rust: arc: add DispatchFromDyn<ArcBorrow<'_, U>> for ArcBorrow<'_, T>

This is part of the effort to minimize the differences of the `rust`
branch with respect to mainline in order to eventually drop it.

It is required to make the (fixed) examples compile.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/sync/arc.rs

@@ -436,6 +436,13 @@ pub struct ArcBorrow<'a, T: ?Sized + 'a> {
 // This is to allow [`ArcBorrow`] (and variants) to be used as the type of `self`.
 impl<T: ?Sized> core::ops::Receiver for ArcBorrow<'_, T> {}
 
+// This is to allow `ArcBorrow<U>` to be dispatched on when `ArcBorrow<T>` can be coerced into
+// `ArcBorrow<U>`.
+impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
+    for ArcBorrow<'_, T>
+{
+}
+
 impl<T: ?Sized> Clone for ArcBorrow<'_, T> {
     fn clone(&self) -> Self {
         *self