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Fix HashStable implementation on InferTy #91892

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Merged
merged 1 commit into from
Dec 14, 2021
Merged

Fix HashStable implementation on InferTy #91892

merged 1 commit into from
Dec 14, 2021
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compiler-errors
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HashStable impl forgot to hash the discriminant.

Fixes #91807

@rustbot rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Dec 14, 2021
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r? @estebank

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@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Dec 14, 2021
@nbdd0121
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I thought about a potential hash collision but scratched my head really hard looking at #88552 (the regressing PR) and couldn't find anything suspicious. Didn't expect the cause to be somewhere that I completely untouched :)

Given that this is a hash collision issue, I wonder if the added test is sufficient to prevent regression.

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@nbdd0121, yeah, the only reason the regression was on your PR was because you introduced a new query that caused us to infer a IntVar and TyVar that collided in HashStable (modulo discriminant).

Not sure what the best way to more thoroughly prevent regressions via a test. On the other hand, I might audit the remainder of the (non-derive) HashStable impls in rustc to check that they're consistent with their Eq implementations.

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dtolnay commented Dec 14, 2021

Is it a known flaw that a hash collision can manifest as an ICE in the first place? I'm wondering whether this PR should close #91807 or keep an issue open to track this. In general it's not correct that unequal items have an unequal hash, even with hash impls that hit all the fields/discriminants. Are there enough bits in this hash that we're happy pretending that's the case?

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dtolnay commented Dec 14, 2021

Either way, this PR looks obviously correct to me. :) Thanks!

Maybe a good follow-up would be to see if there's a good place to insert a tip in the ICE, so that instead of/in addition to "forcing query with already existing DepNode", the panic message would point out you've most likely got a colliding hash.

@bors r+

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bors commented Dec 14, 2021

📌 Commit 75729af has been approved by dtolnay

@bors 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 Dec 14, 2021
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Is it a known flaw that a hash collision can manifest as an ICE in the first place? I'm wondering whether this PR should close #91807 or keep an issue open to track this. In general it's not correct that unequal items have an unequal hash, even with hash impls that hit all the fields/discriminants. Are there enough bits in this hash that we're happy pretending that's the case?

It's intended that hash only is used: https://rustc-dev-guide.rust-lang.org/queries/incremental-compilation-in-detail.html

This approach works rather well but it's not without flaws:

  • There is a small possibility of hash collisions. That is, two different results could have the same fingerprint and the system would erroneously assume that the result hasn't changed, leading to a missed update.

    We mitigate this risk by using a high-quality hash function and a 128 bit wide hash value. Due to these measures the practical risk of a hash collision is negligible.

It works well in practice (modulo incorrect implementation)

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dtolnay commented Dec 14, 2021

Okay, I buy that. Thanks for the link! I agree 128 bits is enough that this won't happen with correct impls.

🙈

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Maybe a good follow-up would be to see if there's a good place to insert a tip in the ICE, so that instead of/in addition to "forcing query with already existing DepNode", the panic message would point out you've most likely got a colliding hash.

Good suggestion! I'll probably turn that assert into a bug! and leave a more detailed error message that the Eq and HashStable implementations probably have diverged for the key type, in case ICEs like this show up in the future.

@matthiaskrgr matthiaskrgr mentioned this pull request Dec 14, 2021
Merged
bors added a commit to rust-lang-ci/rust that referenced this pull request Dec 14, 2021
@bors
Auto merge of rust-lang#91902 - matthiaskrgr:rollup-hjjyhow, r=matthi…
404c847 
…askrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#91529 (add BinaryHeap::try_reserve and BinaryHeap::try_reserve_exact)
 - rust-lang#91820 (Suggest to specify a target triple when lang item is missing)
 - rust-lang#91851 (Make `MaybeUninit::zeroed` `const`)
 - rust-lang#91875 (Use try_normalize_erasing_regions in RevealAllVisitor)
 - rust-lang#91887 (Remove `in_band_lifetimes` from `rustc_const_eval`)
 - rust-lang#91892 (Fix HashStable implementation on InferTy)
 - rust-lang#91893 (Remove `in_band_lifetimes` from `rustc_hir`)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit d0e6bb7 into rust-lang:master Dec 14, 2021
@rustbot rustbot added this to the 1.59.0 milestone Dec 14, 2021
@teohhanhui
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Considering this was a regression, can it be backported to stable?

https://forge.rust-lang.org/release/backporting.html#stable-backporting-in-rust-langrust

@Mark-Simulacrum Mark-Simulacrum added the beta-nominated Nominated for backporting to the compiler in the beta channel. label Dec 15, 2021
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discussed in T-compiler meeting. Beta-accepted.

@pnkfelix pnkfelix added the beta-accepted Accepted for backporting to the compiler in the beta channel. label Dec 16, 2021
@Mark-Simulacrum Mark-Simulacrum modified the milestones: 1.59.0, 1.58.0 Jan 5, 2022
@Mark-Simulacrum Mark-Simulacrum removed the beta-nominated Nominated for backporting to the compiler in the beta channel. label Jan 5, 2022
@Mark-Simulacrum Mark-Simulacrum mentioned this pull request Jan 5, 2022
Merged
bors added a commit to rust-lang-ci/rust that referenced this pull request Jan 8, 2022
@bors
Auto merge of rust-lang#92592 - Mark-Simulacrum:beta-next, r=Mark-Sim…
4aa9d23 
…ulacrum

[beta] backports

Backports these PRs:

* Fix HashStable implementation on InferTy rust-lang#91892
* Revert setting a default for the MACOSX_DEPLOYMENT_TARGET env var for linking rust-lang#91870
* Make rustdoc headings black, and markdown blue rust-lang#91534
* Disable LLVM newPM by default rust-lang#91190
* Deduplicate projection sub-obligations rust-lang#90423
*  Sync portable-simd to remove autosplats rust-lang#91484 by dropping portable_simd entirely (keeping the subtree, just from std/core)
*  Quote bat script command line rust-lang#92208
* Fix failing tests rust-lang#92201 (CI fix)

r? `@Mark-Simulacrum`
@compiler-errors compiler-errors deleted the fix-inferty-hashtable branch April 7, 2022 04:36
@dtolnay dtolnay assigned dtolnay and unassigned estebank Mar 25, 2024
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Regression in Deref<Target = dyn Fn(T)>: forcing query with already existing DepNode
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