async closure parameter type inference does not work anymore · Issue #127425 · rust-lang/rust (original) (raw)

When using async closures, rust since around 1.80 fails to infer parameter type in most scenarios.

#![feature(async_closure)]

use anyhow::Error; use futures::{stream::repeat_with, StreamExt};

fn accept_str(: &str) {} fn accept_string(: &String) {}

#[tokio::main(flavor = "current_thread")] async fn main() -> Result<(), Error> { // when using closure returning async block - everything works repeat_with(|| "foo".to_owned()) .take(1) .for_each(|value| async move { accept_str(&value); // ok accept_str(value.as_str()); // ok accept_string(&value); // ok }) .await;

// when using async closure, most options fail
repeat_with(|| "foo".to_owned())
    .take(1)
    .for_each(async move |value| {
        // error on whole closure, rust thinks that type of value is `str`
        // accept_str(&value);

        // type annotations needed, cannot infer type
        // accept_str(value.as_str());

        // this works
        accept_string(&value); // ok
    })
    .await;

// can be fixed by providing type hint on async closure parameter
repeat_with(|| "foo".to_owned())
    .take(1)
    .for_each(async move |value: String| {
        accept_str(&value); // ok
        accept_str(value.as_str()); // ok
        accept_string(&value); // ok
    })
    .await;

Ok(())

}

I expected to see this happen: All options used to work in past versions of rust

Instead, this happened: Without explicit type hint it fails

Version it worked on

It used to work in the past, around 1.79

Version with regression

rustc 1.81.0-nightly (524d806c6 2024-07-05)
binary: rustc
commit-hash: 524d806c62a82ecc0cf8634b94997ae506f4d6f9
commit-date: 2024-07-05
host: x86_64-pc-windows-msvc
release: 1.81.0-nightly
LLVM version: 18.1.7