unnecessary_to_owned suggestion changes maybe-cloning to always-cloning

[kpreid]

Summary

When iterating over the contents of a Cow<'a, [T]> by value, clippy::unnecessary_to_owned confidently proposes a change which will cause unnecessary element cloning in the case where the Cow is Cow::Owned. This isn't necessarily worse, but it isn't strictly better and shouldn't be recommended without qualification.

Lint Name

unnecessary_to_owned

Reproducer

I tried this code:

use std::borrow::Cow;

struct NoisyClone<T>(T);

impl<T: Clone + std::fmt::Debug> Clone for NoisyClone<T> {
    fn clone(&self) -> Self {
        let value = &self.0;
        println!("Cloning {value:?}");
        Self(value.clone())
    }
}

fn main() {
    case1(Cow::Owned(vec![NoisyClone(100)]));
    case2(Cow::Owned(vec![NoisyClone(200)]));
    case1(Cow::Borrowed(&[NoisyClone(102)]));
    case2(Cow::Borrowed(&[NoisyClone(202)]));
}

/// Original code
fn case1(input: Cow<'_, [NoisyClone<i32>]>) -> Vec<i32> {
    input.into_owned().into_iter().map(|nc| nc.0).collect()
}

/// Code proposed by Clippy
fn case2(input: Cow<'_, [NoisyClone<i32>]>) -> Vec<i32> {
    input.iter().cloned().map(|nc| nc.0).collect()
}

I saw this happen:

warning: unnecessary use of `into_owned`
  --> src/bin/cow_clippy.rs:20:5
   |
20 |     input.into_owned().into_iter().map(|nc| nc.0).collect()
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `input.iter().cloned()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_to_owned
   = note: `#[warn(clippy::unnecessary_to_owned)]` on by default

Both versions of the code have flaws, but neither one is strictly superior to the other, and case1 is likely better, so Clippy should not confidently recommend one.

• case1, when given Borrowed, will unnecessarily create a Vec (rather than only cloning the elements).
• case2, when given Owned, will unnecessarily clone all the elements (rather than moving them).

Version

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-apple-darwin
release: 1.78.0
LLVM version: 18.1.2

Additional Labels

No response