Implement RFC 2645 (transparent enums and unions) #60463

mjbshaw · 2019-05-02T01:15:49Z

Tracking issue: #60405

rust-highfive · 2019-05-02T01:15:53Z

r? @varkor

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

Centril · 2019-05-02T10:38:33Z

cc @pnkfelix

varkor

This looks mostly good. Just a few comments.

src/librustc/hir/check_attr.rs

src/librustc_lint/types.rs

src/librustc_typeck/check/mod.rs

src/librustc_lint/types.rs

src/test/ui/repr/repr-transparent.rs

mjbshaw · 2019-05-08T20:18:43Z

@varkor Thanks for reviewing this! I think I've addressed all the issues you raised. Let me know if there is more.

src/test/ui/repr/repr-transparent.stderr

src/librustc_typeck/check/mod.rs

varkor · 2019-05-10T20:24:33Z

It's looking pretty good now! I've just got a couple more small comments about the diagnostics, making sure that it reads well even in the edge cases.

hanna-kruppe

Sorry for jumping in late, I agree with @varkor that this looks pretty good, but I have a few nits.

src/doc/unstable-book/src/language-features/transparent-enums.md

src/doc/unstable-book/src/language-features/transparent-unions.md

src/librustc_lint/types.rs

src/librustc_typeck/error_codes.rs

src/librustc/ty/mod.rs

src/test/codegen/repr-transparent.rs

Tracking issue: rust-lang#60405

mjbshaw · 2019-06-11T05:08:33Z

@rkruppe Thanks for the review! I think I've addressed all your comments. Fingers crossed all tests pass now...

hanna-kruppe · 2019-06-11T10:29:21Z

LGTM, thanks!

@bors r=varkor,rkruppe rollup=never

(no rollup because IME thes platform-dependent codegen tests for ABIs always take several tries to get right)

bors · 2019-06-11T10:29:22Z

📌 Commit dac1c6a has been approved by varkor,rkruppe

bors · 2019-06-11T10:31:35Z

📌 Commit dac1c6a has been approved by varkor,rkruppe

Centril · 2019-06-11T10:47:25Z

r? @rkruppe

bors · 2019-06-11T11:06:45Z

⌛ Testing commit dac1c6a with merge 8e948df...

Implement RFC 2645 (transparent enums and unions) Tracking issue: #60405

bors · 2019-06-11T13:56:03Z

☀️ Test successful - checks-travis, status-appveyor
Approved by: varkor,rkruppe
Pushing 8e948df to master...

@mjbshaw

…enkov Stabilize `#[repr(transparent)]` on `enum`s in Rust 1.42.0 # Stabilization report The following is the stabilization report for `#![feature(transparent_enums)]`. Tracking issue: #60405 [Version target](https://forge.rust-lang.org/#current-release-versions): 1.42 (2020-01-30 => beta, 2020-03-12 => stable). ## User guide A `struct` with only a single non-ZST field (let's call it `foo`) can be marked as `#[repr(transparent)]`. Such a `struct` has the same layout and ABI as `foo`. Here, we also extend this ability to `enum`s with only one variant, subject to the same restrictions as for the equivalent `struct`. That is, you can now write: ```rust #[repr(transparent)] enum Foo { Bar(u8) } ``` which, in terms of layout and ABI, is equivalent to: ```rust #[repr(transparent)] struct Foo(u8); ``` ## Motivation This is not a major feature that will unlock new and important use-cases. The utility of `repr(transparent)` `enum`s is indeed limited. However, there is still some value in it: 1. It provides conceptual simplification of the language in terms of treating univariant `enum`s and `struct`s the same, as both are product types. Indeed, languages like Haskell only have `data` as the only way to construct user-defined ADTs in the language. 2. In rare occasions, it might be that the user started out with a univariant `enum` for whatever reason (e.g. they thought they might extend it later). Now they want to make this `enum` `transparent` without breaking users by turning it into a `struct`. By lifting the restriction here, now they can. ## Technical specification The reference specifies [`repr(transparent)` on a `struct`](https://doc.rust-lang.org/nightly/reference/type-layout.html#the-transparent-representation) as: > ### The transparent Representation > > The `transparent` representation can only be used on `struct`s that have: > - a single field with non-zero size, and > - any number of fields with size 0 and alignment 1 (e.g. `PhantomData<T>`). > > Structs with this representation have the same layout and ABI as the single non-zero sized field. > > This is different than the `C` representation because a struct with the `C` representation will always have the ABI of a `C` `struct` while, for example, a struct with the `transparent` representation with a primitive field will have the ABI of the primitive field. > > Because this representation delegates type layout to another type, it cannot be used with any other representation. Here, we amend this to include univariant `enum`s as well with the same static restrictions and the same effects on dynamic semantics. ## Tests All the relevant tests are adjusted in the PR diff but are recounted here: - `src/test/ui/repr/repr-transparent.rs` checks that `repr(transparent)` on an `enum` must be univariant, rather than having zero or more than one variant. Restrictions on the fields inside the only variants, like for those on `struct`s, are also checked here. - A number of codegen tests are provided as well: - `src/test/codegen/repr-transparent.rs` (the canonical test) - `src/test/codegen/repr-transparent-aggregates-1.rs` - `src/test/codegen/repr-transparent-aggregates-2.rs` - `src/test/codegen/repr-transparent-aggregates-3.rs` - `src/test/ui/lint/lint-ctypes-enum.rs` tests the interactions with the `improper_ctypes` lint. ## History - 2019-04-30, RFC rust-lang/rfcs#2645 Author: @mjbshaw Reviewers: The Language Team This is the RFC that proposes allowing `#[repr(transparent)]` on `enum`s and `union`. - 2019-06-11, PR #60463 Author: @mjbshaw Reviewers: @varkor and @rkruppe The PR implements the RFC aforementioned in full. - 2019, PR #67323 Author: @Centril Reviewers: @davidtwco The PR reorganizes the static checks taking advantage of the fact that `struct`s and `union`s are internally represented as ADTs with a single variant. - This PR stabilizes `transparent_enums`. ## Related / possible future work The remaining work here is to figure out the semantics of `#[repr(transparent)]` on `union`s and stabilize those. This work continues to be tracked in #60405.

