I tried this code: <a href="https://play.rust-lang.org/?version=stable&mode=debug

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This doesn't use closures and has the same issue. <div class="highlight highlight-

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Bug when using `.flatten()` method that has `Item = &'a T` when calling async function inside loop about rust HOT 5 OPEN

p0lunin commented on August 30, 2024 1

Bug when using `.flatten()` method that has `Item = &'a T` when calling async function inside loop

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Comments (5)

theemathas commented on August 30, 2024

Here's minimized code that causes a different but similarly confusing error message. Unsure if this still preserves the essence of the original code.

async fn listen() {
    let things: Vec<Vec<i32>> = vec![];
    for _ in things.iter().map(|n| n.iter()).flatten() {
        // comment this line and everything compiles
        async {}.await;
    }
}

fn require_send<T: Send>(_x: T) {}

fn foo() {
    let future = listen();
    require_send(future);
}

Playground link

Error message:

   Compiling playground v0.0.1 (/playground)
error: implementation of `FnOnce` is not general enough
  --> src/lib.rs:13:5
   |
13 |     require_send(future);
   |     ^^^^^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
   |
   = note: closure with signature `fn(&'0 Vec<i32>) -> std::slice::Iter<'_, i32>` must implement `FnOnce<(&'1 Vec<i32>,)>`, for any two lifetimes `'0` and `'1`...
   = note: ...but it actually implements `FnOnce<(&Vec<i32>,)>`

error: could not compile `playground` (lib) due to 1 previous error

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Kolsky commented on August 30, 2024

Both fn iter<T>(v: &Vec<T>) -> Iter<'_, T> { v.iter() } replacing closure and .flat_map(|n| n.iter()) make the code compile. The difference between FlatMap and Flatten is that the first one unifies lifetimes of returned U: IntoIterators, and the other tries to unify Map::<I, F: Fn(_) -> _>::Items, but F isn't higher ranked closure, and as such it fails to do so. Feel free to correct me if I'm wrong.

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veera-sivarajan commented on August 30, 2024

@rustbot label -needs-triage +A-diagnostics +A-Async-Await +A-auto-traits +A-traits +D-confusing

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theemathas commented on August 30, 2024

This doesn't use closures and has the same issue.

async fn listen() {
    let things: Vec<Vec<i32>> = vec![];
    for _ in things.iter().map(id).flatten() {
        // comment this line and everything compiles
        async {}.await;
    }
}

fn id<T>(x: T) -> T { x }

fn require_send<T: Send>(_x: T) {}

fn foo() {
    let future = listen();
    require_send(future);
}

   Compiling playground v0.0.1 (/playground)
error: implementation of `FnOnce` is not general enough
  --> src/lib.rs:15:5
   |
15 |     require_send(future);
   |     ^^^^^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
   |
   = note: `fn(&'0 Vec<i32>) -> &'0 Vec<i32> {id::<&'0 Vec<i32>>}` must implement `FnOnce<(&'1 Vec<i32>,)>`, for any two lifetimes `'0` and `'1`...
   = note: ...but it actually implements `FnOnce<(&Vec<i32>,)>`

error: could not compile `playground` (lib) due to 1 previous error

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traviscross commented on August 30, 2024

@rustbot labels +AsyncAwait-Triaged

We discussed this in the async meeting today. We think this is probably related to:

#110338

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Bug when using `.flatten()` method that has `Item = &'a T` when calling async function inside loop about rust HOT 5 OPEN

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