recommend using a HashMap if a HashSet's second generic parameter doesn't implement BuildHasher #147171

Qelxiros · 2025-09-30T03:46:10Z

~~The suggestion span is wrong, but I'm not sure how to find the right one.~~ fixed

I'm relatively new to the diagnostics ecosystem, so I'm not sure if span_help is the right choice. span_suggestion_* might be better, but the output from x test looks weird in that case.

rustbot · 2025-09-30T03:46:15Z

r? @fee1-dead

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fee1-dead · 2025-10-01T23:56:12Z

tests/ui/hashmap/hashset_generics.stderr

+note: the foreign item type `String` doesn't implement `BuildHasher`
+  --> $SRC_DIR/alloc/src/string.rs:LL:COL
+   |
+   = note: not implement `BuildHasher`


note to self, this probably needs improvement.

fee1-dead · 2025-10-02T00:00:27Z

tests/ui/hashmap/hashset_generics.rs

Hmm. Could there be other tests that showcase when this help message could appear? For example when I try to insert, or when I try to create a new value of the HashSet type.

Also I think it might be better to put this under the less crowded tests/ui/hashmap directory.

fee1-dead · 2025-10-07T17:25:45Z

Still waiting on other tests.

Also, use [@]rustbot ready if you want to resubmit for review

Qelxiros · 2025-10-15T12:59:52Z

I've added a test for the insert case. It seems pretty representative of most instances of this error, since the HashSet bounds are on methods. If we want to lint (not error) on HashSet construction with a second parameter that doesn't impl BuildHasher (and I'm not convinced we do), that's a separate conversation.

@rustbot ready

fee1-dead · 2025-10-15T19:34:39Z

tests/ui/hashmap/hashset_generics.stderr

+help: you might have intended to use a HashMap instead
+  --> $DIR/hashset_generics.rs:8:5
+   |
+LL | #[derive(PartialEq)]
+   |          --------- in this derive macro expansion
+...
+LL |     pub parameters: HashSet<String, String>,
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


Can we just use err.help() to avoid highlighting the same thing again?

bors · 2025-10-22T10:44:22Z

☔ The latest upstream changes (presumably #147957) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-10-30T14:06:07Z

This PR was rebased onto a different master 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.

Qelxiros · 2025-10-30T14:06:30Z

@rustbot ready

tests/ui/hashmap/hashset_generics.stderr

Qelxiros · 2025-11-05T22:46:03Z

@rustbot ready

fee1-dead

had one last nit, otherwise this is good to go!

View changes since this review

fee1-dead · 2025-11-09T05:07:50Z

compiler/rustc_hir_typeck/src/method/suggest.rs

+            if foreign_preds.iter().any(|&(root_pred, pred)| {
+                if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(root_pred)) =
+                    root_pred.kind().skip_binder()
+                    && let Some(root_adt) = root_pred.self_ty().ty_adt_def()
+                {
+                    self.tcx.is_diagnostic_item(sym::HashSet, root_adt.did())
+                        && self.tcx.is_diagnostic_item(sym::BuildHasher, pred.def_id())
+                } else {
+                    false
+                }
+            }) {
+                err.help("you might have intended to use a HashMap instead");
+            }


Might be nice to extract this to a helper function that deduplicates the logic between this one and the one above, maybe named something like "suggest_hashmap_on_unsatisfied_hashset_buildhasher"

…sn't implement BuildHasher

Qelxiros · 2025-11-18T01:41:37Z

@rustbot ready

fee1-dead · 2025-11-18T18:39:12Z

@bors r+ rollup

bors · 2025-11-18T18:39:14Z

📌 Commit 754a82d has been approved by fee1-dead

It is now in the queue for this repository.

Rollup of 7 pull requests Successful merges: - #147171 (recommend using a HashMap if a HashSet's second generic parameter doesn't implement BuildHasher) - #147421 (Add check if span is from macro expansion) - #147521 (Make SIMD intrinsics available in `const`-contexts) - #148201 (Start documenting autodiff activities) - #148797 (feat: Add `bit_width` for unsigned `NonZero<T>`) - #148798 (Match <OsString as Debug>::fmt to that of str) - #149082 (autodiff: update formating, improve examples for the unstable-book) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #147171 - Qelxiros:hashmap_diag, r=fee1-dead recommend using a HashMap if a HashSet's second generic parameter doesn't implement BuildHasher closes #147147 ~The suggestion span is wrong, but I'm not sure how to find the right one.~ fixed I'm relatively new to the diagnostics ecosystem, so I'm not sure if `span_help` is the right choice. `span_suggestion_*` might be better, but the output from `x test` looks weird in that case.

Rollup of 7 pull requests Successful merges: - rust-lang/rust#147171 (recommend using a HashMap if a HashSet's second generic parameter doesn't implement BuildHasher) - rust-lang/rust#147421 (Add check if span is from macro expansion) - rust-lang/rust#147521 (Make SIMD intrinsics available in `const`-contexts) - rust-lang/rust#148201 (Start documenting autodiff activities) - rust-lang/rust#148797 (feat: Add `bit_width` for unsigned `NonZero<T>`) - rust-lang/rust#148798 (Match <OsString as Debug>::fmt to that of str) - rust-lang/rust#149082 (autodiff: update formating, improve examples for the unstable-book) r? `@ghost` `@rustbot` modify labels: rollup

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recommend using a HashMap if a HashSet's second generic parameter doe…

754a82d

…sn't implement BuildHasher

Qelxiros force-pushed the hashmap_diag branch from 2656007 to 754a82d Compare November 17, 2025 16:56

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matthiaskrgr mentioned this pull request Nov 19, 2025

Rollup of 7 pull requests #149085

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bors merged commit 847c422 into rust-lang:main Nov 19, 2025
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rustbot added this to the 1.93.0 milestone Nov 19, 2025

recommend using a HashMap if a HashSet's second generic parameter doesn't implement BuildHasher #147171

recommend using a HashMap if a HashSet's second generic parameter doesn't implement BuildHasher #147171

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