[PATCH 2/2] rust: sync: arc: Add UniqueArc<MaybeUninit<T>::assume_init()

[Asahi Lina]

We can already create `UniqueArc<MaybeUninit<T>>` instances with
`UniqueArc::try_new_uninit()` and write to them with `write()`. Add
the missing unsafe `assume_init()` function to promote it to
`UniqueArc<T>`, so users can do piece-wise initialization of the
contents instead of doing it all at once as long as they keep the
invariants (the same requirements as `MaybeUninit::assume_init()`).

This mirrors the std `Arc::assume_init()` function. In the kernel,
since we have `UniqueArc`, arguably this only belongs there since most
use cases will initialize it immediately after creating it, before
demoting it to `Arc` to share it.

Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
---
 rust/kernel/sync/arc.rs | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 752bd7c4699e..b8e9477fe865 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -512,6 +512,15 @@ impl<T> UniqueArc<MaybeUninit<T>> {
     /// Converts a `UniqueArc<MaybeUninit<T>>` into a `UniqueArc<T>` by writing a value into it.
     pub fn write(mut self, value: T) -> UniqueArc<T> {
         self.deref_mut().write(value);
+        // SAFETY: We have just written the contents fully.
+        unsafe { self.assume_init() }
+    }
+
+    /// Returns a UniqueArc<T>, assuming the MaybeUninit<T> has already been initialized.
+    ///
+    /// # Safety
+    /// The contents of the UniqueArc must have already been fully initialized.
+    pub unsafe fn assume_init(self) -> UniqueArc<T> {
         let inner = ManuallyDrop::new(self).inner.ptr;
         UniqueArc {
             // SAFETY: The new `Arc` is taking over `ptr` from `self.inner` (which won't be

-- 
2.35.1