Cannot use moved BufReader after for loop with bufreader.lines() - rust

Question

I'm trying to read some lines from a file, skipping the first few and printing the rest, but I keep getting errors about used value after move:
use std::fs::File;
use std::io::{self, BufRead, BufReader, Read};
use std::path::Path;
fn skip_and_print_file(skip: &usize, path: &Path) {
let mut skip: usize = *skip;
if let Ok(file) = File::open(path) {
let mut buffer = BufReader::new(file);
for (index, line) in buffer.lines().enumerate() {
if index >= skip {
break;
}
}
print_to_stdout(&mut buffer);
}
}
fn print_to_stdout(mut input: &mut Read) {
let mut stdout = io::stdout();
io::copy(&mut input, &mut stdout);
}
fn main() {}
This is the error I'm getting:
error[E0382]: use of moved value: `buffer`
--> src/main.rs:15:30
|
10 | for (index, line) in buffer.lines().enumerate() {
| ------ value moved here
...
15 | print_to_stdout(&mut buffer);
| ^^^^^^ value used here after move
|
= note: move occurs because `buffer` has type `std::io::BufReader<std::fs::File>`, which does not implement the `Copy` trait

Answer 1

In order to avoid the move, use the Read::by_ref() method. That way, you only borrow the BufReader:
for (index, line) in buffer.by_ref().lines().enumerate() { ... }
// ^^^^^^^^^
// you can still use `buffer` here

Answer 2

As Lukas Kalbertodt says, use Read::by_ref.
This prevents lines from consuming the BufReader and instead it consumes a &mut BufReader. The same logic applies to iterators.
Instead of implementing skip yourself, you can use Iterator::take. This has to be driven to completion with a for loop though:
use std::{
fs::File,
io::{self, BufRead, BufReader, Read},
path::Path,
};
fn skip_and_print_file(skip: usize, path: impl AsRef<Path>) {
if let Ok(file) = File::open(path) {
let mut buffer = BufReader::new(file);
for _ in buffer.by_ref().lines().take(skip) {}
// Or: buffer.by_ref().lines().take(skip).for_each(drop);
print_to_stdout(buffer);
}
}
fn print_to_stdout(mut input: impl Read) {
let mut stdout = io::stdout();
io::copy(&mut input, &mut stdout).expect("Unable to copy");
}
fn main() {
skip_and_print_file(2, "/etc/hosts");
}
Note that there's no reason to make the skip variable mutable or even to pass in a reference. You can also take in AsRef<Path> and then callers of skip_and_print_file can just pass in a string literal.