Why can't I collect the Lines iterator into a vector of Strings?

Question

I'm trying to read the lines of a text file into a vector of Strings so I can continually loop over them and write each line to a channel for testing, but the compiler complains about collect:

use std::fs::File;
use std::io::BufRead;
use std::io::BufReader;
use std::path::Path;

fn main() {
    let file = File::open(Path::new("file")).unwrap();
    let reader = BufReader::new(&file);
    let _: Vec<String> = reader.lines().collect().unwrap();
}

The compiler complains:

error[E0282]: type annotations needed
 --> src/main.rs:9:30
  |
9 |     let lines: Vec<String> = reader.lines().collect().unwrap();
  |                              ^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `B`
  |
  = note: type must be known at this point

Without the .unwrap(), compiler says:

error[E0277]: a collection of type `std::vec::Vec<std::string::String>` cannot be built from an iterator over elements of type `std::result::Result<std::string::String, std::io::Error>`
 --> src/main.rs:9:45
  |
9 |     let lines: Vec<String> = reader.lines().collect();
  |                                             ^^^^^^^ a collection of type `std::vec::Vec<std::string::String>` cannot be built from `std::iter::Iterator<Item=std::result::Result<std::string::String, std::io::Error>>`
  |
  = help: the trait `std::iter::FromIterator<std::result::Result<std::string::String, std::io::Error>>` is not implemented for `std::vec::Vec<std::string::String>`

How do I tell Rust the correct type?

Answer 1

Since you want to collect straight into a Vec<String> while the Lines iterator is over Result<String, std::io::Error>, you need to help the type inference a little bit:

let lines: Vec<String> = reader.lines().collect::<Result<_, _>>().unwrap();

or even just:

let lines: Vec<_> = reader.lines().collect::<Result<_, _>>().unwrap();

This way the compiler knows that there is an intermediate step with a Result<Vec<String>, io::Error>. I think this case could be improved in the future, but for now the type inference is not able to deduce this.