[E0670]: `async fn` is not permitted in Rust 2015 (but I'm using Rust 2022)

[amab8901]

Here's my code:

async fn main() {}

Here's the output:

error[E0670]: `async fn` is not permitted in Rust 2015
 --> main.rs:1:1
  |
1 | async fn main() {}
  | ^^^^^ to use `async fn`, switch to Rust 2018 or later
  |
  = help: set `edition = "2021"` in `Cargo.toml`
  = note: for more on editions, read https://doc.rust-lang.org/edition-guide

error[E0752]: `main` function is not allowed to be `async`
 --> main.rs:1:1
  |
1 | async fn main() {}
  | ^^^^^^^^^^^^^^^ `main` function is not allowed to be `async`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0670, E0752.
For more information about an error, try `rustc --explain E0670`.

Here is my Cargo.toml file:

[package]
name = "visualizer"
version = "0.1.0"
edition = '2021'

Here is what it auto-generates in Cargo.lock file:

# This file is automatically @generated by Cargo.
# It is not intended for manual editing.
version = 3

[[package]]
name = "visualizer"
version = "0.1.0"

ATTEMPTED SOLUTION # 1:

I removed the async keyword in async fn main() {} so it became fn main() {} in main.rs. Then I did the following in the terminal:

$ cargo fix --edition
    Checking visualizer v0.1.0 ( // I manually removed the path for privacy reasons )
warning: `src/main.rs` is already on the latest edition (2021), unable to migrate further

If you are trying to migrate from the previous edition (2018), the
process requires following these steps:

1. Start with `edition = "2018"` in `Cargo.toml`
2. Run `cargo fix --edition`
3. Modify `Cargo.toml` to set `edition = "2021"`
4. Run `cargo build` or `cargo test` to verify the fixes worked

More details may be found at
https://doc.rust-lang.org/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html

    Finished dev [unoptimized + debuginfo] target(s) in 0.29s

So I changed edition = "2021" into edition = "2015" in Cargo.toml. Then I did the following in the terminal:

$ cargo fix --edition
    Checking visualizer v0.1.0 ( // I manually removed the path for privacy reasons  )
   Migrating src/main.rs from 2015 edition to 2018
    Finished dev [unoptimized + debuginfo] target(s) in 0.95s

Then I edited back from edition = "2015" to edition = "2021" and did the following in the terminal:

$ cargo build
   Compiling visualizer v0.1.0 ( // I manually removed the path for privacy reasons )
    Finished dev [unoptimized + debuginfo] target(s) in 0.37s
$ cargo test
   Compiling visualizer v0.1.0 ( // I manually removed the path for privacy reasons  )
    Finished test [unoptimized + debuginfo] target(s) in 0.26s
     Running unittests ( // I manually removed the path for privacy reasons  )

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Then I changed back from fn main() {} to async fn main() {} in main.rs, and here's the output I get:

error[E0670]: `async fn` is not permitted in Rust 2015
 --> main.rs:1:1
  |
1 | async fn main() {}
  | ^^^^^ to use `async fn`, switch to Rust 2018 or later
  |
  = help: set `edition = "2021"` in `Cargo.toml`
  = note: for more on editions, read https://doc.rust-lang.org/edition-guide

error[E0752]: `main` function is not allowed to be `async`
 --> main.rs:1:1
  |
1 | async fn main() {}
  | ^^^^^^^^^^^^^^^ `main` function is not allowed to be `async`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0670, E0752.
For more information about an error, try `rustc --explain E0670`.

[Done] exited with code=1 in 0.1 seconds

Thus, my attempted solution has failed.

---

ATTEMPTED SOLUTION # 2

I changed this...

async fn main() {}

into this...

fn main() {}

async fn hello() {}

I run it, and here's the output:

error[E0670]: `async fn` is not permitted in Rust 2015
 --> main.rs:3:1
  |
3 | async fn hello() {}
  | ^^^^^ to use `async fn`, switch to Rust 2018 or later
  |
  = help: set `edition = "2021"` in `Cargo.toml`
  = note: for more on editions, read https://doc.rust-lang.org/edition-guide

error: aborting due to previous error

For more information about this error, try `rustc --explain E0670`.

---

ATTEMPTED SOLUTION # 3
I changed main.rs contents into the following:

use tokio::net::TcpListener;
use tokio::io::{AsyncReadExt, AsyncWriteExt};

#[tokio::main]
async fn main() {}

And here is my new Cargo.toml:

[package]
name = "visualizer"
version = "0.1.0"
edition = "2018"

[dependencies]
tokio = { version= "1", features = ["full"] }

Here's the output:

error[E0670]: `async fn` is not permitted in Rust 2015
 --> main.rs:4:1
  |
4 | async fn main() {}
  | ^^^^^ to use `async fn`, switch to Rust 2018 or later
  |
  = help: set `edition = "2021"` in `Cargo.toml`
  = note: for more on editions, read https://doc.rust-lang.org/edition-guide

error[E0432]: unresolved import `tokio`
 --> main.rs:1:5
  |
1 | use tokio::main;
  |     ^^^^^ maybe a missing crate `tokio`?

error[E0433]: failed to resolve: use of undeclared crate or module `tokio`
 --> main.rs:3:3
  |
3 | #[tokio::main]
  |   ^^^^^ use of undeclared crate or module `tokio`

error[E0752]: `main` function is not allowed to be `async`
 --> main.rs:4:1
  |
4 | async fn main() {}
  | ^^^^^^^^^^^^^^^ `main` function is not allowed to be `async`

error: aborting due to 4 previous errors

Some errors have detailed explanations: E0432, E0433, E0670, E0752.
For more information about an error, try `rustc --explain E0432`.

