rust/kernel: define a fallible implementation of Arc::pin

This is convenient to create pinned pointers inside the kernel,
without having to use/prove an `unsafe` block each time.

Defined as a trait, so it can be implemented for `core` pointers
such as `Arc`, `Rc`, or `Box`.

Includes a sample implementation for `Arc`.

Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>

Author: Sven Van Asbroeck

rust/kernel/lib.rs

@@ -44,6 +44,7 @@ pub mod file;
 pub mod file_operations;
 pub mod miscdev;
 pub mod pages;
+pub mod traits;
 
 pub mod linked_list;
 mod raw_list;

rust/kernel/traits.rs

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Traits useful to Drivers, and their implementations for common types.
+
+use core::{ops::Deref, pin::Pin};
+
+use alloc::{alloc::AllocError, sync::Arc};
+
+/// Trait which implements a fallible version of `pin()` for any pointer type.
+///
+/// Common pointer types which implement `pin()` include `Box`, `Arc` and `Rc`.
+pub trait TryPin<P: Deref> {
+    /// Constructs a new Pin<pointer<T>>. If T does not implement Unpin, then data
+    /// will be pinned in memory and unable to be moved. An error will be returned
+    /// if allocation fails.
+    fn try_pin(data: P::Target) -> core::result::Result<Pin<P>, AllocError>;
+}
+
+impl<T> TryPin<Arc<T>> for Arc<T> {
+    fn try_pin(data: T) -> core::result::Result<Pin<Arc<T>>, AllocError> {
+        // SAFETY: the data T is exposed only through a `Pin<Arc<T>>`, which
+        // does not allow data to move out of the `Arc`. Therefore it can
+        // never be moved.
+        Ok(unsafe { Pin::new_unchecked(Arc::try_new(data)?) })
+    }
+}