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

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Asahi Lina <lina@asahilina.net>
---
 rust/kernel/sync/arc.rs            |  2 ++
 rust/kernel/sync/arc/std_vendor.rs | 28 ++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index f2f1c83d72ba..b904ed898d3a 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -522,3 +522,5 @@ impl<T: ?Sized> DerefMut for UniqueArc<T> {
         unsafe { &mut self.inner.ptr.as_mut().data }
     }
 }
+
+mod std_vendor;
diff --git a/rust/kernel/sync/arc/std_vendor.rs b/rust/kernel/sync/arc/std_vendor.rs
new file mode 100644
index 000000000000..a66a0c2831b3
--- /dev/null
+++ b/rust/kernel/sync/arc/std_vendor.rs
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! The contents of this file come from the Rust standard library, hosted in
+//! the <https://github.com/rust-lang/rust> repository, licensed under
+//! "Apache-2.0 OR MIT" and adapted for kernel use. For copyright details,
+//! see <https://github.com/rust-lang/rust/blob/master/COPYRIGHT>.
+
+use crate::sync::{arc::ArcInner, Arc};
+use core::any::Any;
+
+impl Arc<dyn Any + Send + Sync> {
+    /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
+    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)
+        }
+    }
+}

-- 
2.40.0