lines example in docs doesn't work

[kespindler]

In https://docs.rs/async-std/0.99.3/async_std/io/trait.BufRead.html#examples-2, this example is provided:

use async_std::fs::File;
use async_std::io::BufReader;
use async_std::prelude::*;

let file = File::open("a.txt").await?;
let mut lines = BufReader::new(file).lines();
let mut count = 0;

for line in lines.next().await {
    line?;
    count += 1;
}

I used it in this full program:

#![feature(async_await)]

use std::env::args;

use async_std::fs::File;
use async_std::io::{self, BufReader};
use async_std::prelude::*;
use async_std::task;

fn main() -> io::Result<()> {
    let path = args().nth(1).expect("missing path argument");

    let mut count = 0u64;
    task::block_on(async {
        //jlet file = File::open(&path).await?;
        //let mut lines = BufReader::new(file).lines();
        
        let file = File::open(&path).await?;
        let mut lines = BufReader::new(file).lines();
        let mut count = 0;

        for line in lines.next().await {
            line?;
            count += 1;
        }
        println!("The file contains {} lines.", count);
        Ok(())
    })
}

However, running that counts 1 line for any file with >= 1 line that I run it on. In contrast, this full program works correctly:

#![feature(async_await)]

use std::env::args;

use async_std::fs::File;
use async_std::io::{self, BufReader};
use async_std::prelude::*;
use async_std::task;

fn main() -> io::Result<()> {
    let path = args().nth(1).expect("missing path argument");

    let mut count = 0u64;
    task::block_on(async {
        let file = File::open(&path).await?;
        let mut lines = BufReader::new(file).lines();

        while let Some(line) = lines.next().await {
            line?;
            count += 1;
        }

        println!("The file contains {} lines.", count);
        Ok(())
    })
}

[skade]

Thanks for reporting :).

[kespindler]

Happy to! I'm curious if there's a for construct that does work in this situation.

[matklad]

@kespindler not at the moment, async for is being designed: https://boats.gitlab.io/blog/post/for-await-i/