Label overwritten for never read assignments by chenyukang · Pull Request #154456 · rust-lang/rust

chenyukang · 2026-03-27T06:32:49Z

rustbot · 2026-03-27T06:32:51Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2026-03-27T06:32:54Z

rustbot has assigned @TaKO8Ki.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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chenyukang · 2026-03-27T06:33:23Z

Let's check whether performance changed,
@bors try @rust-timer queue

…ts, r=<try> Label overwritten assignments for never read assignments

rust-bors · 2026-03-27T08:46:58Z

☀️ Try build successful (CI)
Build commit: eaf85f7 (eaf85f785b04901bc9054e3885e9d0a5570a9a05, parent: ac40f5e105ddc8144904679cb63c8879708a9a26)

rust-timer · 2026-03-27T09:27:22Z

Finished benchmarking commit (eaf85f7): 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 3.6%, secondary 1.7%)

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

	mean	range	count
Regressions ❌ (primary)	3.6%	[3.6%, 3.6%]	1
Regressions ❌ (secondary)	4.4%	[4.4%, 4.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.0%	[-1.0%, -1.0%]	1
All ❌✅ (primary)	3.6%	[3.6%, 3.6%]	1

Cycles

Results (primary 3.0%, secondary 2.6%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[3.0%, 3.0%]	1
Regressions ❌ (secondary)	2.6%	[2.4%, 2.9%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.0%	[3.0%, 3.0%]	1

Binary size

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

Bootstrap: 484.633s -> 485.683s (0.22%)
Artifact size: 397.11 MiB -> 397.14 MiB (0.01%)

chenyukang · 2026-03-27T09:33:49Z

-   = help: maybe it is overwritten before being read?
+   |                 ^ this value is reassigned later and never used
+LL |     b += 1;
+   |     ------ `b` is overwritten here before the previous value is read


label on this seems weird, it's a separate issue(#154457), I will work on it with following up PR.

chenyukang · 2026-03-27T09:42:34Z

Finished benchmarking commit (eaf85f7): 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 3.6%, secondary 1.7%)
A less reliable metric. May be of interest, but not used to determine the overall result above.

mean range count
Regressions ❌
(primary) 3.6% [3.6%, 3.6%] 1
Regressions ❌
(secondary) 4.4% [4.4%, 4.4%] 1
Improvements ✅
(primary) - - 0
Improvements ✅
(secondary) -1.0% [-1.0%, -1.0%] 1
All ❌✅ (primary) 3.6% [3.6%, 3.6%] 1

Cycles

Results (primary 3.0%, secondary 2.6%)
A less reliable metric. May be of interest, but not used to determine the overall result above.

mean range count
Regressions ❌
(primary) 3.0% [3.0%, 3.0%] 1
Regressions ❌
(secondary) 2.6% [2.4%, 2.9%] 2
Improvements ✅
(primary) - - 0
Improvements ✅
(secondary) - - 0
All ❌✅ (primary) 3.0% [3.0%, 3.0%] 1

Binary size

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

Bootstrap: 484.633s -> 485.683s (0.22%) Artifact size: 397.11 MiB -> 397.14 MiB (0.01%)

performance seems trivial enough.

estebank · 2026-03-30T20:20:29Z

+warning: value assigned to `d` is never read
+  --> $DIR/overwritten-before-read-note-issue-154434.rs:5:17
+   |
+LL |     let mut d = String::from("hello");
+   |                 ^^^^^^^^^^^^^^^^^^^^^ this value is reassigned later and never used
+LL |     d = String::from("ahoy");
+   |     - `d` is overwritten here before the previous value is read
+   |
+   = note: requested on the command line with `-W unused-assignments`


Do we need the additional test given the existing ones?

yes, seems it's unnecessary, I will remove it in follow-up PR.

estebank · 2026-03-30T20:24:26Z

@bors r+

rust-bors · 2026-03-30T20:24:31Z

📌 Commit 5e0ff58 has been approved by estebank

It is now in the queue for this repository.

