from_ref)]
+#![feature(waker_getters)]
 #![deny(unsafe_op_in_unsafe_fn)]
 
 extern crate test;
@@ -131,3 +132,4 @@ mod task;
 mod time;
 mod tuple;
 mod unicode;
+mod waker;

library/core/tests/waker.rs

@@ -0,0 +1,22 @@
+use std::ptr;
+use std::task::{RawWaker, RawWakerVTable, Waker};
+
+#[test]
+fn test_waker_getters() {
+    let raw_waker = RawWaker::new(42usize as *mut (), &WAKER_VTABLE);
+    assert_eq!(raw_waker.data() as usize, 42);
+    assert!(ptr::eq(raw_waker.vtable(), &WAKER_VTABLE));
+
+    let waker = unsafe { Waker::from_raw(raw_waker) };
+    let waker2 = waker.clone();
+    let raw_waker2 = waker2.as_raw();
+    assert_eq!(raw_waker2.data() as usize, 43);
+    assert!(ptr::eq(raw_waker2.vtable(), &WAKER_VTABLE));
+}
+
+static WAKER_VTABLE: RawWakerVTable = RawWakerVTable::new(
+    |data| RawWaker::new((data as usize + 1) as *mut (), &WAKER_VTABLE),
+    |_| {},
+    |_| {},
+    |_| {},
+);