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

[p0lunin]

I tried this code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f560d75b3eb32ab6d2e0e51590b0337f

I expected to see this happen: code compiles

Instead, this happened: error: implementation of std::marker::Send is not general enough

Meta

rustc --version --verbose:

rustc 1.77.1 (7cf61ebde 2024-03-27)
binary: rustc
commit-hash: 7cf61ebde7b22796c69757901dd346d0fe70bd97
commit-date: 2024-03-27
host: x86_64-unknown-linux-gnu
release: 1.77.1
LLVM version: 17.0.6

Bug exists on beta and nightly versions also.

Backtrace

error: implementation of `std::marker::Send` is not general enough
  --> src/main.rs:63:5
   |
63 |     tokio::spawn(join_with_cancellation(f, cancellation.clone()));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `std::marker::Send` is not general enough
   |
   = note: `std::marker::Send` would have to be implemented for the type `&CancellationToken`
   = note: ...but `std::marker::Send` is actually implemented for the type `&'0 CancellationToken`, for some specific lifetime `'0`

I have working example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=19183aa875e3dadf501f3b6bdfb61ce4.

It seems like the reason is combination of .flatten() method that has Item = &'a T when calling async function inside loop. If you try to remove .await from the line 33, it compiles. If you try to remove .flatten() as in second link - it compiles.

Also, the error seems kinda random. The pattern is as follows:

   = note: ...but `std::marker::Send` is actually implemented for the type `&'0 T`, for some specific lifetime `'0`

But every time I change the code, the compiler shows another T type in error hint.

[theemathas]

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

[Kolsky]

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.

[veera-sivarajan]

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

[theemathas]

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

[traviscross]

@rustbot labels +AsyncAwait-Triaged

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

• Tracking issue for incorrect lifetime bound errors in async #110338