std: optimise IO error formatting by joboet · Pull Request #161945 · rust-lang/rust

joboet · 2026-08-28T17:48:01Z

The current OS error formatting logic goes through a bit of trouble (the format! macro, temporary allocations and unnecessary copying) to create a String for Formatter::write_str. It's more efficient and arguably simpler to have the formatting logic write into the Formatter directly instead.

rustbot · 2026-08-28T17:48:06Z

r? @Mark-Simulacrum

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hanna-kruppe · 2026-08-28T18:35:02Z

It's not obvious to me that this is meaningfully more efficient on the most important targets, nor that formatting performance matters, but:

I didn't see anything that gives me reason to suspect something will get slower.
Avoiding (all or some) temporary allocations is certainly neat.
I agree it's conceptually more appropriate to write this as formatting code rather than string-producing code.

r? me @bors r+ rollup=iffy (lots of tier 2-3 targets touched, may want more try runs for those not tested in PR CI)

Let's also see if perf has anything to say about this on Linux: @bors try @rust-timer queue

rust-bors · 2026-08-28T18:35:05Z

📌 Commit d20d512 has been approved by hanna-kruppe

It is now in the queue for this repository.

std: optimise IO error formatting

std: optimise IO error formatting The current OS error formatting logic goes through a bit of trouble (the `format!` macro, temporary allocations and unnecessary copying) to create a `String` for `Formatter::write_str`. It's more efficient and arguably simpler to have the formatting logic write into the `Formatter` directly instead.

rust-bors · 2026-08-28T19:40:11Z

☀️ Try build successful (CI)
Build commit: fb25bc1 (fb25bc125ff5e59181244e62cf702d54534fb687)
Base parent: 17fd5b8 (17fd5b8a37b6667b6cc137f3cc35f09759768a3b)

rust-timer · 2026-08-28T20:48:14Z

Finished benchmarking commit (fb25bc1): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	1

Max RSS (memory usage)

Results (primary 6.6%, secondary 1.6%)

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

	mean	range	count
Regressions ❌ (primary)	6.6%	[6.6%, 6.6%]	1
Regressions ❌ (secondary)	6.5%	[6.5%, 6.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	6.6%	[6.6%, 6.6%]	1

Cycles

This perf run didn't have relevant results for this metric.

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

Bootstrap: 474.378s -> 479.281s (1.03%)
Artifact size: 402.85 MiB -> 402.92 MiB (0.02%)

…uwer Rollup of 16 pull requests Successful merges: - #160170 (Refactored docs for `std::fs::set_permissions_nofollow` + fix BSD-based systems to use fchmodat with AT_SYMLINK_NOFOLLOW flag) - #161945 (std: optimise IO error formatting) - #160594 (attach global target features to module-level assembly) - #161577 (implement [u8]::split_ascii_whitespace) - #161858 (fix ICE in generic_const_parameter_types with inherents) - #161377 ([bootstrap] Don't reverse the order of dylib search path entries) - #161804 (Document PartialOrd behavior for Option<T> where T: PartialOrd) - #161865 (loongarch: support passing `u128`/`i128` to inline assembly) - #161877 (Do not load macro metadata for local definitions in rustdoc) - #161880 (fix rustc_lint_defs doctest issues) - #161883 (better deal with internal features being injected into doctests) - #161887 (std: uefi: fix File::seek returning the EOF sentinel) - #161897 (Reject contract attributes without arguments) - #161909 (Report the configured Polonius default in -Z help) - #161910 (Add rustdoc-html regression test for generated macro) - #161914 (Retroactively add relnotes for `bool::{ok_or,ok_or_else}` (1.98.0))

std: optimise IO error formatting The current OS error formatting logic goes through a bit of trouble (the `format!` macro, temporary allocations and unnecessary copying) to create a `String` for `Formatter::write_str`. It's more efficient and arguably simpler to have the formatting logic write into the `Formatter` directly instead.

