permit negative impls for non-auto traits #68004

nikomatsakis · 2020-01-08T11:44:53Z

This is a prototype impl that extends impl !Trait beyond auto traits. It is not integrated with coherence or anything else, and hence only serves to prevent downstream impls (but not to allow downstream crates to rely on the absence of such impls for coherence purposes).

Fixes #66544

TODO:

need a test that you can't rely on negative impls for coherence purposes
test that negative impls cannot specialize positive ones
test that positive impls cannot specialize negative ones
extend negative impl to Clone in order to fully fix Pin is unsound due to rogue Deref/DerefMut implementations #66544
and maybe make CoerceUnsized unsafe? -- that problem is now split out into Pin is unsound due to transitive effects of CoerceUnsized #68015
introduce feature flag and prepare a write-up
improve diagnostics?

rust-highfive · 2020-01-08T11:44:56Z

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

rust-highfive · 2020-01-08T11:44:58Z

r? @varkor

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

Aaron1011 · 2020-01-08T12:24:32Z

@nikomatsakis: How does this relate to reservation impls?

nikomatsakis · 2020-01-08T13:24:02Z

A negative impl is a semver commitment never to implement the given trait for the given type. It should eventually mean that you can rely on the impl not existing for the purposes of negative reasoning in coherence, though this PR doesn't implement that. What this PR does is simply require that no negative and positive impls (for the same trait) can overlap. This means that you can rely on an impl not existing for the purpose of proving soundness, and in particular for proving Pin sound.

A reservation impl is...something harder to express. It is considered a kind of "ambiguous" impl -- it doesn't exist yet, but it might exist in the future. This prevents people from assuming negatively that it would never exist. But it doesn't prevent you from writing overlapping impls.

nikomatsakis · 2020-01-08T14:01:45Z

@comex do you think you could provide me with "clean, minimized versions" of your exploits? The code in this playground is rather complex .. I'd prefer to have three or four simple tests I can add for regression tests. They don't have to be fully weaponized necessarily, either -- something like what @withoutboats did would be perfect.

(In particular, I believe we need a negative Clone impl for &mut T, at least, before this particular PR is done. I guess we also need to update the CoerceUnsized trait to be unsafe for us to fully fix the "rogue impl" problem? That feels like maybe a separate PR, but maybe I'll do it together.)

withoutboats · 2020-01-08T14:17:27Z

@nikomatsakis I can provide minimal examples for both DerefMut and Clone. Here they are:

DerefMut for shared reference: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7d0c6cef76960efecd8e9afd3a851443

Clone for mutable reference: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=55430b595a04fa05c52fb6abe024d5e8

(You'll notice these work almost exactly the same way.)

For CoerceUnsized I don't have a sample ready, but I think fixing CoerceUnsized should be tracked separately, possibly even opening a separate issue from #66544

nikomatsakis · 2020-01-08T14:19:29Z

@withoutboats

For CoerceUnsized I don't have a sample ready, but I think fixing CoerceUnsized should be tracked separately, possibly even opening a separate issue from #66544

I was thinking that might be a good idea as well, as the description of #66544 doesn't seem to match.

withoutboats · 2020-01-08T14:22:08Z

Yea I think it'd be better to track the coerceunsized problem as "we added an unchecked invariant to implementing coerceunsized but didnt mark the trait unsafe," just separate from this coherence hole issue.

nikomatsakis · 2020-01-08T17:01:48Z

I've removed the WIP, I think this PR is roughly ready to land, though I think it merits an FCP and a more detailed write-up of

(a) the semantics of what is happening
(b) the motivation for this change around Pin

I can try to get to that in a bit.

varkor

The implementation looks good. The diagnostics around negative trait bounds in general could be improved, but that can be left as a follow up.

src/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.stderr

varkor · 2020-01-08T17:34:40Z

It might also be worth putting negative impls for non-auto traits behind a separate feature flag to optin_builtin_traits, as they're really distinct features.

nikomatsakis · 2020-01-08T17:50:49Z

Ah yeah, I meant to add "new feature flag" to the checklist

Lokathor · 2020-01-08T18:41:49Z

The "stable" version number should be 1.41 or whenever this actually gets to stable. This might sound silly to say but sometimes version strings like this have gone unnoticed so I thought i'd just throw in a reminder.

