Is it possible to join 2 futures in spawning task?

[liang-kang]

Here is the sample code:

use futures::{TryFutureExt, join};
use tokio::io::AsyncReadExt;

async fn test() -> Result<(), std::io::Error>
{
    let mut c1: Vec<u8> = vec![];
    let mut c2: Vec<u8> = vec![];
    let c1_ref = &mut c1; 
    let c2_ref = &mut c2; 

    let f1 = tokio::fs::File::open("test.rs").and_then(
        |mut myfile| async move { myfile.read_buf(c1_ref).await },
    );  

    let f2 = tokio::fs::File::open("test.rs").and_then(
        |mut myfile| async move { myfile.read_buf(c2_ref).await },
    );  

    let (va, vb) = join!(f1, f2);
    va?;
    vb?;
    let s = String::from_utf8(c1).unwrap();
    println!("content={}", s); 
    Ok(())
}

#[tokio::main]
async fn main()
{
    let handler = tokio::spawn(async move {
        test().await
    }); 

    handler.await;
}

such code will cause the compiling error:

Compiling hello_cargo v0.1.0 (/root/work/rust/project/hello_cargo)
error[E0308]: mismatched types
   --> src/main.rs:30:19
    |
12  |         |mut myfile| async move { myfile.read_buf(c1_ref).await },
    |                                 ---------------------------------
    |                                 |
    |                                 the expected `async` block
    |                                 the found `async` block
...
30  |     let handler = tokio::spawn(async move {
    |                   ^^^^^^^^^^^^ lifetime mismatch
    |
    = note: expected opaque type `impl futures::Future`
               found opaque type `impl futures::Future`
note: the lifetime requirement is introduced here
   --> /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/task/spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ^^^^

error[E0308]: mismatched types
   --> src/main.rs:30:19
    |
16  |         |mut myfile| async move { myfile.read_buf(c2_ref).await },
    |                                 ---------------------------------
    |                                 |
    |                                 the expected `async` block
    |                                 the found `async` block
...
30  |     let handler = tokio::spawn(async move {
    |                   ^^^^^^^^^^^^ lifetime mismatch
    |
    = note: expected opaque type `impl futures::Future`
               found opaque type `impl futures::Future`
note: the lifetime requirement is introduced here
   --> /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.15.0/src/task/spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ^^^^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `hello_cargo` due to 2 previous errors

If replacing join! macro with f1.await and f2.await, there will be no such compiling issue.

[Darksonn]

The error you're getting is one of the few places where the compiler gives a really bad compiler error. I believe there's a bug report for it open already.

Anyway, you should be able to get the error to go away by using an async block instead of and_then.

let f2 = async move {
    let file = tokio::fs::File::open("test.rs").await?;
    myfile.read_buf(c2_ref).await
};

[liang-kang]

Thx!
It works with your walkaround.