[PATCH 1/2] rust: sync: arc: implement Arc<dyn Any + Send + Sync>::downcast()

[Asahi Lina]

This mirrors the standard library's alloc::sync::Arc::downcast().

Based on the Rust standard library implementation, ver 1.62.0,
licensed under "Apache-2.0 OR MIT", from:

    https://github.com/rust-lang/rust/tree/1.62.0/library/alloc/src

For copyright details, please see:

    https://github.com/rust-lang/rust/blob/1.62.0/COPYRIGHT

Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
---
 rust/kernel/sync/arc.rs | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index a94e303217c6..752bd7c4699e 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -22,6 +22,7 @@ use crate::{
 };
 use alloc::boxed::Box;
 use core::{
+    any::Any,
     fmt,
     marker::{PhantomData, Unsize},
     mem::{ManuallyDrop, MaybeUninit},
@@ -220,6 +221,27 @@ impl<T: 'static> ForeignOwnable for Arc<T> {
     }
 }
 
+impl Arc<dyn Any + Send + Sync> {
+    /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
+    // Based on the Rust standard library implementation, ver 1.62.0, which is
+    // Apache-2.0 OR MIT.
+    pub fn downcast<T>(self) -> core::result::Result<Arc<T>, Self>
+    where
+        T: Any + Send + Sync,
+    {
+        if (*self).is::<T>() {
+            // SAFETY: We have just checked that the type is correct, so we can cast the pointer.
+            unsafe {
+                let ptr = self.ptr.cast::<ArcInner<T>>();
+                core::mem::forget(self);
+                Ok(Arc::from_inner(ptr))
+            }
+        } else {
+            Err(self)
+        }
+    }
+}
+
 impl<T: ?Sized> Deref for Arc<T> {
     type Target = T;
 

-- 
2.35.1