Lint `.into_iter()` if that only forwards to `.iter()` #1565

oli-obk · 2017-02-21T14:37:04Z

Types like [T; N] don't implement IntoIter themselves, but instead provide it through their Deref<Target=[T]> impl, which is simply a wrapper around calling .iter().

.into_iter() suggests that some ownership transfer is going on, if that is not the case .iter() should be used.

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clarfonthey · 2017-02-21T18:26:46Z

So to clarify this, would this mean calling .into_iter() on a reference? Because that's what &[T]'s into_iter does.

oli-obk · 2017-02-21T20:33:09Z

Yea. Some care needs to be taken to ensure that the for loop desugaring is not linted

bluss · 2017-02-21T20:47:54Z

That should be simple, the for loop uses IntoIterator::into_iter(<expr>) and this lint should apply exactly to where <expr>.into_iter() does not compile if it were replaced with the former (because of method resolution difference).

clarfonthey · 2018-04-09T18:01:38Z

rust-lang/rust#49000 also makes it clear that this lint will also help us detect incorrect usages of into_iter() on slices, which currently forwards to iter() but would forward to a dedicated into_iter() if it were provided.

cuviper · 2018-04-09T18:35:42Z

There are actually two ways that array.into_iter() can resolve to slice iteration:

using impl<'a, T> IntoIterator for &'a [T; $N], for $N in 0..=32 (possibly with autoref)
unsize to [T], then autoref using impl<'a, T> IntoIterator for &'a [T]
- note: this is an explicit final step in probe.rs -- arrays don't actually implement Deref!

I think we only need to lint cases that would be intercepted by IntoIterator for [T; N]. Something like array_ref.into_iter() should have no problem to continue using IntoIterator for &'a [T; N]. But it should also be fine to suggest array_ref.iter() in that case anyway.

I was investigating how I might implement such a lint in the compiler, but a library-specific lint like this is probably not a great idea to hard-code. Doing it in clippy seems safer, even though it might not get as much reach. For rustc I was thinking that src/librustc_typeck/check/method/confirm.rs might be the right place to check the method resolution. Can you suggest where I might do similar in clippy?

oli-obk · 2018-04-09T19:45:16Z

I think the best place would be https://github.com/rust-lang-nursery/rust-clippy/blob/c5559c1648b0810e5dff9f94935fba52e9a9e833/clippy_lints/src/methods.rs#L691 which contains all our method call lints

tamird · 2018-06-06T19:24:02Z

@cuviper any plans to return to this? rust-lang/rust#49000 is quite useful, and seems blocked on this.

cuviper · 2018-06-09T06:22:24Z

I do, yes. I managed to prototype the changes already -- I'll try to get back to clean that up and post a PR for comment soon.

kennytm · 2018-06-14T21:43:20Z

I'm not sure if this has been taken into account by #1565 (comment) —

If it is not too difficult to do, I suggest we also lint (&mut [1, 2, 3]).into_iter() to call [1, 2, 3].iter_mut().

Complete list

Type	`(&x).into_iter()` → `x.iter()`	`(&mut x).into_iter()` → `x.iter_mut()`
`[T; n]`	✓	✓
`[T]`	✓	✓
`Option<T>`	✓	✓
`Result<T, E>`	✓	✓
`Vec<T>`	✓	✓
`BTreeSet<T>`	✓	❌
`BTreeMap<K, V>`	✓	✓
`VecDeque<T>`	✓	✓
`LinkedList<T>`	✓	✓
`LinkedList<T>`	✓	✓
`BinaryHeap<T>`	✓	❌
`HashMap<K, V>`	✓	✓
`HashSet<T>`	✓	❌
`PathBuf`	✓	❌
`Path`	✓	❌
`std::sync::mpsc::Receiver`	✓	❌
`std::os::unix::net::UnixListener`	`incoming()`	❌

If we disregard the chance of false positive, we could even check:

Explicit calls of x.into_iter() where into_iter comes from the trait IntoIterator, and
the adjusted type of x is a reference (&T or &mut T or &&&&&T), and
one of:
- the method T::iter_mut exists and the adjusted type is &mut T (or &mut &mut &mut T), or
- the method T::iter exists,
and
the return type of T::iter_mut/T::iter is equivalent to <&T as IntoIterator>::IntoIter, then
We emit a lint suggesting to call x.iter_mut()/x.iter() instead.

cuviper · 2018-07-10T16:45:14Z

Sorry I haven't found the time for this, but I just rebased my branch and pushed it as-is, in case somebody else wants to take over. I haven't gone through to see if I covered all the cases @kennytm mentioned. I'll try to be responsive about questions of what I was thinking, since I didn't get around to writing many comments either.
https://github.com/cuviper/rust-clippy/tree/array-into_iter

cuviper · 2018-07-11T16:34:49Z

One thing in particular -- I did not cover so many collection types, just arrays and slices. My motivation was primarily linting to avoid collisions with a future impl IntoIterator for [T; N].

kennytm · 2018-07-11T16:41:48Z

The generic procedure at the end should be able to cover all collections except std::os::unix::net::UnixListener.

ishitatsuyuki · 2018-07-13T09:48:18Z

@kennytm I think your suggestion is different from the impl IntoIterator for array case.

Your suggestion is more like a style lint (which is like the lints on for loop), and none of them involves deref coercion or unsizing. The array part, however, involves coercion and thus is harder to implement than the one that doesn't.

I guess I'll only implement for the array case for now.

kennytm · 2018-07-13T09:55:21Z

@ishitatsuyuki "adjusted type" means coercion + auto-reference/deref. Check the compiler docs for "adjustment".

ishitatsuyuki · 2018-07-13T10:19:55Z

@kennytm I mean that the motivation/purpose of these two types of lints are different.

kennytm · 2018-07-13T14:37:27Z

@ishitatsuyuki Yes, and I mean if you need to cover the collection types, it is better to generalize the lint than cover each one individually.

…g#1565.

3344: Added lints `into_iter_on_ref` and `into_iter_on_array`. r=phansch a=kennytm Fixes #1565. `into_iter_on_array` is a correctness lint against `into_iter()` on `[T; n]` and `PathBuf` (please provide a concise noun that covers both types 🙃). `into_iter_on_ref` is a style lint against `into_iter()` on `&C` where `C` is `[T]`, `Vec<T>`, `BTreeSet<T>` etc. Both suggests replacing the `into_iter()` with `iter()` or `iter_mut()`. `into_iter_on_array` is a correctness lint since it is very likely the standard library would provide an `into_iter()` method for `[T; n]` (rust-lang/rust#25725) and existing type inference of `[a, b, c].into_iter()` will be broken. `PathBuf` is also placed under this lint since `PathBuf::into_iter` currently doesn't exist and it makes some sense to implement `IntoIterator` on it yielding `OsString`s. Co-authored-by: kennytm <kennytm@gmail.com>

Should fix clippy toolstat. Changes: ```` rustup rust-lang/rust#55330 Update stderr Rename test files Also lint cfg_attr(.., rustfmt::skip) Add tests from rustfmt::skip test file Run update_lints.py script Add test for non-crate-level inner attributes Differ between inner and outer attributes Add tests Add cfg_attr(rustfmt) lint Addressed comments. Fix dogfood error. Added lints `into_iter_on_ref` and `into_iter_on_array`. Fix rust-lang#1565. Allow single_match_else Update stderr Add copyright statement© Fix typos Fix dogfood error Fix typo and indentation run update_lints script Add tests for unknwon_clippy_lints lint Add new lint: unknwon_clippy_lintsg clippy: fix pedantic warnings and run clippy::pedantic lints on the codebase. Fix a false-positive of needless_borrow UI test cleanup: Extract match_overlapping_arm tests UI test cleanup: Extract expect_fun_call tests Add missing code of conduct file Use slice patterns instead of padding Fix dogfood and pedantic lints ci: when installing rust-toolchain-installer-master, install it in debug mode to save some time in ci. RIIR update lints: Generate deprecated lints Replace big if/else expression with match ````

