Confusing error message on error [E0599]

[Razican]

Hi!
I was doing some work with async_std when I got a confusing error message. With this minimal code:

use async_std::{stream::Stream, task::Poll};

pub fn test_poll_next<F, T>(first: F) -> Poll<Option<T>>
where
    F: Stream<Item = T>,
{
    first.poll_next()
}

I got this error:

error[E0599]: no method named `poll_next` found for type `F` in the current scope
 --> src/lib.rs:7:11
  |
7 |     first.poll_next()
  |           ^^^^^^^^^ method not found in `F`
  |
  = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following traits define an item `poll_next`, perhaps you need to restrict type parameter `F` with one of them:
  |
3 | pub fn test_poll_next<F: futures_core::stream::Stream, T>(first: F) -> Poll<Option<T>>
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
3 | pub fn test_poll_next<F: futures_core::stream::Stream, T>(first: F) -> Poll<Option<T>>
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

If I follow the suggestion, import futures_core and so on, I get the following error message:

error[E0599]: no method named `poll_next` found for type `F` in the current scope
 --> src/lib.rs:7:11
  |
7 |     first.poll_next()
  |           ^^^^^^^^^ method not found in `F`
  |
  = help: items from traits can only be used if the type parameter is bounded by the trait
help: the following traits define an item `poll_next`, perhaps you need to restrict type parameter `F` with one of them:
  |
3 | pub fn test_poll_next<F: futures_core::stream::Stream + futures_core::stream::Stream, T>(first: F) -> Poll<Option<T>>
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
3 | pub fn test_poll_next<F: futures_core::stream::Stream + futures_core::stream::Stream, T>(first: F) -> Poll<Option<T>>
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

Which seems to be redundant. So, this seems not to be the issue I'm facing. What is really happening here?

[TimoFreiberg]

The error message is not that helpful, but the problem is that poll_next takes a Pin<&mut Self>, not a Self.

Adding the unstable feature and changing the function signature to

fn test_poll_next<F,T>(first: Pin<&mut F>) -> Poll<Option<T>> 
where
    F: Stream<Item = T>
{
    first.poll_next()
}

, I get the following error:

error[E0061]: this function takes 1 parameter but 0 parameters were supplied
  --> src/main.rs:11:11
   |
11 |     first.poll_next()
   |           ^^^^^^^^^ expected 1 parameter

error: aborting due to previous error

For more information about this error, try `rustc --explain E0061`.
error: could not compile `test-66289`.

To learn more, run the command again with --verbose.

Which makes sense as poll_next also takes a &mut Context argument.

[nagisa]

Duplicate of #65149