iter_overeager_cloned suggests code that doesn't compile

[carols10cents]

Summary

With clippy 0.1.61 (2022-03-01 f0c4da4), the iter_overeager_cloned lint suggests switching cloned().flatten() to flatten().cloned() on an iterator of a slice of ranges.

This doesn't work because &std::ops::Range<{integer}> doesn't implement Iterator; only std::ops::Range<{integer}> does (so the cloned is important).

Reproducer

I tried this code that compiles:

fn main() {
    let ranges = vec![1..3, 4..8];
    let _something: Vec<_> = ranges.iter().cloned().flatten().collect();
}

and clippy warned:

warning: called `cloned().flatten()` on an `Iterator`. It may be more efficient to call `flatten().cloned()` instead
 [--> src/main.rs:3:30
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)  |
3 |     let _something: Vec<_> = ranges.iter().cloned().flatten().collect();
  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `ranges.iter().flatten().cloned()`
  |
  = note: `#[warn(clippy::iter_overeager_cloned)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_overeager_cloned

so I changed my code to:

fn main() {
    let ranges = vec![1..3, 4..8];
    let _something: Vec<_> = ranges.iter().flatten().cloned().collect();
}

and got these compiler errors:

error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): `&std::ops::Range<{integer}>` is not an iterator
    [--> src/main.rs:3:44
](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021#)     |
3    |     let _something: Vec<_> = ranges.iter().flatten().cloned().collect();
     |                                            ^^^^^^^ `&std::ops::Range<{integer}>` is not an iterator
     |
     = help: the trait `Iterator` is not implemented for `&std::ops::Range<{integer}>`
     = note: required because of the requirements on the impl of `IntoIterator` for `&std::ops::Range<{integer}>`
note: required by a bound in `flatten`

error[[E0599]](https://doc.rust-lang.org/stable/error-index.html#E0599): the method `cloned` exists for struct `Flatten<std::slice::Iter<'_, std::ops::Range<{integer}>>>`, but its trait bounds were not satisfied
   [--> src/main.rs:3:54
](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021#)    |
3   |       let _something: Vec<_> = ranges.iter().flatten().cloned().collect();
    |                                                        ^^^^^^ method cannot be called on `Flatten<std::slice::Iter<'_, std::ops::Range<{integer}>>>` due to unsatisfied trait bounds
    |
    = note: the following trait bounds were not satisfied:
            `<&std::ops::Range<{integer}> as IntoIterator>::IntoIter = _`
            which is required by `Flatten<std::slice::Iter<'_, std::ops::Range<{integer}>>>: Iterator`
            `<&std::ops::Range<{integer}> as IntoIterator>::Item = _`
            which is required by `Flatten<std::slice::Iter<'_, std::ops::Range<{integer}>>>: Iterator`
            `&std::ops::Range<{integer}>: IntoIterator`
            which is required by `Flatten<std::slice::Iter<'_, std::ops::Range<{integer}>>>: Iterator`
            `Flatten<std::slice::Iter<'_, std::ops::Range<{integer}>>>: Iterator`
            which is required by `&mut Flatten<std::slice::Iter<'_, std::ops::Range<{integer}>>>: Iterator`

Some errors have detailed explanations: E0277, E0599.

I expected to get a suggestion from clippy that would compile.

Instead, I got a suggestion that doesn't compile.

Version

rustc 1.59.0 (9d1b2106e 2022-02-23)
binary: rustc
commit-hash: 9d1b2106e23b1abd32fce1f17267604a5102f57a
commit-date: 2022-02-23
host: aarch64-apple-darwin
release: 1.59.0
LLVM version: 13.0.0

Additional Labels

No response

[audunska]

This happens to me (albeit with custom types) on rust beta 1.60 too.