[WIP] provide defaults for DefaultHandler and HardFault

`exception!(HardFault, ..)` and `exception!(*, ..)` can now be omitted from
programs to pick up the default behavior of an infinite loop. Existing programs
that define these exception handlers will continue to work w/o any functional
change.

closes #72

Author: japaric

cortex-m-rt/link.x.in

@@ -45,6 +45,9 @@ PROVIDE(DebugMonitor = DefaultHandler);
 PROVIDE(PendSV = DefaultHandler);
 PROVIDE(SysTick = DefaultHandler);
 
+PROVIDE(DefaultHandler = DefaultDefaultHandler);
+PROVIDE(UserHardFault = DefaultUserHardFault);
+
 /* # Interrupt vectors */
 EXTERN(__INTERRUPTS); /* `static` variable similar to `__EXCEPTIONS` */
 

cortex-m-rt/src/lib.rs

@@ -390,7 +390,7 @@
 
 extern crate r0;
 
-use core::fmt;
+use core::{fmt, ptr};
 
 /// Registers stacked (pushed into the stack) during an exception
 #[derive(Clone, Copy)]
@@ -511,6 +511,26 @@ pub unsafe extern "C" fn Reset() -> ! {
     }
 }
 
+#[doc(hidden)]
+#[no_mangle]
+pub unsafe extern "C" fn DefaultDefaultHandler() {
+    loop {
+        // add some side effect to prevent this from turning into a UDF instruction
+        // see rust-lang/rust#28728
+        ptr::read_volatile(&0u8);
+    }
+}
+
+#[doc(hidden)]
+#[no_mangle]
+pub unsafe extern "C" fn DefaultUserHardFault() {
+    loop {
+        // add some side effect to prevent this from turning into a UDF instruction
+        // see rust-lang/rust#28728
+        ptr::read_volatile(&0u8);
+    }
+}
+
 /// Macro to define the entry point of the program
 ///
 /// **NOTE** This macro must be invoked once and must be invoked from an accessible module, ideally