defmt::export::acquire appears twice in optimized machine code

[japaric]

Steps to reproduce

Start from app-template, configure it to use the main branch of defmt then modify hello to look like this

fn main() -> ! {
    app::export::acquire();
    app::exit()
}

$ cargo build --bin hello --release

$ arm-none-eabi-objdump -Cd target/thumbv7em-none-eabihf/release/hello

 000001ca <defmt::export::acquire>:
  1ca:   b5f0        push    {r4, r5, r6, r7, lr}
  1cc:   af03        add r7, sp, #12
(..)
 252:   f7ff ffae   bl  1b2 <core::panicking::panic>
 256:   defe        udf #254    ; 0xfe

(..)

00000356 <_defmt_acquire>:
 356:   b5f0        push    {r4, r5, r6, r7, lr}
 358:   af03        add r7, sp, #12
(..)
 3de:   f7ff fee8   bl  1b2 <core::panicking::panic>
 3e2:   defe        udf #254    ; 0xfe

Above _defmt_acquire and defmt::export::acquire are exact duplicates.

---

defmt::export::acquire is defined like this:

#[inline(never)]
pub fn acquire() {
    extern "Rust" {
        fn _defmt_acquire();
    }
    unsafe { _defmt_acquire() }
}

_defmt_acquire is defined in defmt-rtt. My guess is that _defmt_acquire gets inlined in the export::acquire BUT a full copy of it stays around because it has both #[no_mangle] on it and we have EXTERN(_defmt_acquire) in the defmt.x linker script.

Unsure how to fix. I wouldn't expect EXTERN to act like KEEP (LLD issue?). Removing EXTERN is likely to cause linker errors in some cases.

I think this issue also applies to other _defmt_* symbols but haven't checked

[jonas-schievink]

Could we make export::acquire #[inline] and _defmt_acquire #[inline(never)]?

[japaric]

That should work!

(Marking export::acquire inline(never), as it is right now, does seems odd. _defmt_acquire is the "binary interface" and that's what should be kept around as a symbol so marking that one inline(never) makes more sense)

This also appears to affect release v0.2.3