---

Rust Version
rust-analyzer 2022-05-09

Editor:
VSCode

Operating System:
Archlinux

rust-analyzer version: rust-analyzer version: 5d5bbec 2022-05-09 stable

rustc version: rustc 1.59.0 (9d1b2106e 2022-02-23)

relevant settings: (eg. client settings, or environment variables like CARGO, RUSTUP_HOME or CARGO_HOME)

[bjorn3]

That is not an error produced by rust-analyzer. It is produced by rustc. Do you have edition = "2021" in your Cargo.toml. If the edition field is missing, cargo will default to the 2015 edition for backwards compatibility.

[amab8901]

That is not an error produced by rust-analyzer. It is produced by rustc. Do you have edition = "2021" in your Cargo.toml. If the edition field is missing, cargo will default to the 2015 edition for backwards compatibility.

I have edition 2021. I added a copy of my Cargo.toml in the main post

[lnicola]

main can't be async, regardless of the edition. The article you've originally linked to has a #[tokio::main] attribute on main, which turns it into a non-async function:

[amab8901]

main can't be async, regardless of the edition. The article you've originally linked to has a #[tokio::main] attribute on main, which turns it into a non-async function:

I tried new solutions based on your suggestions, but it still doesn't work. See the recent additions on the post.

[lnicola]

Are you sure you're pasting the right code? Looking at the errors:

 --> main.rs:1:5
  |
1 | use tokio::main;
  |     ^^^^^ maybe a missing crate `tokio`?

But your line 1 is use tokio::net::TcpListener;:

use tokio::net::TcpListener;
use tokio::io::{AsyncReadExt, AsyncWriteExt};

#[tokio::main]
async fn main() {}

So the error is for a different version of the code.

And the unresolved import error means that tokio isn't in Cargo.toml, or that the full feature isn't set.

[flodiebold]

What do you mean by "output"? Where do you see this output / what are you doing to get it? I thought at first this was a rustfmt error, but you wouldn't get name resolution errors from rustfmt.

[Diggsey]

@amab8901 this is not really a rust-analyzer issue - for general help with Rust there are a few channels where you might get a better response:

• Rust User Forum: https://users.rust-lang.org/
• Rust Discord: https://discord.gg/rust-lang
• Rust subreddit: https://old.reddit.com/r/rust

If you want people to be able to reproduce your issue, then using links to the rust playground is also a good idea:

https://play.rust-lang.org/

[amab8901]

What do you mean by "output"? Where do you see this output / what are you doing to get it? I thought at first this was a rustfmt error, but you wouldn't get name resolution errors from rustfmt.

I'm running the code from main.rs through VSCode text editor, using rust-analyzer to run the .rs files.

My file hierarchy is as follows:
Project
--- src
--- --- main
--- --- main.rs
--- --- utils.rs
--- target
--- Cargo.lock
--- Cargo.toml

[amab8901]

on of the co

---

I tried using Rust playground and it turns out that the following .rs script works fine:

#[tokio::main]
async fn main() {}

So the problem then isn't the script itself, but rather there seems to be a deeper issue with either .toml configuration or perhaps my VSCode editor. But I'm not sure what it is more specifically or how to solve it.

My Cargo.toml file looks like this:

[package]
name = "visualizer"
version = "0.1.0"
edition = "2021"

[dependencies]
tokio = { version= "1", features = ["full"] }

[lnicola]

Try uploading your project somewhere (even here) after cargo clean. Or make a new one.

What's src/main?

[amab8901]

@amab8901 this is not really a rust-analyzer issue - for general help with Rust there are a few channels where you might get a better response:

• Rust User Forum: https://users.rust-lang.org/
• Rust Discord: https://discord.gg/rust-lang
• Rust subreddit: https://old.reddit.com/r/rust

If you want people to be able to reproduce your issue, then using links to the rust playground is also a good idea:

https://play.rust-lang.org/

Thank you for the advice on using rust playground. I'm not really sure what the issue is, so I can't exclude rust-analyzer from being a potential explanation for the issue. I tried writing the following code on the rust playground...

#[tokio::main]
async fn main() {}

... and it works fine. This tells me that the main.rs script itself isn't the issue. But rather the issue is something else. Maybe it's the Cargo.toml file. Maybe it's the VSCode text editor itself. Or maybe it's the rust-analyzer (which I'm using for my VSCode to run the .rs files). Or maybe it's something else.

[flodiebold]

Try running cargo run in the terminal from the project folder, outside of VSCode.