Don't promote to 'static the result of dereferences. #47408

eddyb · 2018-01-13T09:27:36Z

This is a breaking change, removing copies out of dereferences from rvalue-to-'static promotion.

With miri we won't easily know whether the dereference itself would see the same value at runtime as miri (e.g. after mutating a static) or even if it can be interpreted (e.g. integer pointers).
One alternative to this ban is defining at least some of those situations as UB, i.e. you shouldn't have a reference in the first place, and you should work through raw pointers instead, to avoid promotion.

EDIT: The other may seem to be to add some analysis which whitelists references-to-constant-values and assume any values produced by arbitrary computation to not be safe to promote dereferences thereof - but that means producing a reference from an associated constant or const fn would necessarily obscure it, and in the former case, this could still impact code that runs on stable today. What we do today to track "references to statics" only works because we restrict taking a reference to a static at all to other statics (which, again, are currently limited in that they can't be read at compile-time) and to runtime-only fns (not const fns).

I'm primarily opening this PR with a conservative first approximation (e.g. &(*r).a is not allowed, only reborrows are, and in the old borrow only implicit ones from adjustments, at that) for cratering.

r? @nikomatsakis

eddyb · 2018-01-13T09:30:41Z

cc @rust-lang/lang @Kimundi - we didn't catch this before stabilization, will have to make a decision.

@bors try

bors · 2018-01-13T09:30:52Z

⌛ Trying commit 121499a with merge e15dd17...

@nikomatsakis

Don't promote to 'static the result of dereferences. **DO NOT MERGE** *(yet)* This is a **breaking change**, removing copies out of dereferences from rvalue-to-`'static` promotion. With miri we won't easily know whether the dereference itself would see the same value at runtime as miri (e.g. after mutating a `static`) or even if it can be interpreted (e.g. integer pointers). The alternative to this ban is defining at least *some* of those situations as UB, i.e. you shouldn't have a reference in the first place, and you should work through raw pointers instead, to avoid promotion. I'm primarily opening this PR with a conservative first approximation (e.g. `&(*r).a` is not allowed, only reborrows are, and in the old borrow only implicit ones from adjustments, at that) for cratering. r? @nikomatsakis

bors · 2018-01-13T11:01:44Z

☀️ Test successful - status-travis
State: approved= try=True

eddyb · 2018-01-13T11:02:29Z

@rust-lang/infra Can I get a crater run on this?

Mark-Simulacrum · 2018-01-17T03:45:12Z

Crater run started (3ish hours ago, actually).

nikomatsakis · 2018-01-22T18:12:22Z

@Mark-Simulacrum results available?

Mark-Simulacrum · 2018-01-22T19:31:00Z

Probably Tuesday mid-day or Wednesday unfortunately; we have problems with crater downloading the wrong CI artifacts that took a couple days to resolve. Crater claims 22 hours left, but I don't really believe it.

alexreg · 2018-01-26T19:42:07Z

@Mark-Simulacrum Update, please?

Mark-Simulacrum · 2018-01-26T21:54:14Z

We have not yet resolved some problem in crater that uses us all available disk space. The problem hasn't been identified, so until it is, we can't move forward unfortunately.

aidanhs · 2018-01-31T02:06:36Z

We've bitten the bullet and have got a disk with much more space - I anticipate the run succeeding now (just started).

aidanhs · 2018-02-04T14:21:56Z

At long last!

Crater results: http://cargobomb-reports.s3.amazonaws.com/pr-47408/index.html. 'Blacklisted' crates (spurious failures etc) can be found here. If you see any spurious failures not on the list, please make a PR against that file.

kennytm · 2018-02-04T14:25:59Z

Looks like all regressions are spurious

eddyb · 2018-02-04T16:19:26Z

@aidanhs @kennytm Thanks! I agree that there appear to only be spurious regressions.
This PR can only cause less code to compile, but there's no regressions due to compile errors.

eddyb · 2018-02-04T16:25:37Z

@rfcbot pr merge

I'm proposing to merge this PR, retroactively restricting rvalue-to-'static promotion to not promote any copy through a dereference (for which we'd need intrusive points-to analyses post-miri).

