Audit and handle remaining cases of rustc::potential_query_instability lint (iterating HashMaps)

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The iteration order of a HashMap depends on the Hash value of the keys. In rustc, the Hash value of a key can change between compilation sessions, even if the HashStable value remains the same (e.g. DefId, Symbol, etc.). This means that iterating over a HashMap can cause a query to produce different results given the same (as determined by HashStable) input. This can be quite subtle, as demonstrated by #82272 (comment)

To eliminate this source of issues, we should make an internal rustc-only lint for iterating over a HashMap in any way. This will need to cover explicit method calls (e.g. .keys() and .values()), as well as any usage of the IntoIterator impl for HashMap.

Note that we don't want to ban the usage of HashMaps with unstable keys - as long as they're only used for lookup (and not iteration), it's much more efficient than repeatedly converting to the stable form of a key (e.g. DefId -> DefPathHash or Symbol -> String).

Note that we don't want to ban the usage of HashMaps with unstable keys - as long as they're only used for lookup (and not iteration),

#64131 added the StableMap wrapper for FxHashMap that doesn't expose the iteration order.

@bjorn3

@bjorn3 see #84446

The lint exists since #89558, and has allowed on all compiler crates as rustc::potential_query_instability.
We now need to stop allowing it in the compiler.

Steps, for each crate except rustc_data_structures:

remove the allow(rustc::potential_query_instability) in lib.rs;
replace the FxHash{Map,Set} by FxIndex{Map,Set} where necessary.

Please reach out on zulip if there are any questions.

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@rustbot

It seems like no one is assigned to this and I'd like to give it a try so assigning myself.

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@ismailarilik

Poking this since it still unresolved. What work still needs be done to finish this? @ismailarilik @Aaron1011?

Reading through this issue, and the PR's / merges above, and probing around with grep, my current understanding is that there are still some areas with #[allow(...)], without justifying comments.

Specifically, these are the files with local allows missing justifying comments

compiler/rustc_type_ir/src/search_graph/mod.rs (7 occurrences)
- Line 250
- Line 504
- Line 512
- Line 713
- Line 904
- Line 962
- Line 1146
compiler/rustc_mir_dataflow/src/value_analysis.rs (3 occurrences)
- Line 70
- Line 128
- Line 584
compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs (2 occurrences)
- Line 437
- Line 445
compiler/rustc_lint/src/early/diagnostics/check_cfg.rs (1 occurrence)
- Line 118
compiler/rustc_interface/src/interface.rs (1 occurrence)
- Line 307
compiler/rustc_expand/src/mbe/macro_rules.rs (1 occurrence)
- Line 625
compiler/rustc_codegen_ssa/src/target_features.rs (1 occurrence)
- Line 193

[Updated, the list above, assuming github links to comments count as valid justifications]

Is adding justifying comments (or converting to FxIndexMap where necessary) all that remains for this to be resolved?

Auto merge of #125928 - michaelwoerister:fix-cgu-hashstable, r=oli-obk

Add rationale for intentional potential_query_instability allows

Rollup merge of rust-lang#153479 - xfq:issue-84447-20260306, r=davidtwco

Unrolled build for #153479

Rollup merge of #153479 - xfq:issue-84447-20260306, r=davidtwco

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Audit and handle remaining cases of `rustc::potential_query_instability` lint (iterating `HashMap`s) #84447

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Audit and handle remaining cases of rustc::potential_query_instability lint (iterating HashMaps) #84447

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bjorn3 commented on Apr 23, 2021

jyn514 commented on Apr 23, 2021

cjgillot commented on Jul 5, 2022

fee1-dead commented on Jul 7, 2022

NiklasJonsson commented on Jul 8, 2022

126 remaining items

Caylub commented on Jan 24, 2026

ismailarilik commented on May 23, 2026

JinRudy commented on Jun 3, 2026

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Audit and handle remaining cases of `rustc::potential_query_instability` lint (iterating `HashMap`s) #84447