Implement InPlaceWrite<T> for &'static mut MaybeUninit<T>

This feature allows users to use `&'static mut MaybeUninit<T>` as a
place to initialize the value. It mirrors an existing implemetation
for `Box<MaybeUninit>`, but enables users to use external allocation
mechanisms such as `static_cell`.

Signed-off-by: Oleksandr Babak <[email protected]>

Author: Ddystopia

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- `&'static mut MaybeUninit<T>` now implements `InPlaceWrite`. This enables users to use external
+  allocation mechanisms such as `static_cell`.
+
 ### Changed
 
 - `#[pin_data]` now generates a `*Projection` struct similar to the `pin-project` crate.

src/lib.rs

@@ -1449,6 +1449,30 @@ pub trait InPlaceWrite<T> {
     fn write_pin_init<E>(self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E>;
 }
 
+impl<T> InPlaceWrite<T> for &'static mut MaybeUninit<T> {
+    type Initialized = &'static mut T;
+
+    fn write_init<E>(self, init: impl Init<T, E>) -> Result<Self::Initialized, E> {
+        let slot = self.as_mut_ptr();
+
+        // SAFETY: `slot` is a valid pointer to uninitialized memory.
+        unsafe { init.__init(slot)? };
+
+        // SAFETY: The above call initialized the memory.
+        unsafe { Ok(self.assume_init_mut()) }
+    }
+
+    fn write_pin_init<E>(self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
+        let slot = self.as_mut_ptr();
+
+        // SAFETY: `slot` is a valid pointer to uninitialized memory.
+        unsafe { init.__init(slot)? };
+
+        // SAFETY: The above call initialized the memory.
+        unsafe { Ok(Pin::static_mut(self.assume_init_mut())) }
+    }
+}
+
 /// Trait facilitating pinned destruction.
 ///
 /// Use [`pinned_drop`] to implement this trait safely: