Closed
Description
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 givenBorrowed, will unnecessarily create aVec(rather than only cloning the elements).case2, when givenOwned, 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