rust: Gate panic messages behind a feature

The ability to get the panic message in a custom handler has only been
stabilized in Rust 1.81.  Gate this behind a feature so that CI, which
is using an older version of Rust will still be able to compile.

Users wishing to have panic messages can enable this feature in their
app's Cargo.toml.  This will require them to have a newer Rust
toolchain.

Signed-off-by: David Brown <[email protected]>

lib/rust/zephyr/Cargo.toml

@@ -20,3 +20,11 @@ version = "0.3.7"
 # used by the core Zephyr tree, but are needed by zephyr applications.
 [build-dependencies]
 zephyr-build = { version = "0.1.0", path = "../zephyr-build" }
+
+[features]
+default = []
+
+# The panic message is only stabilizesd in Rust 1.81.  Gate this behind a feature, as the CI is
+# using an older version of Rustc.  Users can use this functionality by setting this feature in
+# their Cargo.toml.
+panic-message = []

lib/rust/zephyr/src/lib.rs

@@ -28,10 +28,11 @@ use core::panic::PanicInfo;
 /// Override rust's panic.  This simplistic initial version just hangs in a loop.
 #[panic_handler]
 fn panic(info :&PanicInfo) -> ! {
-    #[cfg(CONFIG_PRINTK)]
+    #[cfg(all(CONFIG_PRINTK, feature = "panic-message"))]
     {
         printkln!("panic: {}", info.message());
     }
+    let _ = info;
     loop {
     }
 }