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Return false from needs_drop for all zero-sized arrays. - #65389

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ecstatic-morse:zero-sized-array-no-drop
Oct 16, 2019
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Return false from needs_drop for all zero-sized arrays.#65389
bors merged 3 commits into
rust-lang:masterfrom
ecstatic-morse:zero-sized-array-no-drop

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@ecstatic-morse

@ecstatic-morseecstatic-morse commented Oct 13, 2019•
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Resolves#65348.

This changes the result of the needs_drop query from true to false for types such as [Box<i32>; 0]. I believe this change to be sound because a zero-sized array can never actually hold a value. This is an elegant way of resolving #65348 and #64945, but obviously it has much broader implications.

@rust-highfive

rust-highfive commented Oct 13, 2019

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r? @davidtwco

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

@rust-highfiverust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 13, 2019
@ecstatic-morse

ecstatic-morse commented Oct 13, 2019•
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Not sure that this is ultimately the way to go, but may be worth discussing. We may want a more targeted fix.

cc @eddyb@oli-obk@RalfJung.

@davidtwco

davidtwco commented Oct 13, 2019

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r? @eddyb

@rust-highfiverust-highfive assigned eddyb and unassigned davidtwcoOct 13, 2019
@Centril

Centril commented Oct 13, 2019

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We already special case zero sized arrays in other places so I'm comfortable with doing it here as well. However, cc @rust-lang/lang

@ecstatic-morse
ecstatic-morseforce-pushed the zero-sized-array-no-drop branch from 82e9edc to c08a871CompareOctober 13, 2019 23:01
@ecstatic-morse

ecstatic-morse commented Oct 13, 2019•
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I added a regression test for the underlying const validation issue as well. A perf run might be informative here. This will result in more calls to try_eval_usize, but will also emit fewer drop terminators in code that constructs an empty array or slice whose element type may have a Drop impl.

@Centril

Centril commented Oct 14, 2019

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@bors try @rust-timer queue

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rust-timer commented Oct 14, 2019

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Awaiting bors try build completion

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bors commented Oct 14, 2019

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⌛ Trying commit c08a871 with merge e823bc1...

bors added a commit that referenced this pull request Oct 14, 2019
@bors
Auto merge of#65389- ecstatic-morse:zero-sized-array-no-drop, r=<try>
e823bc1
Return `false` from `needs_drop` for all zero-sized arrays.
Resolves#65348.
This changes the result of the `needs_drop` query from `true` to `false` for types such as `[Box<i32>; 0]`. I believe this change to be sound because a zero-sized array can never actually hold a value. This is an elegant way of resolving #65348 and #64945, but obviously it has much broader implications.
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bors commented Oct 14, 2019

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☀️ Try build successful - checks-azure
Build commit: e823bc1 (e823bc1930f285a41408feda9a2dfc80bffbb5b8)

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rust-timer commented Oct 14, 2019

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Queued e823bc1 with parent 36d4506, future comparison URL.

eddyb
eddyb approved these changes Oct 14, 2019
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r=me

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rust-timer commented Oct 14, 2019

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Finished benchmarking try commit e823bc1, comparison URL.

@ecstatic-morse

ecstatic-morse commented Oct 15, 2019•
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Perf impact is negligible. Just needs an r+.

@eddyb

eddyb commented Oct 15, 2019

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@bors r+

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bors commented Oct 15, 2019

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📌 Commit c08a871 has been approved by eddyb

@borsbors 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 Oct 15, 2019
Centril added a commit to Centril/rust that referenced this pull request Oct 15, 2019
@Centril
Rollup merge ofrust-lang#65389- ecstatic-morse:zero-sized-array-no-…
39e8aa1
…drop, r=eddyb
Return `false` from `needs_drop` for all zero-sized arrays.
Resolvesrust-lang#65348.
This changes the result of the `needs_drop` query from `true` to `false` for types such as `[Box<i32>; 0]`. I believe this change to be sound because a zero-sized array can never actually hold a value. This is an elegant way of resolving rust-lang#65348 and rust-lang#64945, but obviously it has much broader implications.
@CentrilCentril mentioned this pull request Oct 15, 2019
Closed
Centril added a commit to Centril/rust that referenced this pull request Oct 15, 2019
@Centril
Rollup merge ofrust-lang#65389- ecstatic-morse:zero-sized-array-no-…
2feabbb
…drop, r=eddyb
Return `false` from `needs_drop` for all zero-sized arrays.
Resolvesrust-lang#65348.
This changes the result of the `needs_drop` query from `true` to `false` for types such as `[Box<i32>; 0]`. I believe this change to be sound because a zero-sized array can never actually hold a value. This is an elegant way of resolving rust-lang#65348 and rust-lang#64945, but obviously it has much broader implications.
@CentrilCentril mentioned this pull request Oct 15, 2019
Closed
tmandry added a commit to tmandry/rust that referenced this pull request Oct 15, 2019
@tmandry
Rollup merge ofrust-lang#65389- ecstatic-morse:zero-sized-array-no-…
bbcf66a
…drop, r=eddyb
Return `false` from `needs_drop` for all zero-sized arrays.
Resolvesrust-lang#65348.
This changes the result of the `needs_drop` query from `true` to `false` for types such as `[Box<i32>; 0]`. I believe this change to be sound because a zero-sized array can never actually hold a value. This is an elegant way of resolving rust-lang#65348 and rust-lang#64945, but obviously it has much broader implications.
@tmandrytmandry mentioned this pull request Oct 15, 2019
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bors added a commit that referenced this pull request Oct 15, 2019
@bors
Auto merge of#65454- tmandry:rollup-0k6jiik, r=tmandry
f54911c
Rollup of 14 pull requests
Successful merges:
- #64603 (Reducing spurious unused lifetime warnings.)
- #64623 (Remove last uses of gensyms)
- #65235 (don't assume we can *always* find a return type hint in async fn)
- #65242 (Fix suggestion to constrain trait for method to be found)
- #65265 (Cleanup librustc mir err codes)
- #65293 (Optimize `try_expand_impl_trait_type`)
- #65307 (Try fix incorrect "explicit lifetime name needed")
- #65308 (Add long error explanation for E0574)
- #65353 (save-analysis: Don't ICE when resolving qualified type paths in struct members)
- #65389 (Return `false` from `needs_drop` for all zero-sized arrays.)
- #65402 (Add troubleshooting section to PGO chapter in rustc book.)
- #65425 (Optimize `BitIter`)
- #65438 (Organize `never_type` tests)
- #65444 (Implement AsRef<[T]> for List<T>)
Failed merges:
- #65390 (Add long error explanation for E0576)
r? @ghost
@bors
bors merged commit c08a871 into rust-lang:masterOct 16, 2019
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bors commented Oct 16, 2019

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☔ The latest upstream changes (presumably #65454) made this pull request unmergeable. Please resolve the merge conflicts.

@borsbors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Oct 16, 2019
@ecstatic-morse
ecstatic-morse deleted the zero-sized-array-no-drop branch October 16, 2019 05:28
@ecstatic-morseecstatic-morse mentioned this pull request Oct 25, 2019
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