Allow constants to refer statics by spastorino · Pull Request #66302 · rust-lang/rust

spastorino · 2019-11-11T17:33:48Z

r? @oli-obk, now @eddyb will be happy ❤️

rust-highfive · 2019-11-11T19:24:35Z

The job x86_64-gnu-llvm-6.0 of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

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2019-11-11T17:34:25.4383043Z ##[command]git remote add origin https://github.com/rust-lang/rust
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2019-11-11T19:06:54.0287116Z Testing term stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
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2019-11-11T19:24:28.9158611Z local time: Mon Nov 11 19:24:28 UTC 2019
2019-11-11T19:24:28.9915974Z network time: Mon, 11 Nov 2019 19:24:28 GMT
2019-11-11T19:24:28.9921675Z == end clock drift check ==
2019-11-11T19:24:30.6097960Z 2019-11-11T19:24:30.6188349Z ##[error]Bash exited with code '1'.
2019-11-11T19:24:30.6247270Z ##[section]Starting: Checkout
2019-11-11T19:24:30.6248733Z ==============================================================================
2019-11-11T19:24:30.6248792Z Task : Get sources
2019-11-11T19:24:30.6248831Z Description : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

Centril · 2019-11-12T07:02:32Z

I'm not sure why we are doing this change. If we are to do so, I think an RFC would be in order. For the time being, we have an RFC that explicitly states that this should not be allowed.

Also, do we have any test which forbids the following?

extern{staticFOO:u8;}constX:u8 = unsafe{FOO};

spastorino · 2019-11-12T12:48:19Z

@Centril I've just added the test you mentioned, good point ;).

Centril · 2019-11-12T13:02:13Z

Thanks!

Since the main purpose of this work is to simplify compiler internals rather than as a new language feature, I would suggest moving the existing check to HIR. That shouldn't be too difficult I think -- could be its own pass or we could do it in typeck maybe.

oli-obk · 2019-11-12T13:10:53Z

I'm not sure why we are doing this change. If we are to do so, I think an RFC would be in order.

For the time being, we have an RFC that explicitly states that this should not be allowed.

We have? I was under the assumption this is just a restriction left over from the old const evaluator. The miri-based const eval can handle this without any problems. Which RFC are you referring to?

oli-obk · 2019-11-12T15:16:32Z

@Centril and I discussed this a bit via chat. Here's the summary:

This does not introduce any new runtime behaviour, because one could always have used a static item instead of a const item and it would have worked just as well
This does allow array lengths, enum discriminants and const items to take references to statics and even dereference them to get a value out.
While const FOO: &AtomicUsize = &AtomicUsize::new(42); is illegal because the atomic may be arbitrarily duplicated or deduplicated, const FOO: &AtomicUsize = &SOME_STATIC; is legal, because the constant referes always to the same named static item's memory
- this extends even to lazy or extern statics, while you can make a constant refer to a them, you can't actually read from them because the const evaluator catches that at runtime.
The only problematic situation that exists is const fn. Right now const fn cannot refer to static items, because that would allow one to write a const fn like
```
constfnfoo() -> usize{staticFOO:AtomicUsize = AtomicUsize::new(0);FOO.fetch_add(1,Ordering::Relaxed)}
```
(assuming powerful enough const eval features).
- Even if we forbade statics, we could use the const BAR: &AtomicUsize = &FOO; indirection to work around the static check.
- While we could just forbid constants in const fn that refer to statics anywhere, that won't work for associated constants like
```
constfnfoo<T:Trait>() -> usize{T::ASSOC_CONST.fetch_add(1,Ordering::Relaxed)}
```
- We could still do the check at monomorphization time, but that would mean that a use of a function would fail to compile depending on the type the user passed for the generic parameter

cc @RalfJung

bors · 2019-11-14T17:15:53Z

☔ The latest upstream changes (presumably #66314) made this pull request unmergeable. Please resolve the merge conflicts.

