Individualize feature gates for const fn invocation #43017

durka · 2017-07-02T20:20:03Z

This PR changes the meaning of #![feature(const_fn)] so it is only required to declare a const fn but not to call one. Based on discussion at #24111. I was hoping we could have an FCP here in order to move that conversation forward.

This sets the stage for future stabilization of the constness of several functions in the standard library (listed below), so could someone please tag the lang team for review.

std::cell
- Cell::new
- RefCell::new
- UnsafeCell::new
std::mem
- size_of
- align_of
std::ptr
- null
- null_mut
std::sync
- atomic
  - Atomic{Bool,Ptr,Isize,Usize}::new
- once
  - Once::new
primitives
- {integer}::min_value
- {integer}::max_value

Some other functions are const but they are also unstable or hidden, e.g. Unique::new so they don't have to be considered at this time.

After this stabilization, the following *_INIT constants in the standard library can be deprecated. I wasn't sure whether to include those deprecations in the current PR.

std::sync
- atomic
  - ATOMIC_{BOOL,ISIZE,USIZE}_INIT
- once
  - ONCE_INIT

rust-highfive · 2017-07-02T20:20:09Z

r? @eddyb

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

eddyb · 2017-07-02T20:26:25Z

cc @rust-lang/lang

sfackler · 2017-07-02T21:03:33Z

WRT deprecations, we prefer to do those a release or two after.

Mark-Simulacrum · 2017-07-02T21:20:00Z

[00:51:26] error: use of unstable library feature 'sort_unstable' (see issue #40585)
[00:51:26]    --> /checkout/src/test/run-pass/vector-sort-panic-safe.rs:174:30
[00:51:26]     |
[00:51:26] 174 |                 test!(input, sort_unstable_by);
[00:51:26]     |                              ^^^^^^^^^^^^^^^^
[00:51:26]     |
[00:51:26]     = help: add #![feature(sort_unstable)] to the crate attributes to enable
[00:51:26] 
[00:51:26] error: use of unstable library feature 'sort_unstable' (see issue #40585)
[00:51:26]    --> /checkout/src/test/run-pass/vector-sort-panic-safe.rs:174:30
[00:51:26]     |
[00:51:26] 174 |                 test!(input, sort_unstable_by);
[00:51:26]     |                              ^^^^^^^^^^^^^^^^
[00:51:26]     |
[00:51:26]     = help: add #![feature(sort_unstable)] to the crate attributes to enable
[00:51:26] 
[00:51:26] error: aborting due to 2 previous errors

durka · 2017-07-02T22:21:34Z

Fixed botched rebase.

withoutboats · 2017-07-02T23:07:53Z

@rfcbot fcp merge

SimonSapin · 2017-07-02T23:34:24Z

After this stabilization, the following *_INIT constants in the standard library can be deprecated. I wasn't sure whether to include those deprecations in the current PR.

Do we have a mechanism yet to make something deprecated in some future version? (In this case 1.22+, I think.) It’s bothersome if Nightly warns and says to use instead something that is not available in the Stable channel yet.

SimonSapin · 2017-07-02T23:43:54Z

#42859 adds size_of and align_of to the list.

rfcbot · 2017-07-02T23:44:29Z

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

Concerns:

~~constness~~ resolved by Individualize feature gates for const fn invocation #43017 (comment)

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.

brson · 2017-07-04T00:40:43Z

After this stabilization, the following *_INIT constants in the standard library can be deprecated. I wasn't sure whether to include those deprecations in the current PR.

No, please don't deprecate during the same release as the replacement. Give it a while.

joshtriplett · 2017-07-06T19:34:15Z

This is a bit of a tangent, but I'd assume this applies to core::mem::size_of and similar, not just std::mem::size_of?

nikomatsakis · 2017-07-06T19:38:49Z

@rfcbot concern constness

In discussion in the @rust-lang/lang meeting, the question was raised whether we want to have the ability to control whether to stabilize the "constness" of individual functions. So, for example, one might imagine making size_of and align_of usable in constants, but not necessarily every const fn (since some of them might rely on impl details that we are not sure if we will want to stabilize).

joshtriplett · 2017-07-06T19:39:13Z

@rust-lang/lang was generally in favor of making this change, and stabilizing size_of and align_of as const functions; the only concern was whether this might open the floodgates to stabilize numerous other library functions as const as well, and whether in adding all such functions, we might unnecessarily constrain ourselves in the development of const_fn. We'd recommend being very conservative about what in the standard library should be stabilized as const.

There was also some discussion about whether this automatically makes the constness of all const functions in the library immediately stable, which might not have been anticipated. There was discussion elsewhere about whether there might be future extensions of some such functions/traits that may be non-const.

eddyb · 2017-07-06T19:45:04Z

@joshtriplett std::mem is a mere reexport of core::mem so yes, they are the same functions.

durka · 2017-07-06T20:10:36Z

As written, yes, it stabilizes the constness of const fns in core/std. It seems rather useless to stabilize uses of const fn, but not declarations, and then not declare any const fns :) If we have to be hyper-conservative and start with just size_of/align_of, I guess that's OK, but I'm also excited to get rid of tons of lazy_statics which only exist in order to call Cell::new etc! I'm not 100% sure how we would implement stabilizable-constness. One approach that comes to mind is a build script for core and std that checks which train we are compiling, and sets a cfg flag. But then for a user if you *are* on nightly, there's no opt-in, you just get a const.