Aaron1011 · 2020-01-08T18:53:25Z

Do we want to do a crater run for this? On one hand, I would be shocked if anyone ever wrote this kind of bizarre impl in real code. On the other hand, if someone did write such an impl, it would be good to know that there exists code in the wild that will be permanently broken by this.

nikomatsakis · 2020-03-26T11:42:49Z

@bors r=varkor

bors · 2020-03-26T11:42:50Z

📌 Commit 0d07026 has been approved by varkor

bors · 2020-03-26T12:28:55Z

⌛ Testing commit 0d07026 with merge 8599f89a0f86493bfe6d12bc1a1f0a09d86818eb...

Centril · 2020-03-26T12:33:13Z

@bors retry

@ghost

Rollup of 5 pull requests Successful merges: - rust-lang#68004 (permit negative impls for non-auto traits) - rust-lang#70385 (Miri nits: comment and var name improvement) - rust-lang#70411 (Fix for rust-lang#62691: use the largest niche across all fields) - rust-lang#70417 (parser: recover on `...` as a pattern, suggesting `..`) - rust-lang#70424 (simplify match stmt) Failed merges: r? @ghost

bors · 2020-03-26T15:39:01Z

⌛ Testing commit 0d07026 with merge 2fbb075...

estebank · 2020-03-26T17:46:30Z

src/test/ui/typeck/typeck-negative-impls-builtin.rs

-#![feature(optin_builtin_traits)]
-
-struct TestType;
-
-trait TestTrait {
-    fn dummy(&self) { }
-}
-
-impl !TestTrait for TestType {}
-//~^ ERROR invalid negative impl
-
-fn main() {}


Shouldn't a test for this without the feature flag remain?

rust-highfive assigned varkor Jan 8, 2020

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nikomatsakis force-pushed the negative-impls branch 3 times, most recently from fa21734 to a10769a Compare January 8, 2020 16:58

nikomatsakis changed the title ~~[WIP] permit negative impls for non-auto traits~~ permit negative impls for non-auto traits Jan 8, 2020

nikomatsakis force-pushed the negative-impls branch from a10769a to 4663d8f Compare January 8, 2020 17:06

varkor approved these changes Jan 8, 2020

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src/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.stderr Outdated Show resolved Hide resolved

nikomatsakis added 7 commits March 26, 2020 06:52

move feature-gate-negative-impls test to traits/negative-impls

f662847

comment the typeck-negative-impls-builtin test

7107f6e

give the negative-impls-builtin test a more sensible name

644b9a0

move stderr file too

f5c09ed

add test for negative specializes negative

b9e09d8

use slice pattern instead of calling is_empty() and [0]

c35801e

pacify the merciless x.py fmt

011215b

nikomatsakis force-pushed the negative-impls branch from 1515754 to 011215b Compare March 26, 2020 10:53

wip pacify the merciless ui tests

0d07026

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 Mar 26, 2020

Centril mentioned this pull request Mar 26, 2020

Rollup of 5 pull requests #70427

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bors merged commit b0a63cb into rust-lang:master Mar 26, 2020

estebank reviewed Mar 26, 2020

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calebcartwright mentioned this pull request Mar 28, 2020

bump rustfmt 1x to rustc-ap v651 rust-lang/rustfmt#4100

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ehuss mentioned this pull request Mar 31, 2020

Fix negative_impls feature. rust-lang/nomicon#205

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clangenb mentioned this pull request Apr 16, 2020

negative_impls are now feature gated kcuzner/nb-sync#1

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Ixrec mentioned this pull request Apr 17, 2020

Need negative trait bound #42721

Open

GuillaumeGomez mentioned this pull request Apr 18, 2020

E0751 explanation is using E0748 error #71304

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nikomatsakis mentioned this pull request Jun 8, 2020

Tracking issue for allowing overlapping implementations for marker trait #29864

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6 tasks

nikomatsakis mentioned this pull request Jun 1, 2021

negative impls integrated into coherence rust-lang/lang-team#96

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crlf0710 added the F-negative_impls #![feature(negative_impls)] label Sep 11, 2021

lcnr mentioned this pull request May 6, 2022

[EXPERIMENT] disable orphan check for marker traits #96766

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permit negative impls for non-auto traits #68004

permit negative impls for non-auto traits #68004

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