rust-lang / rust-lang/rust-clippy

Incorrect suggestion for `unnecessary_to_owned` when moving into `'static` closure

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C-bug I-suggestion-causes-error
Dominant language
Rust
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Description

Summary

Clippy gives an incorrect suggestion for a unnecessary_to_owned lint, when the item being cloned is moved into a closure which has a 'static lifetime.

Reproducer

Running Clippy against this code, which compiles succesfully:

fn do_something(x: &str) { println!("{x}") }
fn handle_func(f: impl Fn() + 'static) { f() }

fn main() {
	let items = vec!["foo".to_string(), "bar".to_string(), "baz".to_string()];

	for x in items.iter().cloned() {
		handle_func(move || do_something(&x))
	}
}

Gives this suggestion:

warning: unnecessary use of `cloned`
 --> src/main.rs:7:11
  |
7 |     for x in items.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
help: use
  |
7 |     for x in items.iter() {
  |              ~~~~~~~~~~~~
help: remove this `&`
  |
8 -         handle_func(move || do_something(&x))
8 +         handle_func(move || do_something(x))
  |

Doing this gives the following compilation error:

error[[E0597]](https://doc.rust-lang.org/stable/error-index.html#E0597): `items` does not live long enough
  --> src/main.rs:7:11
   |
7  |     for x in items.iter() {
   |              ^^^^^^^^^^^^ borrowed value does not live long enough
8  |         handle_func(move || do_something(x))
   |         ------------------------------------ argument requires that `items` is borrowed for `'static`
9  |     }
10 | }
   | - `items` dropped here while still borrowed

If handle_func does not have the 'static lifetime bound on the closure, Clippy's suggestion is valid.

Version
rustc 1.67.0-nightly (b28d30e1e 2022-12-06)
binary: rustc
commit-hash: b28d30e1e3c2b90fd08b7dd79d8e63884d1e0339
commit-date: 2022-12-06
host: aarch64-apple-darwin
release: 1.67.0-nightly
LLVM version: 15.0.4
Additional Labels

@rustbot label +I-suggestion-causes-error

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the reproducer in the issue and inspect the implementation and regression tests for the unnecessary_to_owned lint. Run Clippy on the shown code, then ensure the suggested rewrite remains compilable when the closure has a 'static bound and add coverage for this case.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
devtools
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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