Clarify some interning details by nnethercote · Pull Request #158642 · rust-lang/rust

nnethercote · 2026-07-01T06:03:29Z

Details in the commit message.

nnethercote · 2026-07-01T06:03:41Z

petrochenkov · 2026-07-01T06:21:10Z

    fn alloc_import(&'ra self, import: ImportData<'ra>) -> Import<'ra> {
+        // FIXME: `Interned::new_unchecked`'s argument must be unique (e.g. via interning) but it's
+        // not clear that this value is unique! And yet, somehow things seem to work ok. Perhaps
+        // the IDs and/or the spans within `import` guarantee uniqueness?


There isn't much to fix here (and in the other 3 cases), different Imports are unique by definition, even if they have the same data.
The address of the ImportData on the arena is the ID.
In theory we could introduce some separate ImportId(u32)s and add them to ImportData, and alloc_import would generate a fresh ImportId every time, but why doing that if the pointer itself is enough.

Previously arena allocated data in the resolver didn't even use Interned, it used a different type named PtrKey that simply provided pointer-like comparison and hashing for references.
But then PtrKey was merged into Interned (maybe by you actually), despite slightly different requirements.

Upd, relevant PR:

Overhaul interning. #93148

View changes since the review

different Imports are unique by definition, even if they have the same data.

Can you expand on that? I don't understand.

If you write a SAFETY: <comment> explanation to replace the FIXME comments I'm happy to change them.

Suppose DeclData, for example, loses all the auxiliary information and is left only with

struct DeclData { res: Res, span: Span, }

Then you can end up with two different declarations in different modules having the same DeclData (the spans can always be dummy).
But they are still different unique declarations, and the DeclData address on the arena is used as their unique ID, and they can therefore be compared by address.

It can be represented as "desugaring"

struct DeclData { id: *const () = /* address of self */, // this would make the data unique too res: Res, span: Span, }

, except the address is not kept explicitly.
(In C++ maps keyed on pointers like this are used right and left, including in LLVM.)

Thanks for the info. I never really understood this properly. I have adjusted the PR significantly to account for it.

We could consider renaming Interned back to PtrKey, to better reflect that it covers more than just the interning case. But I haven't done that yet.

rustbot · 2026-07-02T02:10:32Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

`Interned` does pointer equality/hashing, which is valid in two cases. - The values are guaranteed to be unique (e.g. via interning, or construction). This is how `rustc_middle` uses `Interned`. - The type has "identity" and different values should be considered distinct even if they are identical. This is how `rustc_resolve` uses `Interned`. PR 137202 tried to clarify things by adding a `T: Hash` constraint to `Interned<'a, T>`. This constraint isn't actually used, because `Interned` is hashed based on pointer value, not contents. But it was intended to communicate the idea that a type stored in `Interned` is actually interned, which is likely to be done with hashing. Panicking impls of `Hash` were added for the relevant `rustc_resolve` types to work around the fact that it doesn't use hashing-based interning. In my opinion PR 137202 didn't improve things. The `T: Hash` constraint is only aimed at the interning case, and even for that case it's not quite right because you could use a `BTreeMap` to intern instead of a `HashMap`. This commit does several things. - Removes the `T: Hash` constraint and the `Hash` impls for `rustc_resolve` types added in PR 137202. - Improves the comments on `Interned` to cover the non-interning cases. - Removes the `PartialOrd`/`Ord` impls on `Interned` because (a) they're not used, and (b) their meaning is unclear for the "identity" case. - Improves the documentation in `rustc_resolve` to explain how `Interned` usage is valid there.

Mark-Simulacrum · 2026-07-05T19:24:37Z

@bors r+

Thanks, this does look better.

rust-bors · 2026-07-05T19:24:40Z

📌 Commit fdfba67 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 2. This pull request will be tested once the tree is reopened.

@Mark-Simulacrum

…n, r=Mark-Simulacrum Clarify some interning details Details in the commit message. r? @Mark-Simulacrum

@Mark-Simulacrum

…n, r=Mark-Simulacrum Clarify some interning details Details in the commit message. r? @Mark-Simulacrum

Rollup of 20 pull requests Successful merges: - #158377 (add `-Zforce-intrinsic-fallback` flag) - #158642 (Clarify some interning details) - #158743 (Look for cdb location in the registry first) - #158775 (bootstrap: only encode RUSTFLAGS when a flag contains a space) - #158782 (Add and use cfg(target_has_threads) to enforce no_thread impl usage) - #158785 (hook intrinsic-test into aarch64-gnu) - #157734 (Stabilize `local_key_cell_update`) - #158183 (std: allocate less memory in `current_exe` for OpenBSD) - #158671 (Move tests batch 17) - #158730 (Update `FIXME(static_mut_refs)` comments) - #158752 (Reorganize `tests/ui/issues` [18/N]) - #158755 (Use `ThinVec` more in the AST) - #158757 (Fix incorrect tracking issue for `read_le`/`read_be`) - #158765 (Fix ICE on non-ident path in `doc(auto_cfg values)`) - #158771 (library: expand HashSet::extract_if coverage) - #158772 (rustc-dev-guide subtree update) - #158776 (fix: emit diagnostic for AVR target without target-cpu) - #158786 (Add regression test for builtin attr macro values) - #158807 (Add regression test for CString::clone_into unwind safety) - #158825 (Fix typo)

