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 <alexanderbabak@proton.me>

Author: Ddystopia

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - `[pin_]init_scope` functions to run arbitrary code inside of an initializer.
+- `&'static mut MaybeUninit<T>` now implements `InPlaceWrite`. This enables users to use external
+  allocation mechanisms such as `static_cell`.
 
 ### Changed
 

src/lib.rs

@@ -1536,6 +1536,33 @@ 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.
+        //
+        // The `'static` borrow guarantees the data will not be
+        // moved/invalidated until it gets dropped (which is never).
+        unsafe { init.__pinned_init(slot)? };
+
+        // SAFETY: The above call initialized the memory.
+        Ok(Pin::static_mut(unsafe { self.assume_init_mut() }))
+    }
+}
+
 /// Trait facilitating pinned destruction.
 ///
 /// Use [`pinned_drop`] to implement this trait safely: