samples: hello_world: Change to logging

Change the hello world sample to use Rust's logging mechanism.  This is
configured to not enable logging in Zephyr, so the output will go to the
same printk mechanism it went to before.

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

Author: d3zd3z

samples/hello_world/Cargo.toml

@@ -14,3 +14,4 @@ crate-type = ["staticlib"]
 
 [dependencies]
 zephyr = "3.7.0"
+log = "0.4.22"

samples/hello_world/prj.conf

@@ -2,3 +2,6 @@
 # SPDX-License-Identifier: Apache-2.0
 
 CONFIG_RUST=y
+
+# CONFIG_RUST_ALLOC=y
+# CONFIG_LOG=y

samples/hello_world/src/lib.rs

@@ -3,14 +3,11 @@
 
 #![no_std]
 
-use zephyr::printkln;
-
-// Reference the Zephyr crate so that the panic handler gets used.  This is only needed if no
-// symbols from the crate are directly used.
-extern crate zephyr;
+use log::info;
 
 #[no_mangle]
 extern "C" fn rust_main() {
-    printkln!("Hello world from Rust on {}",
-              zephyr::kconfig::CONFIG_BOARD);
+    unsafe { zephyr::set_logger().unwrap(); }
+
+    info!("Hello world from Rust on {}", zephyr::kconfig::CONFIG_BOARD);
 }