lint internal hashmap iteration

[lcnr]

Proposal

When iterating over a FxHashMap or FxHashSet in a query, it is easily possible for the iteration order to influence the
behavior of that query. This can lead to either huge difference based on small inputs, which is mostly fine but makes ui tests annoying, if the HashMap hashes something for which the Hash and HashStable implementations differ, e.g. DefId, this can even lead to inconsistent query results, causing a few bugs in the past.

I've implemented a lint in rust-lang/rust#89558 which triggers whenever using methods marked with #[rustc_lint_query_stability] and added that attribute to "unstable" methods of both HashMap and HashSet.

I think that it probably makes sense to use this lint by default and require us to explicitly allow it in all places where the iteration order does not matter. We could also use the results of that lint to fix any issues which exist rn and try to be careful in the future without denying it by default, but that seems slightly worse to me.

It caused about 70 warnings for me locally (errors without a custom config.toml) a bunch of which did not reveal any bugs, but there were definitely a few cases which were either far from trivial or actually incorrect.

Mentors or Reviewers

🤷 someone from @rust-lang/wg-incr-comp probably

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[estebank]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted