Closed
Description
Summary
Consider std::io::Read, which has methods that take self, but the trait is also explicitly implemented for &mut R where R: Read.
Consider the following code:
use std::io::{Cursor, Read};
let mut r = Cursor::new("hello world");
let a = (&mut r).take(3);
// do stuff, drop a
let b = r.take(3);a lets you still use r afterward, but b does not, because it moves r into the wrapper.
Lint Name
needless_borrow
Reproducer
I ran into this problem with code that uses a reader in a loop, using take to read it in chunks, similar to this:
use std::io::{self, Cursor, Read, Write};
fn main() {
let mut data = Cursor::new("line 1\n\
line 2\n\
line 3\n\
line 4\n");
let stdout = io::stdout();
let mut stdout = stdout.lock();
loop {
let mut chunk = (&mut data).take(2); // `(&mut data)` can't be replaced by `data`!
let n = io::copy(&mut chunk, &mut stdout).unwrap();
if n == 0 {
break;
}
stdout.write_all(b".").unwrap();
}
}Clippy says:
warning: this expression borrows a value the compiler would automatically borrow
[--> src/main.rs:11:25
](https://play.rust-lang.org/#) |
11 | let mut chunk = (&mut data).take(2); // `(&mut data)` can't be replaced by `data`!
| ^^^^^^^^^^^ help: change this to: `data`
|
= note: `#[warn(clippy::needless_borrow)]` on by default
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
But that won't compile:
error[[E0382]](https://doc.rust-lang.org/stable/error-index.html#E0382): use of moved value: `data`
[--> src/main.rs:12:25
](https://play.rust-lang.org/#) |
4 | let mut data = Cursor::new("line 1\n\
| -------- move occurs because `data` has type `std::io::Cursor<&str>`, which does not implement the `Copy` trait
...
12 | let mut chunk = data.take(2);
| ^^^^ ------- `data` moved due to this method call, in previous iteration of loop
|
note: this function takes ownership of the receiver `self`, which moves `data`
Version
rustc 1.60.0-nightly (27f5d830e 2022-02-02)
binary: rustc
commit-hash: 27f5d830eb11cd7bdc834d6f0d78120976f75443
commit-date: 2022-02-02
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0
Additional Labels
No response