Suggest using equality comparison instead of pattern matching on non-structural constant in pattern by estebank · Pull Request #154010 · rust-lang/rust

estebank · 2026-03-17T17:54:20Z

When encountering a pattern containing a non-structural constant (not marked as #[derive(PartialEq)] to make it suitable for pattern matching, C in the examples below), we would previously not provide additional guidance. With this PR, the help in the following examples are added:

error: constant of non-structural type `partial_eq::S` in a pattern
  --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:16:18
   |
LL |     struct S;
   |     -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL |     const C: S = S;
   |     ---------- constant defined here
...
LL |             Some(C) => {}
   |                  ^ constant of non-structural type
   |
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
  --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5
   |
LL |     impl PartialEq<S> for S {
   |     ^^^^^^^^^^^^^^^^^^^^^^^
help: add a condition to the match arm checking for equality
   |
LL -             Some(C) => {}
LL +             Some(binding) if binding == C => {}
   |

error: constant of non-structural type `partial_eq::S` in a pattern
  --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:22:18
   |
LL |     struct S;
   |     -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL |     const C: S = S;
   |     ---------- constant defined here
...
LL |         let Some(C) = Some(S) else { return; };
   |                  ^ constant of non-structural type
   |
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
  --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5
   |
LL |     impl PartialEq<S> for S {
   |     ^^^^^^^^^^^^^^^^^^^^^^^
help: check for equality instead of pattern matching
   |
LL -         let Some(C) = Some(S) else { return; };
LL +         if Some(C) == Some(S) { return; };
   |

The suggestion accounts for a few conditions:

if the type is not from the local crate and has no PartialEq impl, the user can't make it structural, so we don't provide the suggestion
regardless of whether the type is local or remote, if it has a manual PartialEq, explain that with a derived PartialEq you could use equality
if the type is local and has no impl, suggest adding a derived PartialEq and use equality check instead of pattern matching
when suggesting equality, account for if-let to suggest chaining (edition dependent), match arm with a present if check, match arm without an existing if check
when encountering let-else, we suggest turning it into an if expression instead (this doesn't check for additional bindings beyond the constant, which would suggest incorrect code in some more complex cases).

Fix #42753.

…strutural constant in pattern ``` error: constant of non-structural type `partial_eq::S` in a pattern --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:16:18 | LL | struct S; | -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns ... LL | const C: S = S; | ---------- constant defined here ... LL | Some(C) => {} | ^ constant of non-structural type | note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5 | LL | impl PartialEq<S> for S { | ^^^^^^^^^^^^^^^^^^^^^^^ help: add a condition to the match arm checking for equality | LL - Some(C) => {} LL + Some(binding) if binding == C => {} | ```

rustbot · 2026-03-17T17:54:25Z

Some changes occurred in match checking

cc @Nadrieril

rustbot · 2026-03-17T17:54:27Z

r? @nnethercote

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…ural type const ``` error: constant of non-structural type `partial_eq::S` in a pattern --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:22:18 | LL | struct S; | -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns ... LL | const C: S = S; | ---------- constant defined here ... LL | let Some(C) = Some(S) else { return; }; | ^ constant of non-structural type | note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5 | LL | impl PartialEq<S> for S { | ^^^^^^^^^^^^^^^^^^^^^^^ help: check for equality instead of pattern matching | LL - let Some(C) = Some(S) else { return; }; LL + if Some(C) == Some(S) { return; }; | ```

nnethercote · 2026-03-19T22:35:07Z

Can you update the PR description to include more detail? I've read #42753, but links to issues aren't always helpful because they often include extraneous details. I also don't know how to interpret the two error outputs in the PR description. Are they new error messages? If so, how do they compare to the old error messages? I don't have enough context to proceed. Thanks.

@rustbot author

rustbot · 2026-03-19T22:35:12Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

estebank · 2026-03-20T15:15:53Z

@nnethercote added some more context to the description, let me know if that sufficient, otherwise I can try to expand further.

nnethercote

The expanded description was extremely helpful, thank you. There is a lot going on with this diagnostic!

