Stabilize FusedIterator #47463

bluss · 2018-01-15T19:04:49Z

FusedIterator is a marker trait that promises that the implementing
iterator continues to return None from .next() once it has returned
None once (and/or .next_back(), if implemented).

The effects of FusedIterator are already widely available through
.fuse(), but with stable FusedIterator, stable Rust users can
implement this trait for their iterators when appropriate.

Closes #35602

rust-highfive · 2018-01-15T19:04:53Z

r? @shepmaster

(rust_highfive has picked a reviewer for you, use r? to override)

sfackler · 2018-01-15T19:36:55Z

This only exists for specialization right? Are we comfortable stabilizing these kinds of things before specialization itself?

bluss · 2018-01-15T20:03:26Z

That is the right question and I think FusedIterator does not depend on any controversial or hard part of specialization, so it seems ok.

meanwhile I can't understand what tidy wants me to fix..

bluss · 2018-01-17T21:20:04Z

rebased to fix newer FusedIterator impls. That should fix tidy.

sfackler · 2018-01-18T05:46:26Z

The worst case is that this becomes a slightly weird/useless trait if we have to do something drastic like roll back specialization entirely. Seems okay to stabilize I guess.

@rfcbot fcp merge

rfcbot · 2018-01-18T05:46:26Z

Team member @sfackler has proposed to merge this. The next step is review by the rest of the tagged teams:

No concerns currently listed.

Once these reviewers reach consensus, this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

withoutboats · 2018-01-18T08:14:46Z

That is the right question and I think FusedIterator does not depend on any controversial or hard part of specialization, so it seems ok.

Maybe I don't understand how it is used, but isn't one of the main use cases essentially something like this:

trait Fuse {
    type Fused;
    fn fuse(self) -> Self::Fused;
}

impl<T: Iterator> Fuse for T { ... }

impl<T: FusedIterator> Fuse for T { ... }

This would involve specializing the associated type, which is one of the trickiest parts of specialization (because it could result in different impl instantiations at typeck time from trans time).

bluss · 2018-01-18T11:17:42Z

The main use is in .fused and it is method specialization only, no associated types.

Your example is interesting in itself but not what std is using now.

bluss · 2018-01-18T11:18:08Z

In Iterator::fuse.

alexcrichton · 2018-01-18T15:59:11Z

Is there perhaps a "banner use case" for this trait right now? I've personally only ever seen it as a pretty niche optimization that almost never comes up in practice (I can't remember the last time I used fuse). That, coupled with the concern about specialization, gives me pause here.

I'm also pretty worried about the specialization here. If we roll it back I feel like it would be a breaking change with a trait like this. Users would use this trait only for the performance improvement (right?) and if we break that then we're effectively breaking the trait?

bluss · 2018-01-18T17:11:43Z

@alexcrichton .fuse() is something itertools uses a lot, so it's typical of that use case -- when we build upon iterators completely generically. For example, a.merge(b) from itertools uses .fuse() on both the component iterators.

The use case would be to implement the marker trait FusedIterator for itertools' own adaptors, where appropriate, which makes the .fuse() usages in itertools or other places much more "pay what you need" instead of paying for that extra flag check per iteration.

As a small nice thing, having FusedIterator stable would make the fusing behaviour of iterator adaptors visible and documented by their trait implementations and if/how those depend on the adapted iterator.

I think the specialization that is assumed here really is the basics and it was my impression that we are pretty certain to ship that part sooner rather than later.

alexcrichton · 2018-01-19T17:07:30Z

@bluss ok cool thanks for the info! To clarify as well, would you consider it a breaking change if we end up later neutering the trait to not actually do anything (if specialization is removed)?

The state of specialization AFAIK is sort of "continuously in the air" where there's always a few known soundness holes with no known fixes, but there's a big chunk that's known as highly desirable and safe but unknown how to stabilize.

bluss · 2018-01-19T17:30:15Z

No, I can't see how it is a breaking change. We would have performance setbacks all over libcore without spec, fuse is not among the most important, and I imagine we would want other mechanisms to get the same effects. There is a risk that the trait becomes obsolete then yes if the new mechanism is not trait based.

alexcrichton · 2018-01-19T22:10:24Z

Hm ok, if you're ok with that outcome so am I!

kennytm · 2018-01-31T12:28:55Z

Triage ping, ticky boxes for you @BurntSushi!

pietroalbini · 2018-02-05T10:56:52Z

@BurntSushi there is a nice checkbox in #47463 (comment) waiting for you!

ollie27 · 2018-02-05T14:03:26Z

src/libcore/char.rs

@@ -904,5 +904,5 @@ impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I> {
    }
 }

-#[unstable(feature = "fused", issue = "35602")]
+#[stable(feature = "fused", since = "1.25.0")]


DecodeUtf8 is still unstable so this should be #[unstable(feature = "decode_utf8", issue = "33906")].

