error: `#[no_mangle]` cannot be used on internal language items
  --> $DIR/no-mangle-on-panic-handler.rs:10:1
   |
LL | #[unsafe(no_mangle)]
   | ^^^^^^^^^^^^^^^^^^^^
LL | #[panic_handler]
LL | pub unsafe fn panic_fmt(pi: &PanicInfo) -> ! {
   | -------------------------------------------- should be the internal language item
   |
   = note: Rustc requires this item to have a specific mangled name.
   = note: If you are trying to prevent mangling to ease debugging, many
   = note: debuggers support a command such as `rbreak rust_begin_unwind` to
   = note: match `.*rust_begin_unwind.*` instead of `break rust_begin_unwind` on a specific name

error: aborting due to 1 previous error