Mark pre_init as deprecated

Author: romancardenas

cortex-m-rt/CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- Mark `pre_init` as deprecated
 - Add `set_msplim` feature to conditionally set the MSPLIM register at device
   reset ([#580]).
 

cortex-m-rt/examples/pre_init.rs

@@ -7,11 +7,16 @@
 extern crate cortex_m_rt as rt;
 extern crate panic_halt;
 
-use rt::{entry, pre_init};
+use rt::entry;
 
-#[pre_init]
-unsafe fn disable_watchdog() {
-    // Do what you need to disable the watchdog.
+// This function is called before the RAM is initialized.
+// For example, it can be used to disable the watchdog.
+core::arch::global_asm! {
+    r#"
+__pre_init:
+    // Do what you need to do before RAM is initialized.
+    bx lr
+    "#
 }
 
 #[entry]

cortex-m-rt/macros/src/lib.rs

@@ -628,6 +628,7 @@ pub fn interrupt(args: TokenStream, input: TokenStream) -> TokenStream {
 }
 
 #[proc_macro_attribute]
+#[deprecated(note = "Use core::arch::global_asm! to define the __pre_init function instead")]
 pub fn pre_init(args: TokenStream, input: TokenStream) -> TokenStream {
     let f = parse_macro_input!(input as ItemFn);
 

cortex-m-rt/tests/compile-fail/pre-init-args.rs

@@ -7,6 +7,7 @@ extern crate panic_halt;
 use cortex_m_rt::{entry, pre_init};
 
 #[pre_init(foo)] //~ ERROR This attribute accepts no arguments
+                 //~^ WARNING Use core::arch::global_asm! to define the __pre_init function instead
 unsafe fn foo() {}
 
 #[entry]