error[E0391]: cycle detected when computing type of async fn

[Diggsey]

I tried this code:

use futures::future::FutureExt;

async fn foo() {
    FutureExt::boxed(async {
        foo().await;
    });
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=19653eea2af1dd97a8d48126dfd7d683

I expected it to compile, but it gave the following error:

error[E0391]: cycle detected when computing type of foo::{{opaque}}#0

edit:
With one minor change, it compiles:

async fn foo() {
    Box::new(async {
        foo().await;
    });
}

There are no rules that would explain why the first should fail when this succeeds.

[camelid]

A bit of a weird error message:

error[E0391]: cycle detected when computing type of `foo::{{opaque}}#0`
 --> src/main.rs:3:16
  |
3 | async fn foo() {
  |                ^
  |
note: ...which requires borrow-checking `foo`...
 --> src/main.rs:3:1
  |
3 | async fn foo() {
  | ^^^^^^^^^^^^^^
note: ...which requires processing `foo`...
 --> src/main.rs:3:1
  |
3 | async fn foo() {
  | ^^^^^^^^^^^^^^
note: ...which requires processing MIR for `foo`...
 --> src/main.rs:3:1
  |
3 | async fn foo() {
  | ^^^^^^^^^^^^^^
note: ...which requires unsafety-checking `foo`...
 --> src/main.rs:3:1
  |
3 | async fn foo() {
  | ^^^^^^^^^^^^^^
note: ...which requires building MIR for `foo`...
 --> src/main.rs:3:1
  |
3 | async fn foo() {
  | ^^^^^^^^^^^^^^
note: ...which requires type-checking `foo`...
 --> src/main.rs:3:1
  |
3 | async fn foo() {
  | ^^^^^^^^^^^^^^
  = note: ...which requires evaluating trait selection obligation `{std::future::ResumeTy, impl futures::Future, ()}: std::marker::Send`...
  = note: ...which again requires computing type of `foo::{{opaque}}#0`, completing the cycle
note: cycle used when checking item types in top-level module
 --> src/main.rs:1:1
  |
1 | / use futures::future::FutureExt;
2 | |
3 | | async fn foo() {
4 | |     FutureExt::boxed(async {
... |
8 | |
9 | | fn main() {}
  | |____________^

error: aborting due to previous error

[jyn514]

I think this is expected behavior? You can't have recursive async functions: https://www.reddit.com/r/rust/comments/cbdxxm/why_are_recursive_async_fns_forbidden/

[jyn514]

The error message is pretty bad, though.

[Diggsey]

@jyn514 this is not a recursive function.

[Diggsey]

For reference, the following very similar code snippet compiles just fine, and should be equivalent (modulo a Pin and some marker traits):

async fn foo() {
    Box::new(async {
        foo().await;
    });
}

[tmandry]

Discussed during the wg-async-foundations meeting.

It seems the difference in the examples could be coming from the signature of FutureExt::Boxed...

fn boxed<'a>(self) -> Pin<Box<dyn Future<Output = Self::Output> + 'a + Send>>

...demanding Self::Output, the return type of the async block, but that's clearly just (). EDIT: Maybe it's trying to make sure the async block body is well-formed before inferring its return type? That would explain why we're checking that the return type of foo is Send.

More spelunking will be needed to figure out where exactly the cycle is coming from and how hard it will be to avoid.