rust: use derive(CoercePointee) on rustc >= 1.84.0

The `kernel` crate relies on both `coerce_unsized` and `dispatch_from_dyn`
unstable features.

Alice Ryhl has proposed [1] the introduction of the unstable macro
`SmartPointer` to reduce such dependence, along with a RFC patch [2].
Since Rust 1.81.0 this macro, later renamed to `CoercePointee` in
Rust 1.84.0 [3], has been fully implemented with the naming discussion
resolved.

This feature is now on track to stabilization in the language.
In order to do so, we shall start using this macro in the `kernel` crate
to prove the functionality and utility of the macro as the justification
of its stabilization.

This patch makes this switch in such a way that the crate remains
backward compatible with older Rust compiler versions,
via the new Kconfig option `RUSTC_HAS_COERCE_POINTEE`.

A minimal demonstration example is added to the
`samples/rust/rust_print_main.rs` module.

Link: https://rust-lang.github.io/rfcs/3621-derive-smart-pointer.html [1]
Link: https://lore.kernel.org/all/20240823-derive-smart-pointer-v1-1-53769cd37239@google.com/ [2]
Link: https://github.com/rust-lang/rust/pull/131284 [3]
Signed-off-by: Xiangfei Ding <dingxiangfei2009@gmail.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20241203205050.679106-2-dingxiangfei2009@gmail.com
[ Fixed version to 1.84. Renamed option to `RUSTC_HAS_COERCE_POINTEE`
  to match `CC_HAS_*` ones. Moved up new config option, closer to the
  `CC_HAS_*` ones. Simplified Kconfig line. Fixed typos and slightly
  reworded example and commit. Added Link to PR. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/samples/rust/rust_print_main.rs b/samples/rust/rust_print_main.rs
index 7935b4772ec6..7e8af5f176a3 100644
--- a/samples/rust/rust_print_main.rs
+++ b/samples/rust/rust_print_main.rs
@@ -34,6 +34,24 @@ fn arc_print() -> Result {
     // Uses `dbg` to print, will move `c` (for temporary debugging purposes).
     dbg!(c);
 
+    {
+        // `Arc` can be used to delegate dynamic dispatch and the following is an example.
+        // Both `i32` and `&str` implement `Display`. This enables us to express a unified
+        // behaviour, contract or protocol on both `i32` and `&str` into a single `Arc` of
+        // type `Arc<dyn Display>`.
+
+        use core::fmt::Display;
+        fn arc_dyn_print(arc: &Arc<dyn Display>) {
+            pr_info!("Arc<dyn Display> says {arc}");
+        }
+
+        let a_i32_display: Arc<dyn Display> = Arc::new(42i32, GFP_KERNEL)?;
+        let a_str_display: Arc<dyn Display> = a.clone();
+
+        arc_dyn_print(&a_i32_display);
+        arc_dyn_print(&a_str_display);
+    }
+
     // Pretty-prints the debug formatting with lower-case hexadecimal integers.
     pr_info!("{:#x?}", a);