core: respect precision in `ByteStr` `Display` by joboet · Pull Request #153025 · rust-lang/rust

joboet · 2026-02-23T17:56:33Z

ByteStr's Display implementation didn't respect the precision parameter. Just like Formatter::pad, this is fixed by counting off the characters in the string and truncating after the requested length – with the added complication that the ByteStr needs to be divided into chunks first. By including a fast path that avoids counting the characters when no parameters were specified this should also fix the performance regressions caused by #152865.

rustbot · 2026-02-23T17:56:39Z

r? @jhpratt

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Owners of files modified in this PR: @scottmcm, libs
@scottmcm, libs expanded to 8 candidates
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Mark-Simulacrum · 2026-02-24T02:59:46Z

@bors try @rust-timer queue

Just to get ahead of potential perf (shouldn't block merging as a correctness fix).

core: respect precision in `ByteStr` `Display`

rust-bors · 2026-02-24T05:10:49Z

☀️ Try build successful (CI)
Build commit: 2448715 (2448715f97539c847c74001978350d9b0ccc64f1, parent: b3869b94cd1ed4bfa2eb28f301535d5e9599c713)

rust-timer · 2026-02-24T05:50:40Z

Finished benchmarking commit (2448715): comparison URL.

Overall result: ✅ improvements - 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

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
Improvements ✅ (primary)	-0.5%	[-0.8%, -0.3%]	2
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	1
All ❌✅ (primary)	-0.5%	[-0.8%, -0.3%]	2

Max RSS (memory usage)

Results (primary 1.1%, secondary 4.8%)

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

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	4.8%	[4.8%, 4.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Cycles

Results (primary 7.1%)

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

	mean	range	count
Regressions ❌ (primary)	7.1%	[7.1%, 7.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	7.1%	[7.1%, 7.1%]	1

Binary size

Results (primary 0.0%, secondary 0.3%)

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)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	4

Bootstrap: 479.41s -> 478.565s (-0.18%)
Artifact size: 395.68 MiB -> 395.73 MiB (0.01%)

jhpratt · 2026-03-01T21:34:18Z

@rustbot reroll

Mark-Simulacrum · 2026-03-08T21:35:03Z

@bors r+

rust-bors · 2026-03-08T21:35:06Z

📌 Commit fa99d5b has been approved by Mark-Simulacrum

It is now in the queue for this repository.

rust-bors · 2026-03-09T07:27:34Z

☀️ Test successful - CI
Approved by: Mark-Simulacrum
Duration: 3h 13m 52s
Pushing 98e7077 to main...

github-actions · 2026-03-09T07:30:24Z

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 eda4fc7 (parent) -> 98e7077 (this PR)

Test differences

Show 3 test diffs

3 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 98e7077b903559d7a4fafb775cd5292cc9427b67 --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-aarch64-apple: 3h 24m -> 1h 53m (-44.4%)
x86_64-rust-for-linux: 53m 7s -> 42m 34s (-19.9%)
dist-apple-various: 1h 49m -> 2h 10m (+19.2%)
x86_64-gnu-miri: 1h 33m -> 1h 17m (-16.3%)
pr-check-1: 33m 12s -> 27m 51s (-16.1%)
i686-gnu-nopt-1: 2h 21m -> 2h 5m (-11.3%)
i686-gnu-2: 1h 40m -> 1h 29m (-10.7%)
dist-powerpc64le-linux-musl: 1h 43m -> 1h 33m (-9.9%)
x86_64-gnu: 2h 30m -> 2h 16m (-9.6%)
x86_64-gnu-llvm-20: 1h 19m -> 1h 11m (-9.3%)

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 · 2026-03-09T08:08:38Z

Finished benchmarking commit (98e7077): comparison URL.

Overall result: ✅ 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
Improvements ✅ (primary)	-0.4%	[-0.8%, -0.2%]	4
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	1
All ❌✅ (primary)	-0.4%	[-0.8%, -0.2%]	4

Max RSS (memory usage)

Results (secondary 1.1%)

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)	1.1%	[1.1%, 1.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.3%, secondary 2.7%)

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)	2.7%	[2.3%, 3.1%]	2
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.3%	[-2.3%, -2.3%]	1

Binary size

Results (primary 0.0%, secondary 0.3%)

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)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	4

Bootstrap: 479.713s -> 479.865s (0.03%)
Artifact size: 396.94 MiB -> 395.05 MiB (-0.48%)

core: respect precision in ByteStr Display

fa99d5b

rustbot assigned jhpratt Feb 23, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Feb 23, 2026

This was referenced Feb 23, 2026

std::path::Display Nightly regression with precision #153022

Closed

core: optimize Display for ByteStr #153021

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

Auto merge of #153025 - joboet:bytestr_precision_display, r=<try>

2448715

core: respect precision in `ByteStr` `Display`

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

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

rustbot assigned Mark-Simulacrum and unassigned jhpratt Mar 1, 2026

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 Mar 8, 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 Mar 9, 2026

rust-bors bot merged commit 98e7077 into rust-lang:main Mar 9, 2026
13 checks passed

rustbot added this to the 1.96.0 milestone Mar 9, 2026

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rust-timer commented Feb 24, 2026

Overall result: ✅ improvements - no action needed

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Test differences

Job duration changes

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rust-timer commented Mar 9, 2026

Overall result: ✅ improvements - no action needed

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joboet commented Feb 23, 2026 •

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