Fix suppression of `unused_assignment` in binding of `unused_variable` #151556

eggyal · 2026-01-23T22:25:16Z

Unused assignments to an unused variable should trigger only the unused_variables lint and not also the unused_assignments lint. This was previously implemented by checking whether the span of the assignee was within the span of the binding pattern, however that failed to capture situations was imported from elsewhere (eg from the input tokenstream of a proc-macro that generates the binding pattern).

By comparing the span of the assignee to those of the variable introductions instead, a reported stable-to-stable regression is resolved.

This fix also impacted some other preexisting tests, which had (undesirably) been triggering both the unused_variables and unused_assignments lints on the same initializing assignment; those tests have therefore now been updated to expect only the former lint.

Fixes #151514
r? cjgillot (as author of reworked liveness testing in #142390)

Unused assignments to an unused variable should trigger only the `unused_variables` lint and not also the `unused_assignments` lint. This was previously implemented by checking whether the span of the assignee was within the span of the binding pattern, however that failed to capture situations was imported from elsewhere (eg from the input tokenstream of a proc-macro that generates the binding pattern). By comparing the span of the assignee to those of the variable introductions instead, a reported stable-to-stable regression is resolved. This fix also impacted some other preexisting tests, which had (undesirably) been triggering both the `unused_variables` and `unused_assignments` lints on the same initializing assignment; those tests have therefore now been updated to expect only the former lint.

rustbot · 2026-01-23T22:25:19Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

eggyal · 2026-01-23T22:28:06Z

tests/ui/pattern/bindings-after-at/bind-by-copy.rs

@@ -53,7 +53,6 @@ pub fn main() {
    }
    match (E::E { a: 10, e: C { c: 20 } }) {
        mut x @ E::E{ a, e: C { mut c } } => {
-            //~^ WARN value assigned to `a` is never read


This line does still trigger the unused_variables lint in respect of a, but that lint is allowed by the test and so is not shown here. I'd be happy to enable it, but there are a bunch of other occasions in the test that would then trigger it (if enabled test-wide; of course I could just enable it for this scope).

Tuntii

The resulting diagnostics are more precise without weakening either lint.

View changes since this review

Tuntii · 2026-01-23T23:57:10Z

compiler/rustc_mir_transform/src/liveness.rs

            // from the list of assignments the ones that happen at the definition site.
            statements.retain(|source_info, _| {
-                source_info.span.find_ancestor_inside(binding.pat_span).is_none()
+                !binding.introductions.iter().any(|intro| intro.span == source_info.span)


This additional filter against binding.introductions makes sense, especially for proc-macro generated bindings where spans don’t line up lexically. It avoids double warnings without weakening the lint.

Tuntii · 2026-01-23T23:58:17Z

tests/ui/lint/unused/auxiliary/unused_assignment_proc_macro.rs

+    quote! {
+        impl Drop for $name {
+            fn drop(&mut self) {
+                let Self { $field } = self;


Without the fix, the compiler would incorrectly emit both unused_variables and unused_assignments. This test clearly asserts the intended behavior.

cjgillot · 2026-01-24T20:33:08Z

r=me if perf is clean

@bors try @rust-timer queue

…r=<try> Fix suppression of `unused_assignment` in binding of `unused_variable`

rust-bors · 2026-01-24T22:49:28Z

☀️ Try build successful (CI)
Build commit: 38bad8b (38bad8be43d60c5e464300a5658806ef6ef23a6c, parent: 021fc25b7a48f6051bee1e1f06c7a277e4de1cc9)

rust-timer · 2026-01-24T23:49:37Z

Finished benchmarking commit (38bad8b): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (primary 2.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.4%, 2.4%]	1

Cycles

Results (primary -1.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.7%	[-2.9%, -0.4%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.7%	[-2.9%, -0.4%]	2

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 472.296s -> 472.772s (0.10%)
Artifact size: 383.54 MiB -> 383.54 MiB (-0.00%)

cjgillot · 2026-01-25T05:12:05Z

@bors r+ rollup

rust-bors · 2026-01-25T05:12:09Z

📌 Commit 22b3f59 has been approved by cjgillot

It is now in the queue for this repository.

rust-bors · 2026-01-25T16:30:29Z

☀️ Test successful - CI
Approved by: cjgillot
Duration: 3h 19m 48s
Pushing 9415853 to main...

github-actions · 2026-01-25T16:33:37Z

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 4d38622 (parent) -> 9415853 (this PR)

Test differences

Show 5 test diffs

Stage 1

[ui] tests/ui/lint/unused/unused_assignment.rs: [missing] -> pass (J1)

Stage 2

[ui] tests/ui/lint/unused/unused_assignment.rs: [missing] -> pass (J0)

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

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 9415853279dda1394c1f7d4114e1fe1e3ced76ec --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-x86_64-apple: 6657.5s -> 8340.7s (+25.3%)
x86_64-gnu-llvm-20-3: 6061.4s -> 6686.7s (+10.3%)
aarch64-msvc-2: 5528.7s -> 6088.0s (+10.1%)
i686-gnu-nopt-1: 7254.3s -> 7949.1s (+9.6%)
x86_64-mingw-1: 10261.6s -> 9283.9s (-9.5%)
x86_64-gnu-llvm-21-2: 5435.7s -> 4937.2s (-9.2%)
dist-loongarch64-linux: 5096.3s -> 5549.0s (+8.9%)
pr-check-2: 2496.0s -> 2292.6s (-8.1%)
x86_64-gnu-stable: 8043.7s -> 7392.1s (-8.1%)
x86_64-gnu-llvm-21-3: 5881.4s -> 6327.7s (+7.6%)

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 · 2026-01-25T17:12:28Z

Finished benchmarking commit (9415853): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.0%, 0.2%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -1.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.4%	[1.6%, 6.7%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.8%	[-5.4%, -4.2%]	6
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 0.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	4.8%	[4.7%, 5.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.4%]	3
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 473.581s -> 470.507s (-0.65%)
Artifact size: 383.57 MiB -> 383.59 MiB (0.01%)

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 Jan 23, 2026

rustbot assigned cjgillot Jan 23, 2026

eggyal commented Jan 23, 2026

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rust-bors bot pushed a commit that referenced this pull request Jan 24, 2026

Auto merge of #151556 - eggyal:unused-assignment-to-unused-variable, …

38bad8b

…r=<try> Fix suppression of `unused_assignment` in binding of `unused_variable`

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 24, 2026

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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 Jan 25, 2026

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rust-bors bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jan 25, 2026

rust-bors bot merged commit 9415853 into rust-lang:main Jan 25, 2026
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rustbot added this to the 1.95.0 milestone Jan 25, 2026

eggyal deleted the unused-assignment-to-unused-variable branch January 25, 2026 17:30

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Fix suppression of unused_assignment in binding of unused_variable #151556

Fix suppression of unused_assignment in binding of unused_variable #151556

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Overall result: no relevant changes - no action needed

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Test differences

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Overall result: ❌✅ regressions and improvements - no action needed

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Fix suppression of `unused_assignment` in binding of `unused_variable` #151556

Fix suppression of `unused_assignment` in binding of `unused_variable` #151556

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