Good enough as is, a couple of minor comments to consider, r=me either way.

View changes since this review

nnethercote · 2026-03-24T00:23:17Z

   |
 LL | pub struct CustomEq;
-   | ------------------- `CustomEq` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
+   | ------------------- `CustomEq` is not usable in patterns


This non-local case is tricky, because the other crate might be under the author's control and able to be changed. I guess it's impossible to know for sure.

I was going back and forth on this one. I settled on "we're giving them a link to the docs". I was concerned with giving the impression to the user that they had to somehow fork their dep. We could instead say something like "CustomEq could be used in patterns if it was annotated with #[derive(PartialEq)]", but I fear that has the same problem.

nnethercote · 2026-03-24T00:25:40Z

+help: add a condition to the match arm checking for equality
+   |
+LL -         BAR_BAZ => panic!(),
+LL +         binding if binding == BAR_BAZ => panic!(),


It's a bit confusing that the error message suggests two separate ways of fixing this -- add derive(PartialEq) and use a match arm condition. Could the one-or-the-other nature be made clearer? Maybe by inserting "alternatively, " at the start of the help message?

Ah, I was looking on how to address this. I'll follow up.

nnethercote · 2026-03-26T04:18:01Z

@bors r+

rust-bors · 2026-03-26T04:18:04Z

📌 Commit 801eafd has been approved by nnethercote

It is now in the queue for this repository.

rust-bors · 2026-03-26T08:41:37Z

☀️ Test successful - CI
Approved by: nnethercote
Duration: 3h 11m 23s
Pushing 3ea2fbc to main...

github-actions · 2026-03-26T08:44:27Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 6d8bc65 (parent) -> 3ea2fbc (this PR)

Test differences

Show 5 test diffs

Stage 1

[ui] tests/ui/consts/const_in_pattern/suggest_equality_comparison_instead_of_pattern_matching.rs: [missing] -> pass (J0)

Stage 2

[ui] tests/ui/consts/const_in_pattern/suggest_equality_comparison_instead_of_pattern_matching.rs: [missing] -> pass (J1)

Additionally, 3 doctest diffs were found. These are ignored, as they are noisy.

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Test dashboard

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cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 3ea2fbcb2a872b7e1fa3cada256f8e97f9e6636f --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-apple-various: 1h 49m -> 1h 24m (-22.8%)
pr-check-1: 33m 49s -> 28m 20s (-16.2%)
x86_64-gnu-miri: 1h 34m -> 1h 19m (-16.0%)
aarch64-gnu-debug: 1h 19m -> 1h 7m (-15.5%)
dist-aarch64-msvc: 1h 51m -> 1h 37m (-13.2%)
i686-gnu-2: 1h 43m -> 1h 29m (-13.1%)
dist-x86_64-apple: 2h -> 1h 44m (-12.8%)
aarch64-gnu-llvm-21-2: 54m 25s -> 47m 47s (-12.2%)
test-various: 2h 10m -> 1h 54m (-12.2%)
i686-msvc-2: 2h 1m -> 2h 16m (+11.9%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-03-26T09:23:21Z

Finished benchmarking commit (3ea2fbc): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

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Results (primary 0.6%)

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	mean	range	count
Regressions ❌ (primary)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.6%, 0.6%]	1

Cycles

Results (secondary 3.3%)

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	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.3%	[2.2%, 4.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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Bootstrap: 483.836s -> 485.545s (0.35%)
Artifact size: 395.09 MiB -> 395.12 MiB (0.01%)

rustbot assigned nnethercote Mar 17, 2026

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 Mar 17, 2026

estebank changed the title ~~Suggest using equality comparison instead of pattern matching on non-…~~ Suggest using equality comparison instead of pattern matching on non-structural constant in pattern Mar 17, 2026

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 19, 2026

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nnethercote approved these changes Mar 24, 2026

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rust-bors Bot merged commit 3ea2fbc into rust-lang:main Mar 26, 2026
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