…

On Thu, Jul 6, 2017 at 3:45 PM, Eduard-Mihai Burtescu < ***@***.***> wrote: @joshtriplett <https://github.com/joshtriplett> std::mem is a mere reexport of core::mem so yes, they are the same functions. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#43017 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAC3n9jAY5Oj_QqARZHnm7R08DiCciqWks5sLTllgaJpZM4OLrkp> .

durka · 2017-07-06T20:13:06Z

We could also just un-constify the rest of that list, breaking nightly users until we figure out which library functions can be const. Are there any specific concerns, though?

nikomatsakis · 2017-07-10T16:23:38Z

@durka

If we have to be hyper-conservative and start with just size_of/align_of, I guess that's OK, but I'm also excited to get rid of tons of lazy_statics which only exist in order to call Cell::new etc!

I have no problem being more aggressive than such a minimal starting set, but it seems like it'd be useful to have the ability to add things later that we don't stabilize immediately.

nikomatsakis · 2017-07-10T16:24:06Z

That said, perhaps un-constifying is indeed a viable alternative.

durka · 2017-07-10T20:33:38Z

All I can think of is adding a magic function attribute #[rustc_const_stable(since = "...")], and then basically undoing this PR :( but adding a check for the attribute here https://github.com/rust-lang/rust/pull/43017/files#diff-c2552a106550d05b69d5e07612f0f812L782

bors · 2017-09-16T16:54:14Z

📌 Commit 332c38c has been approved by eddyb

bors · 2017-09-16T17:02:23Z

⌛ Testing commit 332c38c with merge ae8efdc...

Individualize feature gates for const fn invocation This PR changes the meaning of `#![feature(const_fn)]` so it is only required to declare a const fn but not to call one. Based on discussion at #24111. I was hoping we could have an FCP here in order to move that conversation forward. This sets the stage for future stabilization of the constness of several functions in the standard library (listed below), so could someone please tag the lang team for review. - `std::cell` - `Cell::new` - `RefCell::new` - `UnsafeCell::new` - `std::mem` - `size_of` - `align_of` - `std::ptr` - `null` - `null_mut` - `std::sync` - `atomic` - `Atomic{Bool,Ptr,Isize,Usize}::new` - `once` - `Once::new` - primitives - `{integer}::min_value` - `{integer}::max_value` Some other functions are const but they are also unstable or hidden, e.g. `Unique::new` so they don't have to be considered at this time. After this stabilization, the following `*_INIT` constants in the standard library can be deprecated. I wasn't sure whether to include those deprecations in the current PR. - `std::sync` - `atomic` - `ATOMIC_{BOOL,ISIZE,USIZE}_INIT` - `once` - `ONCE_INIT`

bors · 2017-09-16T19:54:51Z

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing ae8efdc to master...

durka · 2017-09-16T22:31:14Z

Thanks for bearing with me @kennytm and @eddyb (and @bors). Next up, figuring out which of these are actually uncontroversial to stabilize!

…

On Sep 16, 2017 15:55, "bors" ***@***.***> wrote: Merged #43017 <#43017>. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#43017 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAC3n6lgClFDCPpVSyAxgnNjIfGr1ev1ks5sjCepgaJpZM4OLrkp> .

Upgrade to rustc 1.22.0-nightly (277476c4f 2017-09-16) rust-lang/rust#43017  --- This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18541)

This seems to be required now, due to rust-lang/rust#43017

…-09-16) (from servo:rustup); r=SimonSapin rust-lang/rust#43017 Source-Repo: https://github.com/servo/servo Source-Revision: c28cf7490f2def7335c7c12e70e12e343eb1ec05 --HG-- extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear extra : subtree_revision : 1e2b6d8c2339a47ed373f4a00dcea43348c04a99

…-09-16) (from servo:rustup); r=SimonSapin rust-lang/rust#43017 Source-Repo: https://github.com/servo/servo Source-Revision: c28cf7490f2def7335c7c12e70e12e343eb1ec05

mystor · 2017-10-29T18:16:54Z

@durka Did you ever open a bug to try to stabilize the uncontroversial const functions? It seemed from reading this that size_of and align_of, for example, are not controversial, yet I couldn't find a bug tracking their stabilization.

durka · 2017-10-29T18:24:08Z

I'll try to get that process started. I guess #24111 is still tracking all the const fns -- maybe that's too confusing.

