Always-inline closure passed via impl FnOnce doesn't inline

[calebzulawski]

An example (try it in debug configuration)

use std::arch::asm;

#[inline(always)]
fn call<T>(f: impl FnOnce() -> T) -> T {
    f()
}

#[inline(always)]
const fn identity<T>(f: T) -> T {
    f
}

pub fn f() {
    let closure = identity(#[inline(always)] || unsafe { asm!("pause") });
    call(closure)
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=17e7dce927938012e15cd503caeb0cce

I expected to see this happen: call_once marked alwaysinline

Instead, this happened: call_once marked inlinehint

The closure itself is inlined, however the call_once is not, and when using impl FnOnce, effectively defeats the purpose of the #[inline(always)]. Fn and FnMut don't appear to have this issue (or LLVM is more likely to inline them, at least).

Meta

rustc --version --verbose:

rustc 1.60.0 (7737e0b5c 2022-04-04)
binary: rustc
commit-hash: 7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c
commit-date: 2022-04-04
host: x86_64-unknown-linux-gnu
release: 1.60.0
LLVM version: 14.0.0

[RalfJung]

FnOnce uses a shim to convert the environment from by-val to by-ref. Probably that shim does not get the inline(always) attribute? But probably it should. Not sure how hard that is, I don't know much about the inner workings of our closure lowering.

Cc @compiler-errors @lcnr

[lcnr]

It should definitely be possible 🤔 we 'just' need InstanceKind::ClosureOnceShim to inherit the codegen_fn_attrs of the closure self_type