rfcbot · 2018-02-04T16:25:37Z

Team member @eddyb 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.

nikomatsakis · 2018-02-06T18:50:35Z

Nominating so that I can badger @rust-lang/lang members into checking their boxes.

nikomatsakis · 2018-02-08T20:13:55Z

I checked for @nrc -- on the premise that he is not objecting but just busy.

aidanhs · 2018-02-08T22:14:43Z

Out of curiosity, can you give a small self-contained example of something that gets broken by this?

bors · 2018-02-16T15:20:07Z

💔 Test failed - status-appveyor

kennytm · 2018-02-16T15:34:27Z

@bors retry #46903

bors · 2018-02-16T18:21:55Z

⌛ Testing commit 121499a with merge 24a82a987f9f716fad83088470137c38badfc4c1...

bors · 2018-02-16T21:23:21Z

💔 Test failed - status-appveyor

kennytm · 2018-02-16T21:34:26Z

@bors retry #46903

bors · 2018-02-16T21:42:21Z

⌛ Testing commit 121499a with merge b6c10e42efef10877fc43c26a5712cd7f78d9533...

bors · 2018-02-16T21:46:11Z

💔 Test failed - status-travis

kennytm · 2018-02-17T14:40:03Z

@bors retry

bors · 2018-02-17T19:32:32Z

⌛ Testing commit 121499a with merge 5313e87...

@nikomatsakis

Don't promote to 'static the result of dereferences. This is a **breaking change**, removing copies out of dereferences from rvalue-to-`'static` promotion. With miri we won't easily know whether the dereference itself would see the same value at runtime as miri (e.g. after mutating a `static`) or even if it can be interpreted (e.g. integer pointers). One alternative to this ban is defining at least *some* of those situations as UB, i.e. you shouldn't have a reference in the first place, and you should work through raw pointers instead, to avoid promotion. **EDIT**: The other *may seem* to be to add some analysis which whitelists references-to-constant-values and assume any values produced by arbitrary computation to not be safe to promote dereferences thereof - but that means producing a reference from an associated constant or `const fn` would necessarily obscure it, and in the former case, this could still impact code that runs on stable today. What we do today to track "references to statics" only works because we restrict taking a reference to a `static` at all to other `static`s (which, again, are currently limited in that they can't be read at compile-time) and to runtime-only `fn`s (*not* `const fn`s). I'm primarily opening this PR with a conservative first approximation (e.g. `&(*r).a` is not allowed, only reborrows are, and in the old borrow only implicit ones from adjustments, at that) for cratering. r? @nikomatsakis

bors · 2018-02-17T22:33:04Z

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 5313e87 to master...

alexreg · 2018-02-19T19:53:54Z

@nikomatsakis Maybe worth taking off the tags (especially FCP) now that this is closed/merged... unless there's a reason it should still be open? :)

Don't promote to 'static the result of dereferences.

121499a

rust-highfive assigned nikomatsakis Jan 13, 2018

kennytm added T-lang Relevant to the language team, which will review and decide on the PR/issue. S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Jan 13, 2018

kennytm added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Feb 4, 2018

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 Feb 4, 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 Feb 4, 2018

bluss added the relnotes Marks issues that should be documented in the release notes of the next release. label Feb 4, 2018

nikomatsakis added the I-nominated label Feb 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 Feb 16, 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 Feb 16, 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 Feb 16, 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 Feb 16, 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 Feb 16, 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 Feb 17, 2018

bors merged commit 121499a into rust-lang:master Feb 17, 2018

bors mentioned this pull request Feb 17, 2018

Replace all const evaluation with miri #46882

Merged

eddyb deleted the deref-danger branch February 19, 2018 21:25

tjkirch mentioned this pull request Jul 25, 2019

Improve errors for misused attributes shepmaster/snafu#109

Merged

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Don't promote to 'static the result of dereferences. #47408

Don't promote to 'static the result of dereferences. #47408

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Don't promote to 'static the result of dereferences. #47408

Don't promote to 'static the result of dereferences. #47408

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