Playground link

In the playground, an async fn is marked #[inline(always)]:

#[inline(always)]
pub async fn test() -> i32 {
    12345
}

However, if you compile it in debug mode (where inlining only happens for #[inline(always)] functions), and search for 12345 in the generated assembly, you can see that it is not inlined.

Indeed, there are multiple levels of function calls that are not inlined:

run_it
→ GenFuture<T>::poll
→ std::future::set_task_context
→ GenFuture<T>::poll::{closure}
→ playground::test::{{closure}}

That last closure is the generator that contains the actual body of test.

#[inline(always)] is taking effect on the post-transformation function test, but all that does is initialize the generator struct.

As long as async is implemented based on generators, this will be hard to fix. Even if the generator itself were marked alwaysinline, that wouldn't affect GenFuture or set_task_context, both of which are from libstd.

Related to #62918, since if you want an async fn to be #[inline(always)], you probably also want to get rid of the TLS usage by set_task_context.