Incorrect needless_borrow

[ritchie46]

Summary

Clippy suggests removing a borrow that would lead to ownership being moved.

Lint Name

needless_borrow

Reproducer

I tried this code:

fn main() {
    let mut iter = vec![1, 2, 3].into_iter();

    let mut sum = 0;
    for v in (&mut iter).enumerate().take(1) {
        sum += v.1;
    }
    
    let b = iter.count();
}

I saw this happen:

warning: this expression borrows a value the compiler would automatically borrow
  --> src/main.rs:65:14
   |
65 |     for v in (&mut iter).enumerate().take(1) {
   |              ^^^^^^^^^^^ help: change this to: `iter`

If we follow the lint:

fn main() {
    let mut iter = vec![1, 2, 3].into_iter();

    let mut sum = 0;
    for v in iter.enumerate().take(1) {
        sum += v.1;
    }

    let b = iter.count();
}

Our code does not compile:

error[E0382]: use of moved value: `iter`
   --> src/main.rs:69:13
    |
62  |     let mut iter = vec![1, 2, 3].into_iter();
    |         -------- move occurs because `iter` has type `std::vec::IntoIter<i32>`, which does not implement the `Copy` trait
...
65  |     for v in iter.enumerate().take(1) {
    |                   ----------- `iter` moved due to this method call
...
69  |     let b = iter.count();
    |             ^^^^ value used here after move
    |
note: this function takes ownership of the receiver `self`, which moves `iter`
   --> /home/ritchie46/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:928:18
    |
928 |     fn enumerate(self) -> Enumerate<Self>
    |                  ^^^

Version

rustc 1.60.0-nightly (e7aca8959 2022-02-09)
binary: rustc
commit-hash: e7aca895980f25f6d2d3c48e10fd04656764d1e4
commit-date: 2022-02-09
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0

Additional Labels

No response

[chris-morgan]

I just observed much the same thing. Fairly sure it’s a recent regression, as I’m pretty sure I would have run Clippy multiple times between when I wrote this code a few weeks back and last rustup updated a few days ago.

impl Type {
    fn method(&mut self, …) -> … {
        // …
        self.tar_file.seek(SeekFrom::Start(offset))?;
        let mut archive = Archive::new((&mut self.tar_file).take(size));
        // …
    }
}

std::io::Read::take takes by value. self.take(…) could be needless, but (&mut self.tar_file).take(…) is certainly not.