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.

Co-authored-by: Benno Lossin <y86-dev@protonmail.com>
Signed-off-by: Benno Lossin <y86-dev@protonmail.com>
Signed-off-by: Asahi Lina <lina@asahilina.net>
---
 rust/kernel/sync/arc.rs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index b904ed898d3a..b664b41e473d 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -489,6 +489,17 @@ 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() }
+    }
+
+    /// Unsafely assume that `self` is initialized.
+    ///
+    /// # Safety
+    ///
+    /// The caller guarantees that the value behind this pointer has been initialized. It is
+    /// *immediate* UB to call this when the value is not 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

-- 
2.40.0

On Mon, 03 Apr 2023 19:01:12 +0900
Asahi Lina <lina@asahilina.net> wrote:

> 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.
> 
> Co-authored-by: Benno Lossin <y86-dev@protonmail.com>
> Signed-off-by: Benno Lossin <y86-dev@protonmail.com>
> Signed-off-by: Asahi Lina <lina@asahilina.net>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/sync/arc.rs | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index b904ed898d3a..b664b41e473d 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -489,6 +489,17 @@ 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() }
> +    }
> +
> +    /// Unsafely assume that `self` is initialized.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller guarantees that the value behind this pointer has been initialized. It is
> +    /// *immediate* UB to call this when the value is not 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
>