Implement the `min_const_fn` feature gate #53604

oli-obk · 2018-08-22T14:02:22Z

implements the feature gate for #53555

I added a hack so the const_fn feature gate also enables the min_const_fn feature gate. This ensures that nightly users of const_fn don't have to touch their code at all.

The min_const_fn checks are run first, and if they succeeded, the const_fn checks are run additionally to ensure we didn't miss anything.

src/libsyntax/feature_gate.rs

Centril · 2018-08-22T15:53:59Z

src/libsyntax/feature_gate.rs

We will need to ensure that the standard library has not enabled #![feature(const_fn)]; this includes libstd, liballoc, libcore.

Well, we will want it for NonZero::new_unchecked, right?

Hmm... I was hoping on using the stabilization of min_const_fn but exclusion of const_fn from libstd so that standard library functions could be const-ified en masse instead of piecemeal. This kinda relies on not having #![feature(const_fn)] enabled so that accidental things don't happen.

Could we handle new_unchecked through some other internal means instead temporarily?

EDIT: like rustc_const_stable or something...

I'd say:

const fn without #[rustc_const_unstable] is stable if it passes min_const_fn

const fn not passing min_const_fn can be forced stable with #[rustc_const_fn_force_stable]

This way the active feature gates have no influence on stability, but we're checking the right thing and can't accidentally do anything wrong.

@oli-obk this sounds good to me :)

src/libsyntax/feature_gate.rs

src/test/ui/consts/min_const_fn/min_const_fn.rs

src/librustc_mir/transform/qualify_min_const_fn.rs

src/test/ui/consts/min_const_fn/min_const_fn.rs

RalfJung · 2018-08-22T18:39:06Z

src/librustc_mir/transform/qualify_min_const_fn.rs

What is const_raw_ptr_deref?

Can't we somehow check unsafety, instead of trying to get an exhaustive list of unsafe behavior?

If it aint possible to detect this in MIR perhaps do it in HIR instead?

EDIT: Add a test with an empty unsafe { ... } block in it

AFAIK MIR has some unsafety information; we do some unsafety checks in MIR.

Done as in, the reference to const_raw_ptr_deref is gone? If not, please mention the filename where that function is defined...^^

As in there's a test. And const_raw_ptr_deref is a feature gate.

Oh, I see. That's not clear.^^

But the feature is ruled out independent of whether that gate is set...?

well... it's additionally prevented in min_const_fn due to its unsafety. The same doesn't hold for casting *const T to usize. I'll add some more sanity checks. This will annoy the heck out of nightly users since they are now getting a lot of messages twice in different wording (already happening with a bunch of things in this PR)

That's a consequence of doing multiple analyses, right?

I do not think that is the right approach... even if we have separate analyses, we should only call one.

src/librustc_mir/transform/qualify_min_const_fn.rs

RalfJung · 2018-08-23T11:41:24Z

Is it really better to have all that code duplicated into a second analysis, than changing the existing analysis to qualify every const fn as "minimal, normal, invalid"?

oli-obk · 2018-08-23T11:54:52Z

Is it really better to have all that code duplicated into a second analysis, than changing the existing analysis to qualify every const fn as "minimal, normal, invalid"?

The existing analysis is supposed to be split up into three separate analyses. Me adding a fourth one to be split out in the future would not help the process.

RalfJung · 2018-08-23T12:06:03Z

Three?!? I can see that promotion is separate, but other than that everything else should be just "const checking", which should (eventually) be the same for const and const fn. What am I missing?

oli-obk · 2018-08-23T12:09:37Z

What am I missing?

The checks for Drop and Freeze/UnsafeCell to make sure statics don't contain Freeze and constants don't contain Drop values are two further passes.

oli-obk · 2018-08-24T12:19:51Z

src/librustc_mir/transform/check_unsafety.rs

fun side effect: the two cases below which are future compat lints are hard errors inside const fns

src/test/ui/consts/min_const_fn/min_const_fn.rs

src/libsyntax/diagnostic_list.rs

src/test/ui/consts/min_const_fn/min_const_fn.rs

src/librustc/ty/context.rs

src/librustc_mir/transform/qualify_min_const_fn.rs

Centril

Some more tests... :)

src/test/ui/consts/min_const_fn/min_const_fn_dyn.rs

src/test/ui/consts/min_const_fn/min_const_fn_fn_ptr.rs

Centril · 2018-08-24T16:21:53Z

src/test/ui/consts/min_const_fn/min_const_fn_fn_ptr.rs

I specifically do want a test here that ensures that even without access, const fn no_inner_fn_ptr2(x: Hide) { } is banned.

That requires a lot of code... I'll impl it, but I don't think it's a good addition to rustc

So you already have the erroring tests:

const fn no_dyn_trait(_x: &dyn std::fmt::Debug) {} const fn no_fn_ptrs(_x: fn()) {}

This is good, because it leaves open the possibility that this means &dyn const std::fmt::Debug or const fn(); If we didn't reject this, then users could pass in values where you don't have a const implementation of std::fmt::Debug, and so we wouldn't be able to change this in the future. However, as for situations where you have:

struct Hide<'a>(&'a dyn Debug); const fn foo(_x: Hide) {}

... this would only prevent a situation in the future where we decided to split up our nominal type universe into one for the const restriction and one for impure... This seems like a very unlikely future... as such... I think the tests you have currently are sufficient.