…-09-16) (from servo:rustup); r=SimonSapin rust-lang/rust#43017 Source-Repo: https://github.com/servo/servo Source-Revision: c28cf7490f2def7335c7c12e70e12e343eb1ec05 UltraBlame original commit: 14d7326af64d60a6be900427b468f94c7a2d2fda

rust-highfive assigned eddyb Jul 2, 2017

durka mentioned this pull request Jul 2, 2017

const fn tracking issue (RFC 911) #24111

Closed

3 tasks

eddyb added I-nominated T-lang Relevant to the language team, which will review and decide on the PR/issue. labels Jul 2, 2017

durka force-pushed the stabilize-const-invocation branch from 358d2d2 to d0fc8fc Compare July 2, 2017 22:21

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 Jul 2, 2017

durka mentioned this pull request Jul 3, 2017

Implement const fn {size,align}_of. #42859

Merged

eddyb added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Jul 3, 2017

brson added the relnotes Marks issues that should be documented in the release notes of the next release. label Jul 4, 2017

aidanhs added the S-waiting-on-team Status: Awaiting decision from the relevant subteam (see the T-<team> label). label Jul 5, 2017

nikomatsakis removed the I-nominated label Jul 6, 2017

bors merged commit 332c38c into rust-lang:master Sep 16, 2017

This was referenced Sep 16, 2017

Implement RFC 1861: Extern types #44295

Merged

Allow writing metadata without llvm #44085

Merged

Rollup of 25 pull requests #44623

Closed

Rollup of 19 pull requests #44634

Merged

stabilized iterator_for_each (closes #42986) #44567

Merged

This was referenced Sep 17, 2017

yansi Fails to Compile Using cargo rwf2/Rocket#433

Closed

Update for latest nightly, bump version number. Amanieu/parking_lot#42

Merged

durka mentioned this pull request Sep 17, 2017

Document the new individual const fn feature gates #44644

Closed

This was referenced Sep 17, 2017

Fix build on rustc nightly-2017-09-16 mvdnes/spin-rs#42

Merged

Build failed on rustc nightly-2017-09-16 PyO3/pyo3#79

Closed

SimonSapin mentioned this pull request Sep 17, 2017

Upgrade to rustc 1.22.0-nightly (277476c4f 2017-09-16) servo/servo#18541

Merged

ids1024 added a commit to ids1024/newlib that referenced this pull request Sep 18, 2017

Add const_ptr_null feature

1df1ef7

This seems to be required now, due to rust-lang/rust#43017

phil-opp mentioned this pull request Oct 3, 2017

Add new const_unique_new feature gate phil-opp/blog_os#355

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Individualize feature gates for const fn invocation #43017

Individualize feature gates for const fn invocation #43017

durka commented Jul 2, 2017 •

edited

Loading

rust-highfive commented Jul 2, 2017

eddyb commented Jul 2, 2017

sfackler commented Jul 2, 2017 •

edited

Loading

Mark-Simulacrum commented Jul 2, 2017

durka commented Jul 2, 2017

withoutboats commented Jul 2, 2017

SimonSapin commented Jul 2, 2017

SimonSapin commented Jul 2, 2017

rfcbot commented Jul 2, 2017 •

edited

Loading

brson commented Jul 4, 2017

joshtriplett commented Jul 6, 2017

nikomatsakis commented Jul 6, 2017

joshtriplett commented Jul 6, 2017

eddyb commented Jul 6, 2017

durka commented Jul 6, 2017 via email

durka commented Jul 6, 2017

nikomatsakis commented Jul 10, 2017

nikomatsakis commented Jul 10, 2017

durka commented Jul 10, 2017

bors commented Sep 16, 2017

bors commented Sep 16, 2017

bors commented Sep 16, 2017

durka commented Sep 16, 2017 via email

mystor commented Oct 29, 2017

durka commented Oct 29, 2017

Individualize feature gates for const fn invocation #43017

Individualize feature gates for const fn invocation #43017

Conversation

durka commented Jul 2, 2017 • edited Loading

rust-highfive commented Jul 2, 2017

eddyb commented Jul 2, 2017

sfackler commented Jul 2, 2017 • edited Loading

Mark-Simulacrum commented Jul 2, 2017

durka commented Jul 2, 2017

withoutboats commented Jul 2, 2017

SimonSapin commented Jul 2, 2017

SimonSapin commented Jul 2, 2017

rfcbot commented Jul 2, 2017 • edited Loading

brson commented Jul 4, 2017

joshtriplett commented Jul 6, 2017

nikomatsakis commented Jul 6, 2017

joshtriplett commented Jul 6, 2017

eddyb commented Jul 6, 2017

durka commented Jul 6, 2017 via email

durka commented Jul 6, 2017

nikomatsakis commented Jul 10, 2017

nikomatsakis commented Jul 10, 2017

durka commented Jul 10, 2017

bors commented Sep 16, 2017

bors commented Sep 16, 2017

bors commented Sep 16, 2017

durka commented Sep 16, 2017 via email

mystor commented Oct 29, 2017

durka commented Oct 29, 2017

durka commented Jul 2, 2017 •

edited

Loading

sfackler commented Jul 2, 2017 •

edited

Loading

rfcbot commented Jul 2, 2017 •

edited

Loading