Revert "Changed Hardfault's and DefaultHander's default implementations to panic"

This reverts commit ffffb7b.

Author: jonas-schievink

cortex-m-rt/src/lib.rs

@@ -199,15 +199,14 @@
 //! won't find it.
 //!
 //! - `DefaultHandler`. This is the default handler. If not overridden using `#[exception] fn
-//! DefaultHandler(..` this will cause a panic with the message "DefaultHandler #`i`", where `i` is
-//! the number of the interrupt handler.
+//! DefaultHandler(..` this will be an infinite loop.
 //!
 //! - `HardFaultTrampoline`. This is the real hard fault handler. This function is simply a
 //! trampoline that jumps into the user defined hard fault handler named `HardFault`. The
 //! trampoline is required to set up the pointer to the stacked exception frame.
 //!
 //! - `HardFault`. This is the user defined hard fault handler. If not overridden using
-//! `#[exception] fn HardFault(..` it will default to a panic with message "HardFault".
+//! `#[exception] fn HardFault(..` it will default to an infinite loop.
 //!
 //! - `__STACK_START`. This is the first entry in the `.vector_table` section. This symbol contains
 //! the initial value of the stack pointer; this is where the stack will be located -- the stack
@@ -442,6 +441,7 @@ extern crate cortex_m_rt_macros as macros;
 extern crate r0;
 
 use core::fmt;
+use core::sync::atomic::{self, Ordering};
 
 /// Attribute to declare an interrupt (AKA device-specific exception) handler
 ///
@@ -990,17 +990,21 @@ pub unsafe extern "C" fn Reset() -> ! {
 #[link_section = ".HardFault.default"]
 #[no_mangle]
 pub unsafe extern "C" fn HardFault_(ef: &ExceptionFrame) -> ! {
-    panic!("HardFault");
+    loop {
+        // add some side effect to prevent this from turning into a UDF instruction
+        // see rust-lang/rust#28728 for details
+        atomic::compiler_fence(Ordering::SeqCst);
+    }
 }
 
 #[doc(hidden)]
 #[no_mangle]
 pub unsafe extern "C" fn DefaultHandler_() -> ! {
-    const SCB_ICSR: *const u32 = 0xE000_ED04 as *const u32;
-
-    let irqn = core::ptr::read(SCB_ICSR) as u8 as i16 - 16;
-
-    panic!("DefaultHandler #{}", irqn);
+    loop {
+        // add some side effect to prevent this from turning into a UDF instruction
+        // see rust-lang/rust#28728 for details
+        atomic::compiler_fence(Ordering::SeqCst);
+    }
 }
 
 #[doc(hidden)]