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iter_cloned_collect false positive with custom From/IntoIterator impl #9119

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Dentosal opened this issue Jul 4, 2022 · 1 comment · Fixed by #14473
Closed

iter_cloned_collect false positive with custom From/IntoIterator impl #9119

Dentosal opened this issue Jul 4, 2022 · 1 comment · Fixed by #14473
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C-bug Category: Clippy is not doing the correct thing I-false-positive Issue: The lint was triggered on code it shouldn't have

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@Dentosal
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Dentosal commented Jul 4, 2022

Summary

When FromIterator is implemented for a type inside an another iterator, .collect() is able to do run the inner iterator as well. Clippy doesn't recognize this case, and instead assumes that .iter().cloned().collect() to a Vec<_> always has the same resulting type than to_vec(). This also occurs with .iter().copied().collect().

Lint Name

iter_cloned_collect and iter_cloned_collect

Reproducer

I tried this code:

use std::iter;

#[derive(Clone)]
struct Example(u16);

impl iter::FromIterator<Example> for Vec<u8> {
    fn from_iter<T>(iter: T) -> Self
    where
        T: IntoIterator<Item = Example>,
    {
        iter.into_iter().flat_map(|e| e.0.to_le_bytes()).collect()
    }
}

fn main() {
    let examples = vec![Example(1), Example(0x1234)];
    let encoded: Vec<u8> = examples.iter().cloned().collect();
    assert_eq!(encoded, vec![0x01, 0x00, 0x34, 0x12]);
}

I saw this happen:

warning: called `iter().copied().collect()` on a slice to create a `Vec`. Calling `to_vec()` is both faster and more readable
  --> src/main.rs:17:36
   |
17 |     let encoded: Vec<u8> = examples.iter().copied().collect();
   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `.to_vec()`
   |
   = note: `#[warn(clippy::iter_cloned_collect)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_cloned_collect

I expected to see this happen:

No warning, as the to_vec cannot do the required conversion.

Version

rustc 1.62.0 (a8314ef7d 2022-06-27)
binary: rustc
commit-hash: a8314ef7d0ec7b75c336af2c9857bfaf43002bfc
commit-date: 2022-06-27
host: x86_64-unknown-linux-gnu
release: 1.62.0
LLVM version: 14.0.5

Additional Labels

@rustbot label +l-suggestion-causes-error
@Dentosal Dentosal added C-bug Category: Clippy is not doing the correct thing I-false-positive Issue: The lint was triggered on code it shouldn't have labels Jul 4, 2022
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rustbot commented Jul 4, 2022

Error: Label l-suggestion-causes-error can only be set by Rust team members

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

@Dentosal Dentosal mentioned this issue Jul 4, 2022
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@profetia profetia mentioned this issue Mar 25, 2025
Merged
github-merge-queue bot pushed a commit that referenced this issue Apr 8, 2025
@samueltardieu
fix: iter_cloned_collect FP with custom From/IntoIterator impl (#…
1a1ad5e 
…14473)

Closes #9119

changelog: [`iter_cloned_collect`]: fix FP with custom
`From`/`IntoIterator` impl
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Labels
C-bug Category: Clippy is not doing the correct thing I-false-positive Issue: The lint was triggered on code it shouldn't have
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fix: iter_cloned_collect FP with custom From/IntoIterator impl profetia/rust-clippy
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@Dentosal @rustbot