ollie27 · 2018-02-05T14:05:10Z

src/libcore/iter/range.rs

@@ -420,5 +420,5 @@ impl<A: Step> DoubleEndedIterator for ops::RangeInclusive<A> {
    }
 }

-#[unstable(feature = "fused", issue = "35602")]
+#[stable(feature = "fused", since = "1.25.0")]


RangeInclusive is still unstable so this should be #[unstable(feature = "inclusive_range", reason = "recently added, follows RFC", issue = "28237")].

ollie27 · 2018-02-05T14:07:00Z

src/libcore/slice/mod.rs

@@ -2494,7 +2494,7 @@ impl<'a, T> ExactSizeIterator for ExactChunks<'a, T> {
    }
 }

-#[unstable(feature = "fused", issue = "35602")]
+#[stable(feature = "fused", since = "1.25.0")]


ExactChunks is still unstable so this should be #[unstable(feature = "exact_chunks", issue = "47115")].

ollie27 · 2018-02-05T14:07:09Z

src/libcore/slice/mod.rs

@@ -2591,7 +2591,7 @@ impl<'a, T> ExactSizeIterator for ExactChunksMut<'a, T> {
    }
 }

-#[unstable(feature = "fused", issue = "35602")]
+#[stable(feature = "fused", since = "1.25.0")]


ExactChunksMut is still unstable so this should be #[unstable(feature = "exact_chunks", issue = "47115")].

ollie27 · 2018-02-05T14:11:10Z

src/libstd_unicode/u_str.rs

@@ -127,7 +127,7 @@ impl<I> Iterator for Utf16Encoder<I>
    }
 }

-#[unstable(feature = "fused", issue = "35602")]
+#[stable(feature = "fused", since = "1.25.0")]


Utf16Encoder is still unstable so this line can be removed.

ollie27 · 2018-02-05T14:14:47Z

src/libcore/slice/mod.rs

@@ -1847,7 +1847,7 @@ impl<'a, T, P> SplitIter for RSplit<'a, T, P> where P: FnMut(&T) -> bool {
    }
 }

-//#[unstable(feature = "fused", issue = "35602")]
+//#[stable(feature = "fused", since = "1.25.0")]


RSplit is still unstable so this line can just be removed.

ollie27 · 2018-02-05T14:14:56Z

src/libcore/slice/mod.rs

@@ -1906,7 +1906,7 @@ impl<'a, T, P> DoubleEndedIterator for RSplitMut<'a, T, P> where
    }
 }

-//#[unstable(feature = "fused", issue = "35602")]
+//#[stable(feature = "fused", since = "1.25.0")]


RSplitMut is still unstable so this line can just be removed.

rfcbot · 2018-02-06T21:22:43Z

🔔 This is now entering its final comment period, as per the review above. 🔔

alexcrichton · 2018-02-06T21:29:24Z

Ok great! @bluss do you wanna address @ollie27's comments and I'll r+?

bors · 2018-03-05T21:43:39Z

⌛ Testing commit c7c23fe with merge d9455e8a7f272c0fec238dbba7af39b560dbf2f0...

bors · 2018-03-06T00:44:16Z

💔 Test failed - status-travis

alexcrichton · 2018-03-06T02:47:01Z

@bors: retry

dist timeout :(

bors · 2018-03-06T03:47:35Z

⌛ Testing commit c7c23fe with merge 386cd00ba6fa150f64cdae31223f5a648fcf08c4...

bors · 2018-03-06T05:25:53Z

💔 Test failed - status-appveyor

kennytm · 2018-03-06T10:06:56Z

@bors retry #48775

Stabilize FusedIterator FusedIterator is a marker trait that promises that the implementing iterator continues to return `None` from `.next()` once it has returned `None` once (and/or `.next_back()`, if implemented). The effects of FusedIterator are already widely available through `.fuse()`, but with stable `FusedIterator`, stable Rust users can implement this trait for their iterators when appropriate. Closes rust-lang#35602

Rollup of 14 pull requests - Successful merges: #48403, #48432, #48546, #48573, #48590, #48657, #48727, #48732, #48753, #48754, #48761, #48474, #48507, #47463 - Failed merges:

bluss · 2018-03-07T20:26:23Z

Aw, that's quite the fight with bors. You pushed it over the finish line @alexcrichton, thank you!

