CODEWITHSUNDEEP
rustrust-tokio
Ahmed Ghoneim
Ahmed Ghoneim

Reputation: 164

Implementation of `FnOnce` is not general enough with async recursion

I'm trying to write a couple of recursive async functions in Rust. I've created a minimal example of the problem I'm facing:

use std::collections::HashMap;

use anyhow::Result;
use async_recursion::async_recursion;
use futures::StreamExt;

#[async_recursion]
async fn foo() -> Result<()> {
    let map: HashMap<String, String> = HashMap::new();
    futures::stream::iter(map.keys().map(|it: &String| tokio::spawn(bar(it.clone()))))
        .buffer_unordered(2)
        .collect::<Vec<_>>()
        .await;

    Ok(())
}

#[async_recursion]
async fn bar(it: String) -> Result<()> {
    foo().await;
    Ok(())
}

When using an array of string references mapped to the same tokio::spawn call it works fine:

futures::stream::iter(
    [&String::from("hi")].map(|it: &String| tokio::spawn(bar(it.clone()))),
)

When using the HashMap's keys, it gives me the following compiler error at the #[async_recursion] macro:

implementation of `FnOnce` is not general enough
closure with signature `fn(&'0 std::string::String) -> tokio::task::JoinHandle<Result<(), anyhow::Error>>` must implement `FnOnce<(&std::string::String,)>`, for any lifetime `'0`...
...but it actually implements `FnOnce<(&std::string::String,)>`

I'm not very informed about lifetimes, my guess is this has something to do with the HashMap's keys not living long enough or something. How can I fix this error?

Upvotes: 2

Views: 594

Answers (1)

Caesar
Caesar

Reputation: 8494

If you switch to

futures::stream::iter(
    map.into_keys()
        .map(|it: String| tokio::spawn(bar(it.to_owned()))),
)

your code will work.

The reason your original code doesn't work is that rust doesn't know that

.buffer_unordered(2)
.collect::<Vec<_>>()
.await;

forces the spawned futures to finish before your HashMap goes out of scope.

Upvotes: 2

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