clarfonthey mentioned this issue Apr 9, 2018

impl IntoIterator for arrays rust-lang/rust#49000

Closed

kennytm mentioned this issue Oct 20, 2018

IntoIterator should be implemented for [T; N] not just references rust-lang/rust#25725

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kennytm added a commit to kennytm/rust-clippy that referenced this issue Oct 20, 2018

Added lints into_iter_on_ref and into_iter_on_array. Fix rust-lan…

5f5ca79

…g#1565.

kennytm mentioned this issue Oct 20, 2018

Added lints into_iter_on_ref and into_iter_on_array. #3344

Merged

kennytm added a commit to kennytm/rust-clippy that referenced this issue Oct 21, 2018

Added lints into_iter_on_ref and into_iter_on_array. Fix rust-lan…

281ff1d

…g#1565.

cuviper mentioned this issue Oct 23, 2018

impl<T> IntoIterator for [T; $N] rust-lang/rust#32871

Closed

kennytm added a commit to kennytm/rust-clippy that referenced this issue Nov 2, 2018

Added lints into_iter_on_ref and into_iter_on_array. Fix rust-lan…

0d0483a

…g#1565.

kennytm added a commit to kennytm/rust-clippy that referenced this issue Nov 2, 2018

Added lints into_iter_on_ref and into_iter_on_array. Fix rust-lan…

2d1c931

…g#1565.

bors bot closed this as completed in #3344 Nov 2, 2018

audunhalland mentioned this issue Jan 13, 2024

Lazy look_ahead graphql-rust/juniper#1212

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Lint `.into_iter()` if that only forwards to `.iter()` #1565

Lint `.into_iter()` if that only forwards to `.iter()` #1565

oli-obk commented Feb 21, 2017

clarfonthey commented Feb 21, 2017

oli-obk commented Feb 21, 2017

bluss commented Feb 21, 2017

clarfonthey commented Apr 9, 2018

cuviper commented Apr 9, 2018 •

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oli-obk commented Apr 9, 2018

tamird commented Jun 6, 2018

cuviper commented Jun 9, 2018

kennytm commented Jun 14, 2018 •

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cuviper commented Jul 10, 2018

cuviper commented Jul 11, 2018

kennytm commented Jul 11, 2018

ishitatsuyuki commented Jul 13, 2018

kennytm commented Jul 13, 2018

ishitatsuyuki commented Jul 13, 2018

kennytm commented Jul 13, 2018

Lint .into_iter() if that only forwards to .iter() #1565

Lint .into_iter() if that only forwards to .iter() #1565

Comments

oli-obk commented Feb 21, 2017

clarfonthey commented Feb 21, 2017

oli-obk commented Feb 21, 2017

bluss commented Feb 21, 2017

clarfonthey commented Apr 9, 2018

cuviper commented Apr 9, 2018 • edited Loading

oli-obk commented Apr 9, 2018

tamird commented Jun 6, 2018

cuviper commented Jun 9, 2018

kennytm commented Jun 14, 2018 • edited Loading

cuviper commented Jul 10, 2018

cuviper commented Jul 11, 2018

kennytm commented Jul 11, 2018

ishitatsuyuki commented Jul 13, 2018

kennytm commented Jul 13, 2018

ishitatsuyuki commented Jul 13, 2018

kennytm commented Jul 13, 2018

Lint `.into_iter()` if that only forwards to `.iter()` #1565

Lint `.into_iter()` if that only forwards to `.iter()` #1565

cuviper commented Apr 9, 2018 •

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kennytm commented Jun 14, 2018 •

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