I think part of the disconnect is that that what you're calling an "async closure" is more directly an analog of a "regular" function that happens to return a future rather than an "async fn" declared function; you'd need similar syntactic boilerplate for annotating a function that's not declared as "async". Currently, there is no closure version of an "async fn" in Rust, but it's arguably not particularly necessary because you can use a Future as the async version of a closure in a lot of cases due to them being lazy already. For example, spawning a task with tokio just takes a Future, not a closure like spawning a sync thread.

You can annotate the return type inside the closure in those cases, for example:

    let x = || async {
        let file = std::fs::read_to_string("foo.txt")?;
        Ok::<_, Box<dyn std::error::Error>>(file)
    };