Chunks misbehaves for zero-sized item types

[benw]

Calling .chunks(n) on a stream of zero-sized items produces a stream that waits until the original stream terminates, and then yields all the items at once, regardless of the value of n.

use futures::stream::{self, Stream};

fn main() {
    let val = ();
    for i in stream::repeat::<_, ()>(val).take(5).chunks(3).wait() {
        dbg!(i).ok();
    }
}

Actual output:

[src/main.rs:6] i = Ok(
    [
        (),
        (),
        (),
        (),
        ()
    ]
)

Expected output:

[src/main.rs:6] i = Ok(
    [
        (),
        (),
        ()
    ]
)
[src/main.rs:6] i = Ok(
    [
        (),
        ()
    ]
)

Chunks should probably hold its own cap: usize field rather than relying on items.capacity(), because Vec::<()> reports capacity of usize::MAX regardless of what capacity was requested.

[Nemo157]

0.3 has the same misbehaviour, tested with:

#![feature(async_await, await_macro, futures_api)]

use futures::stream::{self, StreamExt};

fn main() {
    futures::executor::block_on(async {
        let mut stream = stream::repeat(()).take(5).chunks(3);
        while let Some(i) = await!(stream.next()) {
            dbg!(i);
        }
    });
}

[cramertj]

seems like a bug in Vec, IMO.