Rollup of 24 pull requests Successful merges: - #158377 (add `-Zforce-intrinsic-fallback` flag) - #158642 (Clarify some interning details) - #158694 (Positive test for closures needing expectations) - #158743 (Look for cdb location in the registry first) - #158775 (bootstrap: only encode RUSTFLAGS when a flag contains a space) - #158782 (Add and use cfg(target_has_threads) to enforce no_thread impl usage) - #158785 (hook intrinsic-test into aarch64-gnu) - #158819 (Put `InhabitedPredicate::NotInModule` earlier in disjunction since it can be a lot faster) - #157734 (Stabilize `local_key_cell_update`) - #158183 (std: allocate less memory in `current_exe` for OpenBSD) - #158310 (Remove unexpected usage of Unambig in non-infer variants) - #158671 (Move tests batch 17) - #158730 (Update `FIXME(static_mut_refs)` comments) - #158752 (Reorganize `tests/ui/issues` [18/N]) - #158755 (Use `ThinVec` more in the AST) - #158757 (Fix incorrect tracking issue for `read_le`/`read_be`) - #158765 (Fix ICE on non-ident path in `doc(auto_cfg values)`) - #158771 (library: expand HashSet::extract_if coverage) - #158772 (rustc-dev-guide subtree update) - #158776 (fix: emit diagnostic for AVR target without target-cpu) - #158786 (Add regression test for builtin attr macro values) - #158810 (Add supplementary information for get_unchecked(mut)) - #158825 (Fix typo) - #158838 (tidy: Use `empty_alternate = true` for triagebot mention glob check)

@Mark-Simulacrum

Rollup merge of #158642 - nnethercote:interning-clarification, r=Mark-Simulacrum Clarify some interning details Details in the commit message. r? @Mark-Simulacrum

Rollup of 24 pull requests Successful merges: - rust-lang/rust#158377 (add `-Zforce-intrinsic-fallback` flag) - rust-lang/rust#158642 (Clarify some interning details) - rust-lang/rust#158694 (Positive test for closures needing expectations) - rust-lang/rust#158743 (Look for cdb location in the registry first) - rust-lang/rust#158775 (bootstrap: only encode RUSTFLAGS when a flag contains a space) - rust-lang/rust#158782 (Add and use cfg(target_has_threads) to enforce no_thread impl usage) - rust-lang/rust#158785 (hook intrinsic-test into aarch64-gnu) - rust-lang/rust#158819 (Put `InhabitedPredicate::NotInModule` earlier in disjunction since it can be a lot faster) - rust-lang/rust#157734 (Stabilize `local_key_cell_update`) - rust-lang/rust#158183 (std: allocate less memory in `current_exe` for OpenBSD) - rust-lang/rust#158310 (Remove unexpected usage of Unambig in non-infer variants) - rust-lang/rust#158671 (Move tests batch 17) - rust-lang/rust#158730 (Update `FIXME(static_mut_refs)` comments) - rust-lang/rust#158752 (Reorganize `tests/ui/issues` [18/N]) - rust-lang/rust#158755 (Use `ThinVec` more in the AST) - rust-lang/rust#158757 (Fix incorrect tracking issue for `read_le`/`read_be`) - rust-lang/rust#158765 (Fix ICE on non-ident path in `doc(auto_cfg values)`) - rust-lang/rust#158771 (library: expand HashSet::extract_if coverage) - rust-lang/rust#158772 (rustc-dev-guide subtree update) - rust-lang/rust#158776 (fix: emit diagnostic for AVR target without target-cpu) - rust-lang/rust#158786 (Add regression test for builtin attr macro values) - rust-lang/rust#158810 (Add supplementary information for get_unchecked(mut)) - rust-lang/rust#158825 (Fix typo) - rust-lang/rust#158838 (tidy: Use `empty_alternate = true` for triagebot mention glob check)

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 1, 2026

rustbot assigned Mark-Simulacrum Jul 1, 2026

petrochenkov reviewed Jul 1, 2026

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nnethercote force-pushed the interning-clarification branch from de8a386 to b097cca Compare July 2, 2026 02:10

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nnethercote force-pushed the interning-clarification branch from b097cca to fdfba67 Compare July 2, 2026 07:10

rust-bors Bot 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 Jul 5, 2026

JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Jul 5, 2026

Rollup merge of rust-lang#158642 - nnethercote:interning-clarificatio…

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…n, r=Mark-Simulacrum Clarify some interning details Details in the commit message. r? @Mark-Simulacrum

JonathanBrouwer mentioned this pull request Jul 5, 2026

Rollup of 16 pull requests #158828

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jhpratt added a commit to jhpratt/rust that referenced this pull request Jul 6, 2026

Rollup merge of rust-lang#158642 - nnethercote:interning-clarificatio…

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…n, r=Mark-Simulacrum Clarify some interning details Details in the commit message. r? @Mark-Simulacrum

jhpratt mentioned this pull request Jul 6, 2026

Rollup of 20 pull requests #158837

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jhpratt mentioned this pull request Jul 6, 2026

Rollup of 24 pull requests #158847

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rust-bors Bot merged commit 75bea57 into rust-lang:main Jul 6, 2026
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rustbot added this to the 1.99.0 milestone Jul 6, 2026

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Unrolled build for #158642

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Rollup merge of #158642 - nnethercote:interning-clarification, r=Mark-Simulacrum Clarify some interning details Details in the commit message. r? @Mark-Simulacrum

nnethercote deleted the interning-clarification branch July 6, 2026 23:08

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Clarify some interning details#158642

Clarify some interning details#158642
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