…uwer Rollup of 16 pull requests Successful merges: - #161945 (std: optimise IO error formatting) - #160594 (attach global target features to module-level assembly) - #161577 (implement [u8]::split_ascii_whitespace) - #161858 (fix ICE in generic_const_parameter_types with inherents) - #161377 ([bootstrap] Don't reverse the order of dylib search path entries) - #161702 (Use `drop_guard` in some places in {core,alloc,std}) - #161804 (Document PartialOrd behavior for Option<T> where T: PartialOrd) - #161865 (loongarch: support passing `u128`/`i128` to inline assembly) - #161877 (Do not load macro metadata for local definitions in rustdoc) - #161880 (fix rustc_lint_defs doctest issues) - #161883 (better deal with internal features being injected into doctests) - #161887 (std: uefi: fix File::seek returning the EOF sentinel) - #161897 (Reject contract attributes without arguments) - #161909 (Report the configured Polonius default in -Z help) - #161910 (Add rustdoc-html regression test for generated macro) - #161914 (Retroactively add relnotes for `bool::{ok_or,ok_or_else}` (1.98.0))

rust-bors · 2026-08-29T01:35:39Z

⌛ Testing commit d20d512 with merge db9e342...

Workflow: https://github.com/rust-lang/rust/actions/runs/33226819888

std: optimise IO error formatting The current OS error formatting logic goes through a bit of trouble (the `format!` macro, temporary allocations and unnecessary copying) to create a `String` for `Formatter::write_str`. It's more efficient and arguably simpler to have the formatting logic write into the `Formatter` directly instead.

Zalathar · 2026-08-29T01:51:18Z

@bors yield (to enclosing rollup)

rust-bors · 2026-08-29T01:51:21Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/33226819888

The next pull request likely to be tested is #161955.

Rollup of 17 pull requests Successful merges: - #161945 (std: optimise IO error formatting) - #160594 (attach global target features to module-level assembly) - #161577 (implement [u8]::split_ascii_whitespace) - #161858 (fix ICE in generic_const_parameter_types with inherents) - #161377 ([bootstrap] Don't reverse the order of dylib search path entries) - #161804 (Document PartialOrd behavior for Option<T> where T: PartialOrd) - #161865 (loongarch: support passing `u128`/`i128` to inline assembly) - #161877 (Do not load macro metadata for local definitions in rustdoc) - #161880 (fix rustc_lint_defs doctest issues) - #161883 (better deal with internal features being injected into doctests) - #161887 (std: uefi: fix File::seek returning the EOF sentinel) - #161897 (Reject contract attributes without arguments) - #161909 (Report the configured Polonius default in -Z help) - #161910 (Add rustdoc-html regression test for generated macro) - #161914 (Retroactively add relnotes for `bool::{ok_or,ok_or_else}` (1.98.0)) - #161924 (Windows: document that `normalize_lexically` converts `/` to `\`) - #161927 (Change `rustc_middle/src/hooks/mod.rs` to `hooks.rs`)

rust-bors · 2026-08-29T02:57:33Z

⌛ Testing commit d20d512 with merge 7891f5c...

Workflow: https://github.com/rust-lang/rust/actions/runs/33230199594

std: optimise IO error formatting The current OS error formatting logic goes through a bit of trouble (the `format!` macro, temporary allocations and unnecessary copying) to create a `String` for `Formatter::write_str`. It's more efficient and arguably simpler to have the formatting logic write into the `Formatter` directly instead.

Zalathar · 2026-08-29T02:58:26Z

@bors yield (rollup again)

rust-bors · 2026-08-29T02:58:35Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/33230199594

The next pull request likely to be tested is #161955.

