Change exception enum discovery in macros

Similar to the way the cortex-m-rt macros
discover enum variants for interrupts, we update
the #[exception] macro to search for exception
enum variants that may not directly come from
the cortex-m-rt crate. The change allows
developers who are interested in making their
own runtime crates re-use the macros in this
crate, and provide their own implementation of
the exception variants.

macros/src/lib.rs

@@ -462,10 +462,9 @@ pub fn exception(args: TokenStream, input: TokenStream) -> TokenStream {
                 #[export_name = #ident_s]
                 #(#attrs)*
                 pub #unsafety extern "C" fn #hash() {
-                    extern crate cortex_m_rt;
 
                     // check that this exception actually exists
-                    cortex_m_rt::Exception::#ident;
+                    exception::#ident;
 
                     #(#vars)*
 

src/lib.rs

@@ -592,6 +592,8 @@ pub enum Exception {
     SysTick,
 }
 
+pub use self::Exception as exception;
+
 extern "C" {
     fn NonMaskableInt();