permit negative impls for non-auto traits by nikomatsakis · Pull Request #68004 · rust-lang/rust

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 fixPin 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.

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

-#![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

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 8, 2020

This comment has been minimized.
Sign in to view

nikomatsakis force-pushed the negative-impls branch from e3a0b47 to e16b360Compare January 8, 2020 13:35

This comment has been minimized.
Sign in to view

nikomatsakis force-pushed the negative-impls branch from e16b360 to c6875fdCompare January 8, 2020 13:56

nikomatsakis force-pushed the negative-impls branch from c6875fd to e2a82faCompare January 8, 2020 14:02

This comment has been minimized.
Sign in to view

nikomatsakis force-pushed the negative-impls branch from e2a82fa to 83d1c33Compare January 8, 2020 15:00

This comment has been minimized.
Sign in to view

nikomatsakis force-pushed the negative-impls branch 3 times, most recently from fa21734 to a10769aCompare January 8, 2020 16:58

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

nikomatsakis force-pushed the negative-impls branch from a10769a to 4663d8fCompare January 8, 2020 17:06

varkor approved these changes Jan 8, 2020
View reviewed changes

Comment threadsrc/test/ui/traits/negative-impls/pin-unsound-issue-66544-clone.stderr Outdated

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 011215bCompare 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
Merged

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

estebank reviewed Mar 26, 2020
View reviewed changes

calebcartwright mentioned this pull request Mar 28, 2020
bump rustfmt 1x to rustc-ap v651 rust-lang/rustfmt#4100
Merged

ehuss mentioned this pull request Mar 31, 2020
Fix negative_impls feature. rust-lang/nomicon#205
Merged

clangenb mentioned this pull request Apr 16, 2020
negative_impls are now feature gated kcuzner/nb-sync#1
Open

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
Closed

nikomatsakis mentioned this pull request Jun 8, 2020
Tracking issue for allowing overlapping implementations for marker trait #29864
Open
6 tasks

nikomatsakis mentioned this pull request Jun 1, 2021
negative impls integrated into coherence rust-lang/lang-team#96
Closed

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
Closed

Uh oh!

Uh oh!

Conversation

nikomatsakis commented Jan 8, 2020• edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rust-highfive commented Jan 8, 2020

Uh oh!

rust-highfive commented Jan 8, 2020

Uh oh!

This comment has been minimized.

Aaron1011 commented Jan 8, 2020

Uh oh!

This comment has been minimized.

nikomatsakis commented Jan 8, 2020

Uh oh!

This comment has been minimized.

nikomatsakis commented Jan 8, 2020

Uh oh!

withoutboats commented Jan 8, 2020

Uh oh!

nikomatsakis commented Jan 8, 2020

Uh oh!

withoutboats commented Jan 8, 2020

Uh oh!

This comment has been minimized.

This comment has been minimized.

nikomatsakis commented Jan 8, 2020

Uh oh!

varkor left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

varkor commented Jan 8, 2020

Uh oh!

nikomatsakis commented Jan 8, 2020

Uh oh!

Lokathor commented Jan 8, 2020

Uh oh!

Aaron1011 commented Jan 8, 2020• edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

nikomatsakis commented Mar 26, 2020

Uh oh!

bors commented Mar 26, 2020

Uh oh!

bors commented Mar 26, 2020

Uh oh!

Centril commented Mar 26, 2020

Uh oh!

bors commented Mar 26, 2020

Uh oh!

estebankMar 26, 2020

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

17 participants

nikomatsakis commented Jan 8, 2020•
edited
Loading

Aaron1011 commented Jan 8, 2020•
edited
Loading