Rollup of 17 pull requests Successful merges: - #161945 (std: optimise IO error formatting) - #160594 (attach global target features to module-level assembly) - #161577 (implement [u8]::split_ascii_whitespace) - #161858 (fix ICE in generic_const_parameter_types with inherents) - #161377 ([bootstrap] Don't reverse the order of dylib search path entries) - #161804 (Document PartialOrd behavior for Option<T> where T: PartialOrd) - #161865 (loongarch: support passing `u128`/`i128` to inline assembly) - #161877 (Do not load macro metadata for local definitions in rustdoc) - #161880 (fix rustc_lint_defs doctest issues) - #161883 (better deal with internal features being injected into doctests) - #161887 (std: uefi: fix File::seek returning the EOF sentinel) - #161897 (Reject contract attributes without arguments) - #161909 (Report the configured Polonius default in -Z help) - #161910 (Add rustdoc-html regression test for generated macro) - #161914 (Retroactively add relnotes for `bool::{ok_or,ok_or_else}` (1.98.0)) - #161924 (Windows: document that `normalize_lexically` converts `/` to `\`) - #161927 (Change `rustc_middle/src/hooks/mod.rs` to `hooks.rs`)

rust-bors · 2026-08-29T05:00:48Z

⌛ Testing commit d20d512 with merge ab5a24f...

Workflow: https://github.com/rust-lang/rust/actions/runs/33235152627

std: optimise IO error formatting The current OS error formatting logic goes through a bit of trouble (the `format!` macro, temporary allocations and unnecessary copying) to create a `String` for `Formatter::write_str`. It's more efficient and arguably simpler to have the formatting logic write into the `Formatter` directly instead.

jhpratt · 2026-08-29T05:19:20Z

yielding to rollup

@bors yield

rust-bors · 2026-08-29T05:19:23Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/33235152627

The next pull request likely to be tested is #161959.

Rollup of 17 pull requests Successful merges: - #161945 (std: optimise IO error formatting) - #160594 (attach global target features to module-level assembly) - #161577 (implement [u8]::split_ascii_whitespace) - #161858 (fix ICE in generic_const_parameter_types with inherents) - #161377 ([bootstrap] Don't reverse the order of dylib search path entries) - #161804 (Document PartialOrd behavior for Option<T> where T: PartialOrd) - #161865 (loongarch: support passing `u128`/`i128` to inline assembly) - #161877 (Do not load macro metadata for local definitions in rustdoc) - #161880 (fix rustc_lint_defs doctest issues) - #161883 (better deal with internal features being injected into doctests) - #161887 (std: uefi: fix File::seek returning the EOF sentinel) - #161888 (compiler: Allow safestack to be togglable via #[sanitize(safestack = "...")]) - #161897 (Reject contract attributes without arguments) - #161910 (Add rustdoc-html regression test for generated macro) - #161914 (Retroactively add relnotes for `bool::{ok_or,ok_or_else}` (1.98.0)) - #161924 (Windows: document that `normalize_lexically` converts `/` to `\`) - #161927 (Change `rustc_middle/src/hooks/mod.rs` to `hooks.rs`)

Rollup merge of #161945 - joboet:format_error, r=hanna-kruppe std: optimise IO error formatting The current OS error formatting logic goes through a bit of trouble (the `format!` macro, temporary allocations and unnecessary copying) to create a `String` for `Formatter::write_str`. It's more efficient and arguably simpler to have the formatting logic write into the `Formatter` directly instead.

std: optimize IO error formatting

d20d512

rustbot assigned Mark-Simulacrum Aug 28, 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 Aug 28, 2026

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rust-bors Bot added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Aug 28, 2026

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rust-bors Bot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 28, 2026

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 28, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 28, 2026

Auto merge of #161945 - joboet:format_error, r=<try>

fb25bc1

std: optimise IO error formatting

JonathanBrouwer mentioned this pull request Aug 28, 2026

Rollup of 16 pull requests #161949

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 28, 2026

JonathanBrouwer mentioned this pull request Aug 28, 2026

Rollup of 16 pull requests #161952

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Zalathar mentioned this pull request Aug 29, 2026

Rollup of 17 pull requests #161955

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jhpratt mentioned this pull request Aug 29, 2026

Rollup of 17 pull requests #161959

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rust-bors Bot merged commit af94503 into rust-lang:main Aug 29, 2026
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rustbot added this to the 1.100.0 milestone Aug 29, 2026

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rust-timer commented Aug 28, 2026

Overall result: ❌ regressions - no action needed

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