Naked functions don't produce an active frame for asm! .seh_* directives on Windows

[bkolobara]

I'm trying to port some Unix asm! code, using the .cfi_* directives inside naked functions to provide information to the unwinder/debugger about call frames, to Windows.

The following code compiles on MacOs and Linux under rustc 1.47.0-nightly (f44c6e4e2 2020-08-24):

#[naked]
unsafe extern "C" fn a() {
    asm!(".cfi_def_cfa rbp, 16")
}

If I try to do something similar on Windows:

#[naked]
unsafe extern "C" fn a() {
    asm!(".seh_setframe rbp, 16")
}

I get: error: .seh_ directive must appear within an active frame. Only if I add .seh_proc a, .seh_endproc and declare the function #[no_mangle] the code compiles:

#[naked]
#[no_mangle]
unsafe extern "C" fn a() {
    asm!(
        ".seh_proc a",
        ".seh_setframe rbp, 16",
        ".seh_endproc"
    )
}

I don't think this is correct though, as the return statement in the naked function happens after .seh_endproc and is excluded from the frame information.

I expect the #[naked] function to behave the same way on Windows as on Unix platforms, where a cfi_startproc is implicitly added to #[naked] functions.

[mati865]

Could you post full reproduction steps?

[bkolobara]

// lib.rs
#![feature(naked_functions, asm)]

#[naked]
pub unsafe extern "C" fn a() {
    asm!(".seh_setframe rbp, 16")
}

Running: rustc --crate-type=lib lib.rs on Windows produces:

error: .seh_ directive must appear within an active frame
 --> lib.rs:5:11
  |
5 |     asm!(".seh_setframe rbp, 16")
  |           ^
  |
note: instantiated into assembly here
 --> <inline asm>:2:2
  |
2 |     .seh_setframe rbp, 16
  |     ^

error: aborting due to previous error