Tokio equivalent of async_std code

#1

Here is a snippet of code that works using async_std:

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

async fn sum_file_async(file_path: &str) -> Result<f64> {
    let f = File::open(file_path).await?;
    let reader = BufReader::new(f);
    let mut sum = 0.0;
    let mut stream = reader.lines(); // KEY POINT!
    while let Some(Ok(line)) = stream.next().await {
        if let Ok(n) = line.parse::<f64>() {
            println!("{}", n);
            sum += n;
        }
    }
    Ok(sum)
}

I want to do the same thing with tokio. Here's what I tried:

use std::{io::Result, prelude::*};
use tokio::{
    fs::File,
    io::{AsyncBufRead, BufReader},
};

async fn sum_file_async(file_path: &str) -> Result<f64> {
    let f = File::open(file_path).await?;
    let reader = BufReader::new(f);
    let mut sum = 0.0;
    // Error on next line is:
    // method not found in `tokio::io::BufReader<tokio::fs::File>`
    let mut stream = reader.lines(); // KEY POINT!
    while let Some(Ok(line)) = stream.next().await {
        if let Ok(n) = line.parse::<f64>() {
            println!("{}", n);
            sum += n;
        }
    }
    Ok(sum)
}

The tokio docs mention a lines method on tokio::io::AsyncBufReadExt here:
https://docs.rs/tokio/1.2.0/tokio/io/struct.Lines.html
But it's not clear to me from the docs how to create one of those and whether that is an alternative to tokio::io::BufReader.

#2

The compiler gives this error:

error[E0599]: no method named `lines` found for struct `tokio::io::BufReader<tokio::fs::File>` in the current scope
   --> src/lib.rs:13:29
    |
13  |     let mut stream = reader.lines(); // KEY POINT!
    |                             ^^^^^ method not found in `tokio::io::BufReader<tokio::fs::File>`
    | 
   ::: /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.0.2/src/io/util/async_buf_read_ext.rs:248:12
    |
248 |         fn lines(self) -> Lines<Self>
    |            ----- the method is available for `Box<tokio::io::BufReader<tokio::fs::File>>` here
    |
    = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
    |
1   | use tokio::io::AsyncBufReadExt;
    |

Notice how the help section literally tells you why it doesn't work and what to do to fix it.

#3

Ah sorry, I didn't read closely enough. I also had to change this line for async-std:

    while let Some(Ok(line)) = stream.next().await {

to this for tokio:

    while let Ok(Some(line)) = stream.next_line().await {