RalfJung · 2019-11-16T18:43:32Z

The crucial thing to make sure is that CTFE does not read from (let alone write do) mutable statics. We should make sure to have a few miri-unleash tests that cover this. Cnetril mentioned one, @oli-obk mentioned this one:

constFOO:usize{staticFOO:AtomicUsize = AtomicUsize::new(0);FOO.fetch_add(1,Ordering::Relaxed)};

and then there also is

constFOO:usize{staticFOO:AtomicUsize = AtomicUsize::new(0);*(&FOOas*const_as*constusize)};

and

staticmutMUTABLE:u32 = 0;constBAD:u32 = unsafe{MUTABLE};

All of these should fail even with miri-unleashed, to make sure that our dynamic checks are covering all possibilities.

What remains then is the question of whether we want to have static checks on top of the dynamic ones, and what those are good for. Doing only dynamic checks leads to monomorphization-time errors. But from how I understood @oli-obk, we either already have those or ruling those out also rules out way too many useful things, and hence monomorphization-time errors are the better option? I do not have a strong stanza on this, if @oli-obk and @Centril agree on that that's good enough for me. I just want to see the Miri engine do enough checks to rule out all these examples. :) (And I think so far it does not do enough checks, actually.)

JohnCSimon · 2019-11-24T02:04:26Z

Ping from triage
@spastorino Can you please address the merge conflicts?
Thank you!

spastorino · 2019-11-24T13:02:44Z

@JohnCSimon, I don't think it makes sense to rebase this until we don't figure out a way to make this idea work.

Centril · 2019-12-16T05:20:11Z

But from how I understood @oli-obk, we either already have those or ruling those out also rules out way too many useful things, and hence monomorphization-time errors are the better option? I do not have a strong stanza on this, if @oli-obk and @Centril agree on that that's good enough for me.

My stance is that should not be adding post-monomorphization errors and that const fns should remain deterministic when executed at run-time.

oli-obk · 2019-12-16T16:20:37Z

I'm currently unable to come up with a non-postmonomorphization plan that would not end up making const qualif more complex or require new marker traits

Let's close until we have a plan for working this out

RalfJung · 2019-12-16T16:33:44Z

CTFE already causes post-monomorphization errors because some parts of CTFE can only happen after monomorphization and sometimes that can error (causing a panic during CTFE is the most obvious case).

Not sure if I would consider "you read from a static" a * new* post-monomorphization error.

oli-obk · 2019-12-16T17:29:57Z

writing to mutable statics by first taking a reference and then writing through that reference is also only detected post monomorphization

@RalfJung

…t, r=RalfJung Ensure that evaluating or validating a constant never reads from a static r? @RalfJung as per rust-lang#66302 (comment) This does not yet address the fact that evaluation of a constant can read from a static (under unleash-miri)

rust-highfive assigned oli-obk Nov 11, 2019

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Nov 11, 2019

oli-obk reviewed Nov 11, 2019
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Comment threadsrc/test/ui/consts/const-fn-not-safe-for-const.rs Outdated

Centril added needs-rfc This change is large or controversial enough that it should have an RFC accepted before doing it. relnotes Marks issues that should be documented in the release notes of the next release. T-lang Relevant to the language team labels Nov 12, 2019

Centril mentioned this pull request Nov 12, 2019
Remove cannot mutate statics in initializer of another static error #66306
Merged

Allow constants to refer statics
774445d

spastorino force-pushed the constants-can-refer-statics branch from 723630e to 774445dCompare November 12, 2019 11:13

Add a test to be sure that you can't read an extern static from a const
37cbe73

Add test to prove that consts can refer to sync types
5cd73aa

JohnCSimon 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 Nov 24, 2019

spastorino added S-waiting-on-team and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 24, 2019

RalfJung mentioned this pull request Nov 28, 2019
Add memoization for const function evaluations #66294
Merged

oli-obk mentioned this pull request Dec 15, 2019
Ensure that evaluating or validating a constant never reads from a static #67337
Merged

oli-obk closed this Dec 16, 2019

oli-obk mentioned this pull request Apr 14, 2020
Why can consts not refer to statics? rust-lang/const-eval#11
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