BoxFuture.wait() does not work unless futures::Future is imported

[stepancheg]

xx.rs:

use futures::*;

pub fn yy() -> BoxFuture<u32, ::std::io::Error> {
    done(Ok(1)).boxed()
}

main.rs:

extern crate futures;

mod xx;

use futures::Future;

fn main() {
    let f = xx::yy();

    println!("wait: {:?}", f.wait());
}

This code works.

If use futures::Future is commented out in main.rs, compiler complains:

src/main.rs:10:30: 10:34 error: the trait bound `futures::Future<Error=std::io::Error, Item=u32> + Send: std::marker::Sized` is not satisfied [E0277]
src/main.rs:10     println!("wait: {:?}", f.wait());
                                            ^~~~
<std macros>:2:27: 2:58 note: in this expansion of format_args!
<std macros>:3:1: 3:54 note: in this expansion of print! (defined in <std macros>)
src/main.rs:10:5: 10:38 note: in this expansion of println! (defined in <std macros>)
src/main.rs:10:30: 10:34 help: run `rustc --explain E0277` to see a detailed explanation
src/main.rs:10:30: 10:34 note: `futures::Future<Error=std::io::Error, Item=u32> + Send` does not have a constant size known at compile-time
error: aborting due to previous error
error: Could not compile `futures-rs-td`.

I'm not sure if is it a bug, or just a usability problem, is it in futures-rs or in rust language, but it is hard to understand error message.

rustc 1.11.0 (9b21dcd6a 2016-08-15), futures-rs from master

[ahicks92]

Your problem here is that traits are only usable if the trait is imported. You'll have this problem with other modules as well.

See here for documentation of this behavior.

[alexcrichton]

Yes I think that this may be a case of just a bad compiler error? Perhaps a bug could be opened up against rust-lang/rust?

[alexcrichton]

Ok, moved to rust-lang/rust#35976