…r=bluss Unstabilize FusedIterator for Flatten since Flatten is unstable PR rust-lang#47463 made `impl<I, U> FusedIterator for Flatten<I>` stable but shouldn't have since `Flatten` is still unstable. This PR makes the impl unstable again.

rust-highfive assigned shepmaster Jan 15, 2018

bluss added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Jan 15, 2018

bluss force-pushed the fused-iterator branch from 76113a2 to 9273422 Compare January 15, 2018 20:02

kennytm added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 16, 2018

bluss force-pushed the fused-iterator branch 2 times, most recently from 644f111 to 96e6103 Compare January 17, 2018 21:19

rfcbot added the proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. label Jan 18, 2018

kennytm added S-waiting-on-team Status: Awaiting decision from the relevant subteam (see the T-<team> label). and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 31, 2018

ollie27 reviewed Feb 5, 2018

View reviewed changes

rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Feb 6, 2018

bors added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Mar 5, 2018

bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Mar 6, 2018

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-review Status: Awaiting review from the assignee but also interested parties. labels Mar 6, 2018

bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Mar 6, 2018

kennytm mentioned this pull request Mar 6, 2018

Spurious PermissionDenied error with cargo's rename_with_link_search_path test in AppVeyor #48775

Closed

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-review Status: Awaiting review from the assignee but also interested parties. labels Mar 6, 2018

kennytm mentioned this pull request Mar 6, 2018

Rollup of 14 pull requests #48768

Merged

alexcrichton merged commit c7c23fe into rust-lang:master Mar 6, 2018

bluss deleted the fused-iterator branch March 7, 2018 20:16

Centril mentioned this pull request Mar 15, 2018

Unstabilize FusedIterator for Flatten since Flatten is unstable #49049

Merged

orium mentioned this pull request Apr 1, 2018

Implement FusedIterator for our iterators (when it stabilizes) orium/rpds#16

Open

bluss mentioned this pull request Nov 24, 2018

Document optimizations enabled by FusedIterator #55896

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Stabilize FusedIterator #47463

Stabilize FusedIterator #47463

bluss commented Jan 15, 2018

rust-highfive commented Jan 15, 2018

sfackler commented Jan 15, 2018

bluss commented Jan 15, 2018

bluss commented Jan 17, 2018

sfackler commented Jan 18, 2018

rfcbot commented Jan 18, 2018 •

edited by BurntSushi

Loading

withoutboats commented Jan 18, 2018

bluss commented Jan 18, 2018

bluss commented Jan 18, 2018

alexcrichton commented Jan 18, 2018

bluss commented Jan 18, 2018 •

edited

Loading

alexcrichton commented Jan 19, 2018

bluss commented Jan 19, 2018

alexcrichton commented Jan 19, 2018

kennytm commented Jan 31, 2018

pietroalbini commented Feb 5, 2018

ollie27 Feb 5, 2018

ollie27 Feb 5, 2018

ollie27 Feb 5, 2018

ollie27 Feb 5, 2018

ollie27 Feb 5, 2018

ollie27 Feb 5, 2018

ollie27 Feb 5, 2018

rfcbot commented Feb 6, 2018

alexcrichton commented Feb 6, 2018

bors commented Mar 5, 2018

bors commented Mar 6, 2018

alexcrichton commented Mar 6, 2018

bors commented Mar 6, 2018

bors commented Mar 6, 2018

kennytm commented Mar 6, 2018

bluss commented Mar 7, 2018

Stabilize FusedIterator #47463

Stabilize FusedIterator #47463

Conversation

bluss commented Jan 15, 2018

rust-highfive commented Jan 15, 2018

sfackler commented Jan 15, 2018

bluss commented Jan 15, 2018

bluss commented Jan 17, 2018

sfackler commented Jan 18, 2018

rfcbot commented Jan 18, 2018 • edited by BurntSushi Loading

withoutboats commented Jan 18, 2018

bluss commented Jan 18, 2018

bluss commented Jan 18, 2018

alexcrichton commented Jan 18, 2018

bluss commented Jan 18, 2018 • edited Loading

alexcrichton commented Jan 19, 2018

bluss commented Jan 19, 2018

alexcrichton commented Jan 19, 2018

kennytm commented Jan 31, 2018

pietroalbini commented Feb 5, 2018

ollie27 Feb 5, 2018

Choose a reason for hiding this comment

ollie27 Feb 5, 2018

Choose a reason for hiding this comment

ollie27 Feb 5, 2018

Choose a reason for hiding this comment

ollie27 Feb 5, 2018

Choose a reason for hiding this comment

ollie27 Feb 5, 2018

Choose a reason for hiding this comment

ollie27 Feb 5, 2018

Choose a reason for hiding this comment

ollie27 Feb 5, 2018

Choose a reason for hiding this comment

rfcbot commented Feb 6, 2018

alexcrichton commented Feb 6, 2018

bors commented Mar 5, 2018

bors commented Mar 6, 2018

alexcrichton commented Mar 6, 2018

bors commented Mar 6, 2018

bors commented Mar 6, 2018

kennytm commented Mar 6, 2018

bluss commented Mar 7, 2018

rfcbot commented Jan 18, 2018 •

edited by BurntSushi

Loading

bluss commented Jan 18, 2018 •

edited

Loading