rust-highfive assigned varkor May 2, 2019

This comment has been minimized.

Sign in to view

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 2, 2019

GuillaumeGomez mentioned this pull request May 2, 2019

Invalid doc file changes detection rust-lang/highfive#212

Open

This comment has been minimized.

Sign in to view

mjbshaw mentioned this pull request May 6, 2019

Allow null-pointer-optimized enums in FFI if their underlying representation is FFI safe #60300

Merged

This comment has been minimized.

Sign in to view

varkor suggested changes May 6, 2019

View reviewed changes

varkor 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 May 6, 2019

varkor reviewed May 6, 2019

View reviewed changes

src/librustc_lint/types.rs Outdated Show resolved Hide resolved

This comment has been minimized.

Sign in to view

varkor reviewed May 7, 2019

View reviewed changes

src/test/ui/repr/repr-transparent.rs Show resolved Hide resolved

This comment has been minimized.

Sign in to view

varkor reviewed May 10, 2019

View reviewed changes

src/test/ui/repr/repr-transparent.stderr Show resolved Hide resolved

varkor reviewed May 10, 2019

View reviewed changes

src/librustc_typeck/check/mod.rs Outdated Show resolved Hide resolved

hanna-kruppe suggested changes May 12, 2019

View reviewed changes

mjbshaw force-pushed the transparent branch from 2027e37 to 6b74ca1 Compare May 22, 2019 14:31

This comment has been minimized.

Sign in to view

mjbshaw force-pushed the transparent branch 2 times, most recently from afe263a to b0f8f1b Compare May 22, 2019 15:14

This comment has been minimized.

Sign in to view

mjbshaw force-pushed the transparent branch from b0f8f1b to c196d35 Compare June 10, 2019 22:28

This comment has been minimized.

Sign in to view

mjbshaw force-pushed the transparent branch from c196d35 to 84b362d Compare June 10, 2019 23:03

mjbshaw force-pushed the transparent branch from 84b362d to 9944a0b Compare June 10, 2019 23:18

This comment has been minimized.

Sign in to view

mjbshaw force-pushed the transparent branch from 9944a0b to c8848ed Compare June 11, 2019 04:59

Implement RFC 2645 (transparent enums and unions)

dac1c6a

Tracking issue: rust-lang#60405

mjbshaw force-pushed the transparent branch from c8848ed to dac1c6a Compare June 11, 2019 05:07

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 Jun 11, 2019

This comment has been minimized.

Sign in to view

rust-highfive assigned hanna-kruppe and unassigned varkor Jun 11, 2019

bors added a commit that referenced this pull request Jun 11, 2019

Auto merge of #60463 - mjbshaw:transparent, r=varkor,rkruppe

8e948df

Implement RFC 2645 (transparent enums and unions) Tracking issue: #60405

bors added the merged-by-bors This PR was explicitly merged by bors. label Jun 11, 2019

bors merged commit dac1c6a into rust-lang:master Jun 11, 2019

bors mentioned this pull request Jun 11, 2019

Implement arbitrary_enum_discriminant #60732

Merged

Centril mentioned this pull request Jun 11, 2019

Tracking issue for RFC 2645, "Transparent Unions" (formerly: and Enums) #60405

Open

3 tasks

bors mentioned this pull request Jun 11, 2019

Initial implementation of or-patterns #61708

Merged

Centril mentioned this pull request Jan 11, 2020

Stabilize #[repr(transparent)] on enums in Rust 1.42.0 #68122

Merged

Implement RFC 2645 (transparent enums and unions) #60463

Implement RFC 2645 (transparent enums and unions) #60463

Uh oh!

Conversation

mjbshaw commented May 2, 2019

Uh oh!

This comment has been minimized.

rust-highfive commented May 2, 2019

Uh oh!

Centril commented May 2, 2019

Uh oh!

This comment has been minimized.

This comment has been minimized.

varkor left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

This comment has been minimized.

Uh oh!

This comment has been minimized.

mjbshaw commented May 8, 2019

Uh oh!

Uh oh!

Uh oh!

varkor commented May 10, 2019

Uh oh!

hanna-kruppe left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

This comment has been minimized.

This comment has been minimized.

This comment has been minimized.

This comment has been minimized.

This comment has been minimized.

mjbshaw commented Jun 11, 2019

Uh oh!

hanna-kruppe commented Jun 11, 2019 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

bors commented Jun 11, 2019

Uh oh!

This comment has been minimized.

bors commented Jun 11, 2019

Uh oh!

Centril commented Jun 11, 2019

Uh oh!

bors commented Jun 11, 2019

Uh oh!

bors commented Jun 11, 2019

Uh oh!

Uh oh!

hanna-kruppe commented Jun 11, 2019 •

edited

Loading