So you don't need to implement it.

src/test/ui/consts/min_const_fn/min_const_fn_dyn.rs

Centril · 2018-08-24T19:35:04Z

Should be fine to merge once the tests pass. :)
Thanks for doing this!

oli-obk · 2018-08-27T09:08:23Z

@Centril this is ready now I think

src/librustc_mir/transform/qualify_min_const_fn.rs

src/libsyntax/feature_gate.rs

oli-obk · 2018-08-31T10:13:20Z

So... while I could actually implement a check that validates the MIR of constants in min_const_fn terms, this would be a major undertaking, requiring a new query and very hacky inspection of the MIR of constants at weird intervals.

Can we just decide to treat const items as boundaries for stability? Meaning a constant may access unstable constants, even if said constant and const fns is used in a min_const_fn. There's basically no forward compat issue, as we can replace any unstable const fn by a macro and get pretty much the exact same thing. And using an unstable or even internal macro to compute a stable constant's value seems uncontroversial.

Alternatively I can rewrite the entire min_const_fn checker and fiddle it into the const_qualif pass. I don't trust myself to implement that blacklisting scheme correctly though.

RalfJung · 2018-08-31T13:50:22Z

I am perfectly fine with treating const as boundary for all sorts of analysis -- IMHO we should never inspect the code of another const, only ever its type+value. And for the value, we should be able to rely on it being const-safe for the given type.

Centril · 2018-08-31T16:03:53Z

@oli-obk I'm fine with this; I guess T-libs will just be a bit more careful with respect to const items :)

Centril · 2018-09-01T08:35:57Z

@bors r=Centril,varkor

bors · 2018-09-01T08:35:58Z

📌 Commit 2839f4f has been approved by Centril,varkor

bors · 2018-09-01T11:26:32Z

⌛ Testing commit 2839f4f with merge fea32f1...

@RalfJung

Implement the `min_const_fn` feature gate cc @RalfJung @eddyb r? @Centril implements the feature gate for #53555 I added a hack so the `const_fn` feature gate also enables the `min_const_fn` feature gate. This ensures that nightly users of `const_fn` don't have to touch their code at all. The `min_const_fn` checks are run first, and if they succeeded, the `const_fn` checks are run additionally to ensure we didn't miss anything.

bors · 2018-09-01T13:44:50Z

☀️ Test successful - status-appveyor, status-travis
Approved by: Centril,varkor
Pushing fea32f1 to master...

jethrogb · 2018-09-17T21:10:52Z

I had this code in a PR I was working on, how should I write this now?

impl<T> UnsafeList<T> {
    pub const fn new() -> Self {
        unsafe {
            UnsafeList {
                head_tail: NonNull::new_unchecked(1 as _),
                head_tail_entry: None
            }
        }
    }
}

jethrogb · 2018-09-17T21:45:12Z

Minimal repro on playground:

#![feature(staged_api, const_fn)]
use std::ptr::NonNull;

pub struct UnsafeList<T>(NonNull<T>);

impl<T> UnsafeList<T> {
    pub const fn new() -> Self {
        unsafe {
            UnsafeList(NonNull::new_unchecked(1 as _))
        }
    }
}

The only way to get around this is to add an unstable attribute.

Centril · 2018-09-17T23:11:43Z

@jethrogb Is this for a PR to the standard library or the compiler, or is this for code outside of that?
If it is the latter, it is simply unstable and you won't be able to do it.

jethrogb · 2018-09-17T23:14:03Z

PR for std

Centril · 2018-09-17T23:38:39Z

@jethrogb if UnsafeList::new is marked as #[rustc_const_unstable(feature="<name>")] then you can use powers that const_fn gives you, but that will also make the constness unstable. We will not permit any new APIs in the standard library to define stable const fns that do not pass min_const_fn.

jethrogb · 2018-09-17T23:43:40Z

@Centril that is not sufficient. You also need to add an unstable attribute where this wasn't needed before, even if you explicitly set the const_fn feature, see my minimal repro.

Centril · 2018-09-17T23:53:35Z

@jethrogb #[stable] + #[rustc_const_unstable] work together: https://play.rust-lang.org/?gist=fc05cbb77603a742574a3d85e54ecbb5&version=nightly&mode=debug&edition=2015

rust-highfive assigned Centril Aug 22, 2018

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 22, 2018

Centril mentioned this pull request Aug 22, 2018

Tracking issue for a minimal subset of RFC 911, const fn #53555

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oli-obk force-pushed the min_const_fn branch from 7c861bb to 7c9a377 Compare August 24, 2018 12:16

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oli-obk force-pushed the min_const_fn branch from e62488f to e8c7622 Compare August 25, 2018 09:40

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oli-obk force-pushed the min_const_fn branch from 744798a to b68bb63 Compare August 31, 2018 06:40

Get rid of token passing

2839f4f

oli-obk force-pushed the min_const_fn branch from 2894555 to 2839f4f Compare August 31, 2018 09:49

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 Sep 1, 2018

bors merged commit 2839f4f into rust-lang:master Sep 1, 2018

This was referenced Sep 1, 2018

stabilize outlives requirements #53793

Merged

Report const eval error inside the query #53821

Merged

Merge proc_macro_ expansion feature gates as proc_macro_hygiene #52121

Merged

Implement object-safety for arbitrary_self_types #50173

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F001 mentioned this pull request Sep 12, 2018

Implement RFC 2302: tuple_struct_self_ctor #53751

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Centril mentioned this pull request Oct 22, 2018

Add MaybeUninit::new #55252

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Centril mentioned this pull request Jan 17, 2019

Make iter::Empty<T> Send and Sync for any T #57682

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Centril mentioned this pull request Feb 2, 2020

Initializers of stable constants can call const unstable functions #68743

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oli-obk deleted the min_const_fn branch May 31, 2020 13:17

Implement the min_const_fn feature gate #53604

Implement the min_const_fn feature gate #53604

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Implement the `min_const_fn` feature gate #53604

Implement the `min_const_fn` feature gate #53604

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