JonathanBrouwer · 2026-03-31T07:35:11Z

@bors p=6
Scheduling this first while we figure out the rollup situation

…ts, r=estebank Label overwritten for never read assignments Fixes #154434 Closes #144079

chenyukang · 2026-03-31T12:45:53Z

@bors try jobs=x86_64-gnu-llvm-21-3

…ts, r=<try> Label overwritten for never read assignments try-job: x86_64-gnu-llvm-21-3

rust-bors · 2026-03-31T13:09:34Z

💔 Test for 4e89344 failed: CI. Failed job:

try - x86_64-gnu-llvm-21-3 (web logs, enhanced plaintext logs)

chenyukang · 2026-03-31T13:35:21Z

@bors try jobs=x86_64-gnu-llvm-21-3

…ts, r=<try> Label overwritten for never read assignments try-job: x86_64-gnu-llvm-21-3

rust-bors · 2026-03-31T14:00:39Z

💔 Test for 05b552e failed: CI. Failed job:

try - x86_64-gnu-llvm-21-3 (web logs, enhanced plaintext logs)

chenyukang · 2026-03-31T14:20:23Z

@bors try jobs=x86_64-gnu-llvm-21-3

…ts, r=<try> Label overwritten for never read assignments try-job: x86_64-gnu-llvm-21-3

rust-bors · 2026-03-31T16:18:50Z

☀️ Try build successful (CI)
Build commit: f9599e6 (f9599e611f775cab61e208bbcdd9a36c57ca8b66, parent: e4fdb554ad2c0270473181438e338c42b5b30b0c)

JonathanBrouwer · 2026-03-31T21:24:06Z

@bors r=estebank p=0

rust-bors · 2026-03-31T21:24:19Z

📌 Commit ebf22f8 has been approved by estebank

It is now in the queue for this repository.

rust-bors · 2026-04-01T05:47:18Z

☀️ Test successful - CI
Approved by: estebank
Duration: 3h 8m 55s
Pushing 12ab1cf to main...

github-actions · 2026-04-01T05:50:37Z

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 700cfa8 (parent) -> 12ab1cf (this PR)

Test differences

Show 4 test diffs

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

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 12ab1cf1fd3dc925a846b0eb8a4b39393140fdd3 --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

test-various: 1h 40m -> 2h 8m (+27.6%)
optional-x86_64-gnu-parallel-frontend: 2h 12m -> 2h 40m (+21.0%)
pr-check-1: 26m 47s -> 31m 58s (+19.4%)
i686-gnu-1: 1h 56m -> 2h 17m (+18.4%)
dist-apple-various: 1h 53m -> 1h 34m (-17.0%)
x86_64-rust-for-linux: 45m 36s -> 53m 10s (+16.6%)
x86_64-gnu-distcheck: 2h 10m -> 2h 29m (+14.1%)
dist-aarch64-msvc: 1h 45m -> 1h 59m (+13.1%)
dist-aarch64-llvm-mingw: 1h 47m -> 1h 33m (-12.9%)
i686-gnu-nopt-1: 2h 2m -> 2h 17m (+12.2%)

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 · 2026-04-01T06:28:54Z

Finished benchmarking commit (12ab1cf): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary -0.5%, secondary -0.8%)

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

	mean	range	count
Regressions ❌ (primary)	1.7%	[1.1%, 2.4%]	4
Regressions ❌ (secondary)	1.0%	[1.0%, 1.0%]	1
Improvements ✅ (primary)	-3.6%	[-3.9%, -3.3%]	3
Improvements ✅ (secondary)	-1.7%	[-2.0%, -1.4%]	2
All ❌✅ (primary)	-0.5%	[-3.9%, 2.4%]	7

Cycles

Results (primary -3.8%, secondary 2.6%)

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)	2.6%	[2.6%, 2.6%]	1
Improvements ✅ (primary)	-3.8%	[-3.8%, -3.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.8%	[-3.8%, -3.8%]	1

Binary size

Results (primary 0.0%, secondary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	4

Bootstrap: 484.273s -> 482.203s (-0.43%)
Artifact size: 394.91 MiB -> 394.92 MiB (0.00%)

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 27, 2026

rustbot assigned TaKO8Ki Mar 27, 2026