compiler: allow transmute of ZST arrays with generics #114009

dvdhrm · 2023-07-24T12:28:12Z

Extend the SizeSkeleton evaluator to shortcut zero-sized arrays, thus considering [T; 0] to have a compile-time fixed-size of 0.

The existing evaluator already deals with generic arrays under the feature-guard transmute_const_generics. However, it merely allows comparing fixed-size types with fixed-size types, and generic types with generic types. For generic types, it merely compares whether their arguments match (ordering them first). Even if their exact sizes are not known at compile time, it can ensure that they will eventually be the same.

This patch extends this by shortcutting the size-evaluation of zero sized arrays and thus allowing size comparisons of () with [T; 0], where one contains generics and the other does not.

This code is guarded by transmute_const_generics (#109929), even though it is unclear whether it should be. However, this assumes that a separate stabilization PR is required to move this out of the feature guard.

Initially reported in #98104.

rustbot · 2023-07-24T12:28:19Z

r? @TaKO8Ki

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

dvdhrm · 2023-07-24T14:57:20Z

Note that this correctly calculates the size of [T; 0], as well as [[T; 0]; n], etc. However, it does not propagate through structs, newtypes, etc. This would require more changes to SizeSkeleton.

tests/ui/transmute/transmute-zst-generics.rs

apiraino · 2023-08-18T10:50:28Z

Based on this comment, I'm switching to waiting on author to incorporate changes. Feel free to request a review with @rustbot ready, thanks!

@rustbot author

JulianKnodt · 2023-09-06T08:06:00Z

Since I'm not a maintainer (but have worked on the feature prior), should someone who can approve review @apiraino?

apiraino · 2023-11-15T15:36:51Z

Hello, checking the current status.

I'm sorry @JulianKnodt somehow I completely missed your question! If you're familiar with the codebase, feel free to comment, it's always helpful. But I think that the final r+ can only be emitted by someone with approval rights.

@dvdhrm was this comment addressed? Thanks for an update.

Rerolling reviewer since the current one is not available at this time

r? compiler

cjgillot · 2023-11-25T19:29:17Z

r? compiler

apiraino · 2023-11-27T15:37:35Z

~~rerolling because the new assignee is not actively reviewing patches.~~

EDIT: oops

r? @wesleywiser

apiraino · 2024-02-29T16:01:44Z

rolling the dice 🎲 and finding another reviewer

r? compiler

JulianKnodt · 2024-03-01T21:25:38Z

@apiraino can you find another reviewer? cjgillot already was assigned and then reassigned to someone else

apiraino · 2024-03-05T12:35:40Z

fair point, sorry I didnt notice. if this dice throwing does not work I'll ask someone to adopt this review. Thank you for your patience @JulianKnodt

r? compiler

pnkfelix · 2024-03-05T14:36:01Z

This code is guarded by transmute_const_generics (#109929), even though it is unclear whether it should be. However, this assumes that a separate stabilization PR is required to move this out of the feature guard.

This code looks fine to me.

I was for a moment a little worried that it might introduce some insta-stable behavior change, but the fact quoted above(that this change is entirely contained under feature-guarded code, which is trivial to verify due to how the code is structured) was the detail that made me confident in an r+ here.

@bors r+

bors · 2024-03-05T14:36:04Z

📌 Commit 14be58a has been approved by pnkfelix

It is now in the queue for this repository.

JulianKnodt · 2024-03-05T19:01:55Z

@apiraino thanks for the continued support on this PR
More thanks to @dvdhrm who contributed this!

compiler: allow transmute of ZST arrays with generics Extend the `SizeSkeleton` evaluator to shortcut zero-sized arrays, thus considering `[T; 0]` to have a compile-time fixed-size of 0. The existing evaluator already deals with generic arrays under the feature-guard `transmute_const_generics`. However, it merely allows comparing fixed-size types with fixed-size types, and generic types with generic types. For generic types, it merely compares whether their arguments match (ordering them first). Even if their exact sizes are not known at compile time, it can ensure that they will eventually be the same. This patch extends this by shortcutting the size-evaluation of zero sized arrays and thus allowing size comparisons of `()` with `[T; 0]`, where one contains generics and the other does not. This code is guarded by `transmute_const_generics` (rust-lang#109929), even though it is unclear whether it should be. However, this assumes that a separate stabilization PR is required to move this out of the feature guard. Initially reported in rust-lang#98104.

…kingjubilee Rollup of 13 pull requests Successful merges: - rust-lang#113525 (Dynamically size sigaltstk in std) - rust-lang#114009 (compiler: allow transmute of ZST arrays with generics) - rust-lang#116793 (Allow targets to override default codegen backend) - rust-lang#118623 (Improve std::fs::read_to_string example) - rust-lang#120504 (Vec::try_with_capacity) - rust-lang#121089 (Remove `feed_local_def_id`) - rust-lang#121280 (Implement MaybeUninit::fill{,_with,_from}) - rust-lang#122087 (Add missing background color for top-level rust documentation page and increase contrast by setting text color to black) - rust-lang#122104 (Rust is a proper name: rust → Rust) - rust-lang#122110 (Make `x t miri` respect `MIRI_TEMP`) - rust-lang#122114 (Make not finding core a fatal error) - rust-lang#122115 (Cancel parsing ever made during recovery) - rust-lang#122126 (Fix `tidy --bless` on ̶X̶e̶n̶i̶x̶ Windows) r? `@ghost` `@rustbot` modify labels: rollup

compiler-errors

Need to update ui test flag

tests/ui/transmute/transmute-zst-generics.rs

compiler-errors · 2024-03-07T14:49:31Z

@bors r- #122131 (comment)

