linux-kernel - [PATCH v4 10/14] rust: alloc: add `Box::into

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Message-Id: <20241206-hrtimer-v3-v6-12-rc2-v4-10-6cb8c3673682@kernel.org>
Date: Fri, 06 Dec 2024 20:33:02 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Miguel Ojeda <ojeda@...nel.org>, 
 Anna-Maria Behnsen <anna-maria@...utronix.de>, 
 Frederic Weisbecker <frederic@...nel.org>, 
 Thomas Gleixner <tglx@...utronix.de>, Danilo Krummrich <dakr@...nel.org>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
 Gary Guo <gary@...yguo.net>, 
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
 Benno Lossin <benno.lossin@...ton.me>, Alice Ryhl <aliceryhl@...gle.com>, 
 Trevor Gross <tmgross@...ch.edu>, Lyude Paul <lyude@...hat.com>, 
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Andreas Hindborg <a.hindborg@...nel.org>
Subject: [PATCH v4 10/14] rust: alloc: add `Box::into_pin`

Add an associated function to convert a `Box<T>` into a `Pin<Box<T>>`.

Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
---
 rust/kernel/alloc/kbox.rs | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index 9ce414361c2c6dd8eea09b11041f6c307cbc7864..1a993ec8602b37041c192458d8b6acff30769a04 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -245,6 +245,16 @@ pub fn pin(x: T, flags: Flags) -> Result<Pin<Box<T, A>>, AllocError>
         Ok(Self::new(x, flags)?.into())
     }
 
+    /// Convert a [`Box<T,A>`] to a [`Pin<Box<T,A>>`]. If `T` does not implement
+    /// [`Unpin`], then `x` will be pinned in memory and can't be moved.
+    pub fn into_pin(boxed: Self) -> Pin<Self> {
+        // SAFETY: `Self` is guaranteed to be the only pointer to the boxed
+        // value. Thus, if `T: !Unpin`, `T` is guaranteed to stay pinned; there
+        // is no way to get rid of the `Pin` and move out of the returned
+        // `Pin<Box<T>>`.
+        unsafe { Pin::new_unchecked(boxed) }
+    }
+
     /// Forgets the contents (does not run the destructor), but keeps the allocation.
     fn forget_contents(this: Self) -> Box<MaybeUninit<T>, A> {
         let ptr = Self::into_raw(this);

-- 
2.46.0