samples: rust: print: Add sample code for Arc printing

This both demonstrates the usage of different print format in Rust and
serves as a selftest for the `Display` and `Debug` implementation of
`Arc` and its friends.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Reviewed-by: Finn Behrens <fin@nyantec.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230207185216.1314638-3-boqun.feng@gmail.com
[ Applied suggestions and reworded for fixing title typos. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

samples/rust/rust_print.rs

@@ -15,6 +15,30 @@ module! {
 
 struct RustPrint;
 
+fn arc_print() -> Result {
+    use kernel::sync::*;
+
+    let a = Arc::try_new(1)?;
+    let b = UniqueArc::try_new("hello, world")?;
+
+    // Prints the value of data in `a`.
+    pr_info!("{}", a);
+
+    // Uses ":?" to print debug fmt of `b`.
+    pr_info!("{:?}", b);
+
+    let a: Arc<&str> = b.into();
+    let c = a.clone();
+
+    // Uses `dbg` to print, will move `c` (for temporary debugging purposes).
+    dbg!(c);
+
+    // Pretty-prints the debug formatting with lower-case hexadecimal integers.
+    pr_info!("{:#x?}", a);
+
+    Ok(())
+}
+
 impl kernel::Module for RustPrint {
     fn init(_module: &'static ThisModule) -> Result<Self> {
         pr_info!("Rust printing macros sample (init)\n");
@@ -43,6 +67,8 @@ impl kernel::Module for RustPrint {
         pr_cont!(" is {}", "continued");
         pr_cont!(" with {}\n", "args");
 
+        arc_print()?;
+
         Ok(RustPrint)
     }
 }