Any reason why the `#[exception]` macro disallows `extern "C" fn`s?

I have the following signature for my HardFault handler:

#[exception(trampoline = false)]
#[naked]
unsafe fn HardFault() -> ! {
...

This triggers the compiler warning

warning: Rust ABI is unsupported in naked functions
note: #[warn(undefined_naked_function_abi)] on by default

Which could be avoided by modifying the signature to be:

#[exception(trampoline = false)]
#[naked]
unsafe extern "C" fn HardFault() -> ! {
...

But this is not accepted by the macro:

HardFault handler must have signature unsafe fn() -> !

Is there any reason for the macro to not allow specifying the ABI?

Also, could someone point me as to why does the following not produce the same warning?

#[pre_init]
#[naked]
unsafe fn pre_init() {
...

Start by inspecting the signature validation for the #[exception] macro and compare it with the #[pre_init] macro's handling of unsafe naked functions and ABIs. Determine whether extern "C" should be accepted for HardFault handlers and verify the behavior with focused macro tests, including the warning shown for the current signature.