naked_asm! on wasm emits invalid assembly

[tgross35]

#![feature(asm_experimental_arch)]
#![feature(naked_functions)]

#[naked]
pub extern "C" fn foo() {
    unsafe { core::arch::naked_asm!("nop") }
}

This should work (at least, I don't know of any reason it shouldn't), but it looks like the way we wrap it is something that LLVM doesn't like (trimmed output):

error: unknown directive
note: instantiated into assembly here
1 | .pushsection .text._ZN7example3foo17h5bf07194c275cbceE,"ax", @progbits

warning: .size directive ignored for function symbols
7 | .size _ZN7example3foo17h5bf07194c275cbceE, . - _ZN7example3foo17h5bf07194c275cbceE

error: unknown directive
note: instantiated into assembly here
8 | .popsection

error: Unmatched block construct(s) at function end: function
note: instantiated into assembly here
9 |

IR for reference:

module asm ".pushsection .text.foo,\22ax\22, @progbits"
module asm ".balign 4"
module asm ".globl foo"
module asm ".type foo, @function"
module asm "foo:"
module asm "nop"
module asm ".size foo, . - foo"
module asm ".popsection"

I don't know enough about wasm to know what is correct here, but looking at some wasm codegen it seems like .pushsection should become .section, .popsection should be dropped, and we should emit .functype directives (e.g. .functype somefunc (f64) -> (f64))

https://rust.godbolt.org/z/re5sverch

cc @folkertdev for naked functions and @daxpedda for knowing more about wasm-asm

[folkertdev]

Yes this was just missed so far, and I agree that it should work.

Supporting wasm-asm appears mostly straightforward, but emitting .functype is a little tricky (and requires at least this bugfix). Anyway, I'm working on it.