Non-send future produced by chaining `Stream` combinators

[George-Miao]

The future produced by chaining filter_map and chunks with a self reference inside filter_map closure is not Send for some reason, and this can be fixed by adding a .boxed() after the chunks call.

fn assert_send<T: Send>(t: T) -> T { t }
async fn test() {
    struct A;
    impl A {
        async fn a(&self) {
            futures::stream::iter([0u8])
                .filter_map(|_| async {
                    self.c();
                    // self.b().await; // Same as calling `c`
                    Some(0)
                })
                .chunks(1)
                // .boxed() // This will fix the problem
                .for_each(|_| async {}) // This can be anything consumes the stream without pinning it, include for_each_concurrent and collect
                .await;
        }

        async fn b(&self) {}

        fn c(&self) {}
    }
    assert_send(A.a()).await; // This will fail to compile
}

Error:

error[E0308]: mismatched types
   --> [:REDACTED:]/test.rs:252:9
    |
231 |                       .filter_map(|_| async {
    |  _____________________________________-
232 | |                         self.c();
233 | |                         // self.b().await; // Same as calling `c`
234 | |                         Some(0)
235 | |                     })
    | |                     -
    | |                     |
    | |_____________________the expected `async` block
    |                       the found `async` block
...
252 |           assert_send(A.a_consume()).await; // This will fail to compile
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
    |
    = note: expected `async` block `{async block@[:REDACTED:]/test.rs:231:37: 235:22}`
               found `async` block `{async block@[:REDACTED:]/test.rs:231:37: 235:22}`
note: the lifetime requirement is introduced here
   --> [:REDACTED:]/test.rs:223:23
    |
223 |     fn assert_send<T: Send>(t: T) -> T {
    |                       ^^^^

For more information about this error, try `rustc --explain E0308`.

[taiki-e]

This is a rustc bug: rust-lang/rust#104382

Closing in favor of the upstream issue.