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 allows users to avoid allocation.

Signed-off-by: Oleksandr Babak <alexanderbabak@proton.me>

Author: Ddystopia

src/lib.rs

@@ -1449,6 +1449,28 @@ 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: `self` is a valid pointer to uninitialized memory.
+        unsafe { init.__init(slot)? };
+        // SAFETY: The above call initialized the memory.
+        Ok(unsafe { &mut *slot })
+    }
+
+    fn write_pin_init<E>(self, init: impl PinInit<T, E>) -> Result<Pin<Self::Initialized>, E> {
+        let slot = self.as_mut_ptr();
+        // SAFETY: `self` is a valid pointer to uninitialized memory.
+        unsafe { init.__pinned_init(slot)? };
+        // SAFETY:
+        //   - The above call initialized the memory.
+        //   - The 'static borrow guarantees the data will not be moved/invalidated
+        //     until it gets dropped (which is never).
+        Ok(unsafe { Pin::new_unchecked(&mut *slot) })
+    }
+}
+
 /// Trait facilitating pinned destruction.
 ///
 /// Use [`pinned_drop`] to implement this trait safely: