Fix bugs in Peekable and Flatten when using non-fused iterators #68915

timvermeulen · 2020-02-07T02:31:11Z

I fixed a couple of bugs with regard to the Peekable and Flatten/FlatMap iterators when the underlying iterator isn't fused. For testing, I also added a NonFused iterator wrapper that panics when next or next_back is called on an iterator that has returned None before, which will hopefully make it easier to spot these mistakes in the future.

Peekable

Peekable::next_back was implemented as

self.iter.next_back().or_else(|| self.peeked.take().and_then(|x| x))

which is incorrect because when the peeked field is Some(None), then None has already been returned from the inner iterator and what it returns from next_back can no longer be relied upon. test_peekable_non_fused tests this.

Flatten

When a FlattenCompat instance only has a backiter remaining (i.e. self.frontiter is None and self.iter is empty), then next will call self.iter.next() every time, so the iter field needs to be fused. I fixed it by giving it the type Fuse<I> instead of I, I think this is the only way to fix it. test_flatten_non_fused_outer tests this.

Furthermore, previously FlattenCompat::next did not set self.frontiter to None after it returned None, which is incorrect when the inner iterator type isn't fused. I just delegated it to try_fold because that already handles it correctly. test_flatten_non_fused_inner tests this.

r? @scottmcm

rust-highfive · 2020-02-10T12:51:53Z

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Dylan-DPC-zz · 2020-02-13T16:52:14Z

r? @Amanieu

Amanieu · 2020-03-02T11:14:39Z

Furthermore, previously FlattenCompat::next did not set self.frontiter to None after it returned None, which is incorrect when the inner iterator type isn't fused. I just delegated it to try_fold because that already handles it correctly. test_flatten_non_fused_inner tests this.

I am concerned that the generated code is now substantially more complicated. Wouldn't it have been better to simply set frontiter to None after the if?

All the other changes look fine.

JohnCSimon · 2020-03-09T07:08:57Z

Ping from triage:
@timvermeulen can you please address the comments from Amanieu? Thank you.

timvermeulen · 2020-03-13T22:47:24Z

Sorry, yep, I'll address them soon.

timvermeulen · 2020-03-16T23:08:07Z

@Amanieu Thanks for taking a look! I assumed that the generated code for next / next_back would be the same, but benchmarks show that this was indeed a regression, so I've fixed it in the way you suggested.

Amanieu · 2020-03-17T07:26:43Z

@bors r+

bors · 2020-03-17T07:26:45Z

📌 Commit 8cf33b0 has been approved by Amanieu

@scottmcm

Fix bugs in Peekable and Flatten when using non-fused iterators I fixed a couple of bugs with regard to the `Peekable` and `Flatten`/`FlatMap` iterators when the underlying iterator isn't fused. For testing, I also added a `NonFused` iterator wrapper that panics when `next` or `next_back` is called on an iterator that has returned `None` before, which will hopefully make it easier to spot these mistakes in the future. ### Peekable `Peekable::next_back` was implemented as ```rust self.iter.next_back().or_else(|| self.peeked.take().and_then(|x| x)) ``` which is incorrect because when the `peeked` field is `Some(None)`, then `None` has already been returned from the inner iterator and what it returns from `next_back` can no longer be relied upon. `test_peekable_non_fused` tests this. ### Flatten When a `FlattenCompat` instance only has a `backiter` remaining (i.e. `self.frontiter` is `None` and `self.iter` is empty), then `next` will call `self.iter.next()` every time, so the `iter` field needs to be fused. I fixed it by giving it the type `Fuse<I>` instead of `I`, I think this is the only way to fix it. `test_flatten_non_fused_outer` tests this. Furthermore, previously `FlattenCompat::next` did not set `self.frontiter` to `None` after it returned `None`, which is incorrect when the inner iterator type isn't fused. I just delegated it to `try_fold` because that already handles it correctly. `test_flatten_non_fused_inner` tests this. r? @scottmcm

bors · 2020-03-18T03:09:03Z

⌛ Testing commit 8cf33b0 with merge d939f70...

bors · 2020-03-18T06:31:18Z

☀️ Test successful - checks-azure
Approved by: Amanieu
Pushing d939f70 to master...

rust-highfive · 2020-03-18T06:32:47Z

📣 Toolstate changed by #68915!

Tested on commit d939f70.
Direct link to PR: #68915

💔 rls on linux: test-pass → test-fail (cc @Xanewok).

@Xanewok

Tested on commit rust-lang/rust@d939f70. Direct link to PR: <rust-lang/rust#68915> 💔 rls on linux: test-pass → test-fail (cc @Xanewok).

rust-highfive assigned scottmcm Feb 7, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 7, 2020

timvermeulen added 2 commits February 10, 2020 13:32

Fix Peekable::next_back

14bcf0a

Fuse FlattenCompat's inner iterator

16a23e7

rust-highfive assigned Amanieu and unassigned scottmcm Feb 13, 2020

timvermeulen added 2 commits March 17, 2020 00:03

Fix FlattenCompat::{next, next_back}

cff1182

Add tests

8cf33b0

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 17, 2020

Manishearth mentioned this pull request Mar 17, 2020

Rollup of 12 pull requests #70085

Closed

bors added the merged-by-bors This PR was explicitly merged by bors. label Mar 18, 2020

bors merged commit d939f70 into rust-lang:master Mar 18, 2020

timvermeulen deleted the non_fused_iter branch March 18, 2020 14:23

cuviper mentioned this pull request Jan 21, 2021

Flatten and FlatMap have incorrect bounds for their FusedIterator impls #81248

Closed

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Fix bugs in Peekable and Flatten when using non-fused iterators #68915

Fix bugs in Peekable and Flatten when using non-fused iterators #68915

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Fix bugs in Peekable and Flatten when using non-fused iterators #68915

Fix bugs in Peekable and Flatten when using non-fused iterators #68915

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timvermeulen commented Feb 7, 2020

Peekable

Flatten

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