Extend the `SizeSkeleton` evaluator to shortcut zero-sized arrays, thus considering `[T; 0]` to have a compile-time fixed-size of 0. The existing evaluator already deals with generic arrays under the feature-guard `transmute_const_generics`. However, it merely allows comparing fixed-size types with fixed-size types, and generic types with generic types. For generic types, it merely compares whether their arguments match (ordering them first). Even if their exact sizes are not known at compile time, it can ensure that they will eventually be the same. This patch extends this by shortcutting the size-evaluation of zero sized arrays and thus allowing size comparisons of `()` with `[T; 0]`, where one contains generics and the other does not. This code is guarded by `transmute_const_generics` (rust-lang#109929), even though it is unclear whether it should be. However, this assumes that a separate stabilization PR is required to move this out of the feature guard. Initially reported in rust-lang#98104.

dvdhrm · 2024-03-20T10:33:24Z

(force-pushed: updated ui-test-flag and added the previously mentioned tests)

@rustbot ready

pnkfelix · 2024-03-22T16:07:28Z

@bors r+

bors · 2024-03-22T16:07:30Z

📌 Commit 31d23c4 has been approved by pnkfelix

It is now in the queue for this repository.

…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#114009 (compiler: allow transmute of ZST arrays with generics) - rust-lang#122195 (Note that the caller chooses a type for type param) - rust-lang#122651 (Suggest `_` for missing generic arguments in turbofish) - rust-lang#122784 (Add `tag_for_variant` query) - rust-lang#122839 (Split out `PredicatePolarity` from `ImplPolarity`) - rust-lang#122873 (Merge my contributor emails into one using mailmap) - rust-lang#122885 (Adjust better spastorino membership to triagebot's adhoc_groups) - rust-lang#122888 (add a couple more tests) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#114009 - dvdhrm:pr/transmzst, r=pnkfelix compiler: allow transmute of ZST arrays with generics Extend the `SizeSkeleton` evaluator to shortcut zero-sized arrays, thus considering `[T; 0]` to have a compile-time fixed-size of 0. The existing evaluator already deals with generic arrays under the feature-guard `transmute_const_generics`. However, it merely allows comparing fixed-size types with fixed-size types, and generic types with generic types. For generic types, it merely compares whether their arguments match (ordering them first). Even if their exact sizes are not known at compile time, it can ensure that they will eventually be the same. This patch extends this by shortcutting the size-evaluation of zero sized arrays and thus allowing size comparisons of `()` with `[T; 0]`, where one contains generics and the other does not. This code is guarded by `transmute_const_generics` (rust-lang#109929), even though it is unclear whether it should be. However, this assumes that a separate stabilization PR is required to move this out of the feature guard. Initially reported in rust-lang#98104.

…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#114009 (compiler: allow transmute of ZST arrays with generics) - rust-lang#122195 (Note that the caller chooses a type for type param) - rust-lang#122651 (Suggest `_` for missing generic arguments in turbofish) - rust-lang#122784 (Add `tag_for_variant` query) - rust-lang#122839 (Split out `PredicatePolarity` from `ImplPolarity`) - rust-lang#122873 (Merge my contributor emails into one using mailmap) - rust-lang#122885 (Adjust better spastorino membership to triagebot's adhoc_groups) - rust-lang#122888 (add a couple more tests) r? `@ghost` `@rustbot` modify labels: rollup

rustbot assigned TaKO8Ki Jul 24, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 24, 2023

dvdhrm mentioned this pull request Jul 24, 2023

Cannot transmute between ZST and zero-sized generic array #98104

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JulianKnodt reviewed Jul 24, 2023

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rustbot 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-review Status: Awaiting review from the assignee but also interested parties. labels Aug 18, 2023

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rustbot assigned cjgillot and unassigned TaKO8Ki Nov 15, 2023

rustbot assigned wesleywiser and unassigned cjgillot Nov 25, 2023

rustbot assigned b-naber and wesleywiser and unassigned wesleywiser and b-naber Nov 27, 2023

rustbot assigned cjgillot and unassigned wesleywiser Feb 29, 2024

rustbot assigned pnkfelix Mar 5, 2024

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workingjubilee mentioned this pull request Mar 7, 2024

Rollup of 13 pull requests #122131

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compiler-errors requested changes Mar 7, 2024

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bors 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 Mar 7, 2024

dvdhrm force-pushed the pr/transmzst branch from 14be58a to 31d23c4 Compare March 20, 2024 10:32

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matthiaskrgr mentioned this pull request Mar 22, 2024

Rollup of 8 pull requests #122900

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bors merged commit 104c4bc into rust-lang:master Mar 23, 2024
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compiler: allow transmute of ZST arrays with generics #114009

compiler: allow transmute of ZST arrays with generics #114009

dvdhrm commented Jul 24, 2023

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dvdhrm commented Jul 24, 2023

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compiler: allow transmute of ZST arrays with generics #114009

compiler: allow transmute of ZST arrays with generics #114009

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