"cycle detected" with "const async" fn

[matthiaskrgr]

fn main() {
  const async fn a() {}
}

Seems a bit surprising that we don't bail out immediately when jumping from a function to itself? 🤔

rustc 1.66.0-nightly (0ca356586 2022-10-06)

error: functions cannot be both `const` and `async`
 --> src/main.rs:2:3
  |
2 |   const async fn a() {}
  |   ^^^^^-^^^^^----------
  |   |     |
  |   |     `async` because of this
  |   `const` because of this

error[[E0391]](https://doc.rust-lang.org/stable/error-index.html#E0391): cycle detected when computing type of `main::a::{opaque#0}`
 --> src/main.rs:2:22
  |
2 |   const async fn a() {}
  |                      ^
  |
note: ...which requires borrow-checking `main::a`...
 --> src/main.rs:2:3
  |
2 |   const async fn a() {}
  |   ^^^^^^^^^^^^^^^^^^
note: ...which requires processing `main::a`...
 --> src/main.rs:2:3
  |
2 |   const async fn a() {}
  |   ^^^^^^^^^^^^^^^^^^
note: ...which requires const checking `main::a`...
 --> src/main.rs:2:3
  |
2 |   const async fn a() {}
  |   ^^^^^^^^^^^^^^^^^^
  = note: ...which requires computing whether `impl core::future::future::Future<Output = ()>` is freeze...
  = note: ...which requires evaluating trait selection obligation `impl core::future::future::Future<Output = ()>: core::marker::Freeze`...
  = note: ...which again requires computing type of `main::a::{opaque#0}`, completing the cycle
note: cycle used when checking item types in top-level module
 --> src/main.rs:1:1
  |
1 | / fn main() {
2 | |   const async fn a() {}
3 | | }
  | |_^

[aDotInTheVoid]

Fixed, needs a test: https://godbolt.org/z/64TxYTTnd

[Milo123459]

how would i be able to add a test? on the godbolt it still fails and seems to have the same output as the issue.

@rustbot claim

[aDotInTheVoid]

the dev guide has information on how to add a test. The think we are testing for here is that when a const async function is declaired inside another function, the compiller has a parser error, but not a cycle detected error. That's the difference between the two outputs.

[Milo123459]

the stderr from this example in the rest test suite:

error: expected one of `:`, `;`, `<`, `=`, or `where`, found keyword `fn`
  --> $DIR/const-async-fn-in-main.rs:4:17
   |
LL |     const async fn a() {}
   |                 ^^ expected one of `:`, `;`, `<`, `=`, or `where`

error: missing type for `const` item
  --> $DIR/const-async-fn-in-main.rs:4:16
   |
LL |     const async fn a() {}
   |                ^ help: provide a type for the item: `: <type>`

error: aborting due to 2 previous errors

the code:

// Check what happens when a const async fn is in the main function (#102796)

fn main() {
    const async fn a() {}
  }
  

the comand:

 x test tests/ui/const-async/const-async-fn-in-main.rs --stage 1 --bless

[aDotInTheVoid]

I think you need to set the edition to 2018 or later, otherwise async isn't a keyword. If I remember correctly you need // edition:2021 at the top of the file, but check the dev-guide, it'll know

[Milo123459]

error: functions cannot be both `const` and `async`
  --> $DIR/const-async-fn-in-main.rs:5:5
   |
LL |     const async fn a() {};
   |     ^^^^^-^^^^^----------
   |     |     |
   |     |     `async` because of this
   |     `const` because of this

error: aborting due to previous error

error now looks like this. will pr.

[Milo123459]

#116178