Implement write() method for Box<MaybeUninit<T>>

This adds method similar to `MaybeUninit::write` main difference being
it returns owned `Box`. This can be used to elide copy from stack
safely, however it's not currently tested that the optimization actually
occurs.

Analogous methods are not provided for `Rc` and `Arc` as those need to
handle the possibility of sharing. Some version of them may be added in
the future.

This was discussed in #63291 which this change extends.

1 files changed, 2 insertions, 4 deletions

diff --git a/compiler/rustc_data_structures/src/functor.rs b/compiler/rustc_data_structures/src/functor.rs
index 920f7b1ed0a..2703af97a1b 100644
--- a/compiler/rustc_data_structures/src/functor.rs
+++ b/compiler/rustc_data_structures/src/functor.rs
@@ -31,11 +31,9 @@ impl<T> IdFunctor for Box<T> {
             let value = raw.read();
             // SAFETY: Converts `Box<T>` to `Box<MaybeUninit<T>>` which is the
             // inverse of `Box::assume_init()` and should be safe.
-            let mut raw: Box<mem::MaybeUninit<T>> = Box::from_raw(raw.cast());
+            let raw: Box<mem::MaybeUninit<T>> = Box::from_raw(raw.cast());
             // SAFETY: Write the mapped value back into the `Box`.
-            raw.write(f(value)?);
-            // SAFETY: We just initialized `raw`.
-            raw.assume_init()
+            Box::write(raw, f(value)?)
         })
     }
 }