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matthiaskrgr opened this issue Jun 5, 2025 · 2 comments · May be fixed by #15013
Open

iter-on-single-items breaks code in macro context that takes IntoIterator #14981

matthiaskrgr opened this issue Jun 5, 2025 · 2 comments · May be fixed by #15013
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C-bug Category: Clippy is not doing the correct thing I-suggestion-causes-error Issue: The suggestions provided by this Lint cause an ICE/error when applied

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@matthiaskrgr
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matthiaskrgr commented Jun 5, 2025 •
edited by rustbot
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Using the following flags

--force-warn clippy::iter-on-single-items

this code:

use std::option::IntoIter;
fn takes_into_iter(_: impl IntoIterator<Item = i32>) {}

macro_rules! x {
    ($e:expr) => {
        takes_into_iter($e); // lint here because `.into_iter()` is "redundant"
        let _: IntoIter<i32> = $e; // removing `.into_iter()` leads to a type error here
    };
}

fn main() {
    x!(Some(5).into_iter());
}

caused the following diagnostics:

    Checking _clippy11065-3 v0.1.0 (/tmp/icemaker_global_tempdir.t8dYaH8PqaqW/icemaker_clippyfix_tempdir.S1c809nLp6vL/_clippy11065-3)
warning: `into_iter` call on a collection with only one item
  --> src/main.rs:12:8
   |
12 |     x!(Some(5).into_iter());
   |        ^^^^^^^^^^^^^^^^^^^ help: try: `std::iter::once(5)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_on_single_items
   = note: requested on the command line with `--force-warn clippy::iter-on-single-items`

warning: `_clippy11065-3` (bin "_clippy11065-3") generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s

However after applying these diagnostics, the resulting code:

use std::option::IntoIter;
fn takes_into_iter(_: impl IntoIterator<Item = i32>) {}

macro_rules! x {
    ($e:expr) => {
        takes_into_iter($e); // lint here because `.into_iter()` is "redundant"
        let _: IntoIter<i32> = $e; // removing `.into_iter()` leads to a type error here
    };
}

fn main() {
    x!(std::iter::once(5));
}

no longer compiled:

    Checking _clippy11065-3 v0.1.0 (/tmp/icemaker_global_tempdir.t8dYaH8PqaqW/icemaker_clippyfix_tempdir.S1c809nLp6vL/_clippy11065-3)
error[E0308]: mismatched types
  --> src/main.rs:12:8
   |
7  |         let _: IntoIter<i32> = $e; // removing `.into_iter()` leads to a type error here
   |                ------------- expected due to this
...
12 |     x!(std::iter::once(5));
   |        ^^^^^^^^^^^^^^^^^^ expected `IntoIter<i32>`, found `Once<{integer}>`
   |
   = note: expected struct `std::option::IntoIter<i32>`
              found struct `std::iter::Once<{integer}>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `_clippy11065-3` (bin "_clippy11065-3") due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `_clippy11065-3` (bin "_clippy11065-3" test) due to 1 previous error

Version:

rustc 1.89.0-nightly (076ec59ff 2025-06-05)
binary: rustc
commit-hash: 076ec59ff1dcf538b9d3a0b8e0d7f4edd0559959
commit-date: 2025-06-05
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5
@matthiaskrgr
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may be the same problem as here:

use std::option::IntoIter;

fn main() {
    const C: fn(IntoIter<i32>) -> IntoIter<i32> = <IntoIter<i32> as IntoIterator>::into_iter;
    C(Some(5).into_iter()); // ICE here
}

@matthiaskrgr matthiaskrgr added C-bug Category: Clippy is not doing the correct thing I-suggestion-causes-error Issue: The suggestions provided by this Lint cause an ICE/error when applied labels Jun 6, 2025
@profetia
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profetia commented Jun 6, 2025

@rustbot claim

@profetia profetia linked a pull request Jun 7, 2025 that will close this issue
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Fix iter_on_single_items FP on function pointers and let stmts profetia/rust-clippy
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