Allow specifying function alignment

[repnop]

I've been recently writing a kernel for the RISC-V architecture, using strictly as much Rust as I can get away with and during which I've run into the need to have certain functions aligned to specific byte alignments (ex. any function placed in mtvec MUST be 4 byte aligned), and while I can seemingly accomplish this using asm!(".align <alignment>") I think it would make sense for that to be an attribute of the function itself, whether it be limited to extern "C" fns or valid on all ABIs. Example of what I am asking for:

#[no_mangle]
#[repr(align(4))]
extern "C" fn gets_put_in_mtvec() {
    // ... content here ...
}

[hanna-kruppe]

For context, how is this requirement addressed by C toolchains?

[repnop]

Using https://c.godbolt.org/z/E7ETx8 to verify, it looks like both GCC and Clang support using __attribute__((aligned(<alignment>))) to specify function alignment on a per-function basis

[nbdd0121]

I believe you would need to use assembly for interrupt handlers because naked functions don't work as intended anyway.

[Skallwar]

I'm on the exact same situation as @repnop, with the exact same context.
asm!(".align <alignment>") inside the function worked for you?

[Skallwar]

If this is something that should be part of the language, I would gladly try to implement it

[sampersand]

I'd also find this quite helpful for some low-level bit twiddling I'm doing.

[repnop]

@Skallwar oh, somehow I missed the notification from this issue. yeah, currently I'm (ab)using that in a #[naked] fn (I have no idea if it works for non-naked fns).

Opened a PR so we'll see where this goes :)

[memoryruins]

The tracking issue for #![feature(fn_align)] is now #82232