Add by-value iterator for arrays #62959

LukasKalbertodt · 2019-07-24T22:52:15Z

This adds an iterator that can iterate over arrays by value, yielding all elements by value. However, this PR does not add a corresponding IntoIterator impl for arrays. The IntoIterator impl needs some discussion about backwards-compatibility that should take place in a separate PR. With this patch, this code should work (but there is currently still a bug):

#![feature(array_value_iter)]
use std::array::IntoIter;

let arr = [1, 2, 3];
for x in IntoIter::new(arr) {
    println!("{}", x);
}

TODO:

Get initial feedback
Add tests
Figure out why stage1 produces weird bugs (comment)
Add UI tests as mentioned here (will do that soon-ish)
Fix this new bug

Notes for reviewers

Is the use of MaybeUninit correct here? I think it has to be used due to the Clone impl which has to fill the dead array elements with something, but cannot fill it with a correct instance.
Are the unit tests sufficient?

CC #25725

Centril

Overall this looks nice. Here's some food for thought however:

I think some tests should also be added that exercise each impl you've provided here.
In particular, there should be tests collecting into a Vec<...> and then checking the contents.
It would be good to have tests that drop some elements on the floor in some positions (let _ = iter.next();)
Checking the boundary conditions around N and self.pos is a good idea.
Tests that also format!(...) the iterator to exercise Debug would be good.

(Some of these checks can be done in the same tests).

Also cc @scottmcm

src/libcore/array.rs

Centril · 2019-07-24T23:47:32Z

r? @scottmcm

Mark-Simulacrum · 2019-07-24T23:49:19Z

This is unfortunately not backwards compatible. Arrays deref to slices and calling .into_iter currently returns &T but would begin returning T. cc @cuviper who has done investigation into this in the past - we should probably update the relevant issue.

LukasKalbertodt · 2019-07-25T00:14:14Z

@Centril Thanks a lot for the fairly in-depth review already! I will fix all of that once the bigger questions are resolved.

@Mark-Simulacrum I wasn't aware this is a problem. Sigh, that's unfortunate indeed. (I am a bit surprised about quickly closing the PR without a discussion though...)

From #49000 it seems like there has been a fair amount of discussion already. To sidestep the IntoIterator problem entirely for now, what do you think about changing this PR to only add the iterator type and an unstable fn owned_iter(self) -> IntoIter for arrays? That way the actual functionality is already there and we can discuss how we deal with IntoIterator afterwards. Makes sense?

Centril · 2019-07-25T00:28:50Z

I also wonder if there has already been a crater run and what the fallout was? Maybe @cuviper knows.

Ostensibly some minor breakage could (it's a choice!) be justified as "inference breakage" and this is a rather large paper cut in the language which would be good to finally solve. (I'm also thinking of #59500 which technically was not "allowed" due to #[fundamental] but which we merged anyways and did not revert even with the ensuing reported regression).

To sidestep the IntoIterator problem entirely for now, what do you think about changing this PR to only add the iterator type and an unstable fn owned_iter(self) -> IntoIter for arrays? That way the actual functionality is already there and we can discuss how we deal with IntoIterator afterwards. Makes sense?

In my view a solid interim plan! Reopening so that we may consider that.

cuviper

Please compare with #49000 for feature parity of what I did before. Most of it should be similar, apart from having real const generics now. I had as_slice() and as_mut_slice(), Clone, DoubleEndedIterator, ExactSizedIterator, FusedIterator, and TrustedLen. You could take my added tests too.

src/libcore/array.rs

cuviper · 2019-07-25T00:46:02Z

I also wonder if there has already been a crater run and what the fallout was?

I don't think we did a direct crater run with the new IntoIterator, but there was an attempt at running with the clippy lint denied, which failed horribly for unrelated reasons. (The rustflags interfered with build scripts, or something like that.)

Centril · 2019-07-25T01:05:16Z

@cuviper In that case this is as good a time as any to try out a crater run then.

scottmcm · 2019-07-25T01:10:42Z

My personal opinion: having this impl is so obviously the correct thing that I'd be willing to bend stability guarantees to have it, but we don't even need to because adding a new trait impl is an allowed change, no matter whether it breaks code. And the only code that it breaks, today, is code that was doing a.into_iter() when a.iter() would have done exactly the same thing for less typing, so any workaround needed to not trigger this change will make the code strictly better regardless.

src/libcore/array.rs

RalfJung · 2019-07-25T13:25:01Z

And the only code that it breaks, today, is code that was doing a.into_iter() when a.iter() would have done exactly the same thing for less typing,

Is that true? I think it also breaks code that does for x in a because for implicitly does into_iter().

We could probably have a clippy/rustc lint to start moving people away from that, though.

Mark-Simulacrum · 2019-07-25T13:28:28Z

for _ in a doesn't actually go through deref on a as we call IntoIterator::into_iter(a) instead of a.into_iter() in the desugaring, that is, https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a367b046176dcf7a3eb762da2ace045a doesn't compile.

RalfJung · 2019-07-25T13:46:42Z

@Mark-Simulacrum ah, good point. That's great then!

The part about the lint still stands.

Centril · 2019-07-25T14:16:52Z

for _ in a doesn't actually go through deref on a as we call IntoIterator::into_iter(a) instead of a.into_iter() in the desugaring, that is, https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a367b046176dcf7a3eb762da2ace045a doesn't compile.

(Relevant lowering code:

rust/src/librustc/hir/lowering.rs

Lines 4887 to 4894 in eedf6ce

    
           let iter = P(self.expr_ident(head_sp, iter, iter_pat_nid)); 
        
           let ref_mut_iter = self.expr_mut_addr_of(head_sp, iter); 
        
           let next_path = &[sym::iter, sym::Iterator, sym::next]; 
        
           let next_expr = P(self.expr_call_std_path( 
        
               head_sp, 
        
               next_path, 
        
               hir_vec![ref_mut_iter], 
        
           ));

)

cuviper · 2019-07-25T14:55:56Z

The clippy lint exists, although I don't think it was default-deny when we tried the crater run before.

https://rust-lang.github.io/rust-clippy/master/index.html#into_iter_on_array

src/libcore/array/mod.rs

The iterator is implemented using const generics. It implements the traits `Iterator`, `DoubleEndedIterator`, `ExactSizeIterator`, `FusedIterator` and `TrustedLen`. It also contains a public method `new` to create it from an array. `IntoIterator` was not implemented for arrays yet, as there are still some open questions regarding backwards compatibility. This commit only adds the iterator impl and does not yet offer a convenient way to obtain that iterator.

Many tests are based on tests by Josh Stone <cuviper@gmail.com>

This it to make sure traits are implemented for arrays with length 32 and below, while they are not implemented for >= 33.

scottmcm · 2019-10-25T03:45:03Z

Huzzah! Kudos to everyone working on const generics for getting this to work ❤️

@bors r+

bors · 2019-10-25T03:45:04Z

📌 Commit c36b9dd has been approved by scottmcm

…=scottmcm Add by-value iterator for arrays This adds an iterator that can iterate over arrays by value, yielding all elements by value. However, **this PR does _not_ add a corresponding `IntoIterator` impl for arrays**. The `IntoIterator` impl needs some discussion about backwards-compatibility that should take place in a separate PR. With this patch, this code should work (but there is currently still a bug): ```rust #![feature(array_value_iter)] use std::array::IntoIter; let arr = [1, 2, 3]; for x in IntoIter::new(arr) { println!("{}", x); } ``` **TODO**: - [x] Get initial feedback - [x] Add tests - [x] Figure out why stage1 produces weird bugs ([comment](rust-lang#62959 (comment))) - [x] Add UI tests as mentioned [here](rust-lang#62959 (comment)) (will do that soon-ish) - [x] Fix [this new bug](rust-lang#62959 (comment)) **Notes for reviewers** - Is the use of `MaybeUninit` correct here? I think it has to be used due to the `Clone` impl which has to fill the dead array elements with something, but cannot fill it with a correct instance. - Are the unit tests sufficient? CC rust-lang#25725

@ghost

Rollup of 9 pull requests Successful merges: - #62959 (Add by-value iterator for arrays ) - #65390 (Add long error explanation for E0576) - #65408 (reorder config.toml.example options and add one missing option) - #65414 (ignore uninhabited non-exhaustive variant fields) - #65666 (Deprecated proc_macro doesn't trigger warning on build library) - #65742 (Pre-expansion gate most of the things) - #65747 (Adjust the tracking issue for `untagged_unions`.) - #65763 (Changed APIT with explicit generic args span to specific arg spans) - #65775 (Fix more `ReEmpty` ICEs) Failed merges: - #65519 (trait-based structural match implementation) r? @ghost

src/libcore/array/iter.rs

@Centril

…er-tracking-issue, r=Centril Fill tracking issue number for `array_value_iter` Thanks for [noticing](rust-lang#62959 (comment))! r? @Centril

@Centril

…er-tracking-issue, r=Centril Fill tracking issue number for `array_value_iter` Thanks for [noticing](rust-lang#62959 (comment))! r? @Centril

@Centril

…er-tracking-issue, r=Centril Fill tracking issue number for `array_value_iter` Thanks for [noticing](rust-lang#62959 (comment))! r? @Centril

@Centril

…er-tracking-issue, r=Centril Fill tracking issue number for `array_value_iter` Thanks for [noticing](rust-lang#62959 (comment))! r? @Centril

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LukasKalbertodt force-pushed the array-value-iter branch from ac96d77 to cad2150 Compare July 26, 2019 13:01

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LukasKalbertodt added 3 commits October 24, 2019 15:46

Add unit tests for array::IntoIter

5334a30

Many tests are based on tests by Josh Stone <cuviper@gmail.com>

Add UI tests for array::IntoIter impls

c36b9dd

This it to make sure traits are implemented for arrays with length 32 and below, while they are not implemented for >= 33.

LukasKalbertodt force-pushed the array-value-iter branch from 0fe38bd to c36b9dd Compare October 24, 2019 13:47

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Rollup of 9 pull requests #65793

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Tracking issue for by-value array iterator (feature array_value_iter) #65798

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Fill tracking issue number for array_value_iter #65799

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Add IntoIterator impl for arrays by value (for [T; N]) #65819

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IntoIterator should be implemented for [T; N] not just references #25725

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HalidOdat mentioned this pull request Jun 11, 2020

Object specialization boa-dev/boa#419

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Add by-value iterator for arrays #62959

Add by-value iterator for arrays #62959

LukasKalbertodt commented Jul 24, 2019 •

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