improved error message for unused futures

[ibaryshnikov]

Follows #31 (comment)
I guess this variant was merged in rust-lang/rust. What about message for streams?

[yoshuawuyts]

@ibaryshnikov the same message can be used for streams

#[must_use = "streams do nothing unless you `.await` or poll them"]

[taiki-e]

@yoshuawuyts

#[must_use = "streams do nothing unless you `.await` or poll them"]

I think the message may be misleading, as streams cannot use .await directly.

Related: rust-lang/futures-rs#1600

[yoshuawuyts]

@taiki-e ah yeah, fair -- I guess the way we want to go is for x.await in y {} loops, but not sure how to mention that. "polled" is probably accurate (poll_next), but await... is currently only sort of accurate? Do you have a suggestion for something better?

[taiki-e]

@yoshuawuyts
I don't know what the more desirable error message is.
For example, .next().await may be more convenient, but this may not be appropriate because next is not a method of Stream trait. Also, we use must_use for impl Sink types, so I don't know if the message related to a specific method name is really good.
This also depends on how Stream trait is added in the language, so, for now, I think it is better to postpone the stream must_use message problem.

[taiki-e]

By the way, "improwed" in the commit message and the title looks like a typo.

[yoshuawuyts]

for now, I think it is better to postpone the stream must_use message problem.

@taiki-e that sounds reasonable (: