I've created a contrived example to understand an error I'm seeing with an async function using tokio:
pub async fn unique_words_async() -> io::Result<u32> {
let file = File::open("sample.txt").await?;
let reader = BufReader::new(file);
let mut lines = reader.lines();
let mut h = HashMap::new();
h.insert("word", true);
while let Some(line) = lines.next_line().await? {
let mut split = line.trim().split(' ');
while let Some(word) = split.next() {
h.insert(word, true);
}
}
Ok(h.len() as u32)
}
here's the exact error:
16 | let mut split = line.trim().split(' ');
| ^^^^^^^^^^^ borrowed value does not live long enough
...
20 | }
| - `line` dropped here while still borrowed
21 | Ok(h.len() as u32)
| ------- borrow later used here
see running live in playground
The non-async code to accomplish the same task works fine (see playground):
pub fn unique_words() -> u32 {
let input = "this is one line\nhere is another line";
let mut lines = input.lines();
let mut h = std::collections::HashMap::new();
while let Some(line) = lines.next() {
let mut split = line.split(' ');
while let Some(word) = split.next() {
h.insert(word, true);
}
}
h.len() as u32
}
CodePudding user response:
Your non-async translation is incorrect. If you will use File
with BufReader
, you will have the same error:
pub fn unique_words() -> std::io::Result<u32> {
use std::io::BufRead;
let file = std::fs::File::open("sample.txt")?;
let reader = std::io::BufReader::new(file);
let mut lines = reader.lines();
let mut h = std::collections::HashMap::new();
while let Some(line) = lines.next().transpose()? {
let mut split = line.split(' ');
while let Some(word) = split.next() {
h.insert(word, true);
}
}
Ok(h.len() as u32)
}
error[E0597]: `line` does not live long enough
--> src/main.rs:10:25
|
10 | let mut split = line.split(' ');
| ^^^^^^^^^^^^^^^ borrowed value does not live long enough
...
14 | }
| - `line` dropped here while still borrowed
15 | Ok(h.len() as u32)
| ------- borrow later used here
This is because Lines::next()
returns Option<io::Result<String>>
, and the String
is dropped at the end of each loop turn. However, str::trim()
and str::split()
takes &str
and produce &str
with the same lifetime (as they only slice the string, not change it). Thus you push into h
a &str
with the lifetime of one loop turn, that is, a dangling reference.
The reason it worked with str::lines()
is that std::str::Lines::next()
returns Option<&str>
with the reference tied to the original &str
, which is 'static
.
The simplest way to fix that is to convert the &str
to an owned String
:
h.insert(word.to_owned(), true);