Too many `type inside `async` object must be known in this context` errors

[olegnn]

This code (Playground)

use futures::stream::{iter/*, StreamExt*/};

async fn produce_11_errors() {
    iter(vec![1, 2, 3])/*.collect::<Vec<_>>()*/.await;
}

fn main() {}

currently produces 11 errors:

• 1 of the trait bound is not satisfied
• 10 of type inside async object must be known in this context.

If you add elements to vec, it will generate even more.

[doc-jones]

@rustbot claim

[estebank]

Current output since 1.54:

error[E0277]: `futures::stream::Iter<std::vec::IntoIter<{integer}>>` is not a future
 --> src/main.rs:4:5
  |
4 |     iter(vec![1, 2, 3])/*.collect::<Vec<_>>()*/.await;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `futures::stream::Iter<std::vec::IntoIter<{integer}>>` is not a future
  |
  = help: the trait `futures::Future` is not implemented for `futures::stream::Iter<std::vec::IntoIter<{integer}>>`
  = note: futures::stream::Iter<std::vec::IntoIter<{integer}>> must be a future or must implement `IntoFuture` to be awaited
  = note: required because of the requirements on the impl of `std::future::IntoFuture` for `futures::stream::Iter<std::vec::IntoIter<{integer}>>`

@olegnn are there other changes you'd like to see here?

[olegnn]

@estebank now it looks much better!

However, code like this still produces the same error twice

use futures::stream::*;

async fn same_error_twice() -> Vec<u8> {
    once(async { 0.0 }).collect().await
}

fn main() {}

error[E0277]: the trait bound `Vec<u8>: Extend<{float}>` is not satisfied
  --> src/main.rs:4:5
   |
4  |     once(async { 0.0 }).collect().await
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Extend<{float}>` is not implemented for `Vec<u8>`
   |
   = help: the following implementations were found:
             <Vec<T, A> as Extend<&'a T>>
             <Vec<T, A> as Extend<T>>
   = note: required because of the requirements on the impl of `futures::Future` for `Collect<futures::stream::Once<impl futures::Future>, Vec<u8>>`
note: required by `futures::Future::poll`

error[E0277]: the trait bound `Vec<u8>: Extend<{float}>` is not satisfied
 --> src/main.rs:4:25
  |
4 |     once(async { 0.0 }).collect().await
  |                         ^^^^^^^ the trait `Extend<{float}>` is not implemented for `Vec<u8>`
  |
  = help: the following implementations were found:
            <Vec<T, A> as Extend<&'a T>>
            <Vec<T, A> as Extend<T>>

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` due to 2 previous errors

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a63224be65d4e278f12786dc0dd497fd

[estebank]

Current output:

error[E0277]: the trait bound `Vec<u8>: Extend<{float}>` is not satisfied
 --> src/main.rs:4:34
  |
4 |     once(async { 0.0 }).collect().await
  |                                  ^^^^^^ the trait `Extend<{float}>` is not implemented for `Vec<u8>`
  |
  = help: the following other types implement trait `Extend<A>`:
            <Vec<T, A> as Extend<&'a T>>
            <Vec<T, A> as Extend<T>>
  = note: required for `Collect<futures::stream::Once<[async block@src/main.rs:4:10: 4:23]>, Vec<u8>>` to implement `futures::Future`

error[E0277]: the trait bound `Vec<u8>: Extend<{float}>` is not satisfied
   --> src/main.rs:4:25
    |
4   |     once(async { 0.0 }).collect().await
    |                         ^^^^^^^ the trait `Extend<{float}>` is not implemented for `Vec<u8>`
    |
    = help: the following other types implement trait `Extend<A>`:
              <Vec<T, A> as Extend<&'a T>>
              <Vec<T, A> as Extend<T>>
note: required by a bound in `futures::StreamExt::collect`
   --> /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.25/src/stream/stream/mod.rs:516:29
    |
516 |     fn collect<C: Default + Extend<Self::Item>>(self) -> Collect<Self, C>
    |                             ^^^^^^^^^^^^^^^^^^ required by this bound in `StreamExt::collect`

I wonder if we could extend the current deduplication logic to account for expressions and look at earlier errors in method chains...