slice_split_once: bounds check optimization note by ArhanChaudhary · Pull Request #158282 · rust-lang/rust

ArhanChaudhary · 2026-06-23T03:56:51Z

Tracking issue: #112811

~~Use unchecked sub-slicing operations for <T>::split_once and <T>::rsplit_once.~~
Note that unchecked sub-slicing operations are equivalent to the current checked sub-slicing operations.

rustbot · 2026-06-23T03:56:56Z

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @jhpratt (or someone else) some time within the next two weeks.

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Owners of files modified in this PR: libs
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Random selection from Darksonn, JohnTitor, Mark-Simulacrum, clarfonthey, jhpratt

jhpratt

It shouldn't be necessary to explicitly state the start and end for these (in either location).

View changes since this review

tgross35 · 2026-06-23T04:57:52Z

        let index = self.iter().position(pred)?;
-        Some((&self[..index], &self[index + 1..]))
+        // SAFETY: `index` is inside `self`
+        let left = unsafe { self.get_unchecked(0..index) };
+        // SAFETY: `index` is inside `self`
+        let right = unsafe { self.get_unchecked(index + 1..self.len()) };
+        Some((left, right))


I just did a quick check: both versions actually produce identical code at -Copt-level=1 https://rust.godbolt.org/z/n8cTdY4rT. So I think you can actually just leave the code as-is to save the unsafe, but instead add a comment saying the bounds check is trivially optimized out. Unless @vmmon happens to remember more about the circumstances of #112811 (comment).

Sorry about the back and forth here!

View changes since the review

(PR also needs a squash and commit message update before merge)

squashing can be done with bors now :)

The comment you linked makes it sound like the compiler requires providing panic machinery because it's (theoretically) being called, which an embedded user might not want to do because of the possibility of it accidentally being used elsewhere. I could be wildly off on that; embedded is not my forte.

squashing can be done with bors now :)

🎉 I keep forgetting we have a new bors in town

The comment you linked makes it sound like the compiler requires providing panic machinery because it's (theoretically) being called, which an embedded user might not want to do because of the possibility of it accidentally being used elsewhere. I could be wildly off on that; embedded is not my forte.

I dug a bit more and looks like that check wasn't optimized out at -O1 around the time that compiler was written, https://rust.godbolt.org/z/n8fqr61bb has a call to slice_start_index_len_fail. It was at -O2 so maybe some optimizations moved around since then?

Up to you on which you prefer. unsafe is obviously future-proof regarding optimizations, but I also wouldn't expect regressions here.

r=me with the current version unless you have a strong preference, I wouldn't expect anyone concerned about binary size to be running with no optimizations

@bors r=tgross35,jhpratt rollup

I think it would be preferable not to rely on the optimizer to do what we want it to. I would expect that someone would have to regularly go in and fact check that comment, while having unsafe guarantees the behavior we want regardless of how things change.

ArhanChaudhary · 2026-06-23T05:09:55Z

@bors squash msg="slice_split_once: bounds check optimization note"

rust-bors · 2026-06-23T05:10:16Z

🔨 5 commits were squashed into 5d07ed5.

rust-bors · 2026-06-23T05:29:04Z

📌 Commit 5d07ed5 has been approved by tgross35,jhpratt

It is now in the queue for this repository.

… r=tgross35,jhpratt slice_split_once: bounds check optimization note Tracking issue: rust-lang#112811 Use unchecked sub-slicing operations for `<T>::split_once` and `<T>::rsplit_once`.

… r=tgross35,jhpratt slice_split_once: bounds check optimization note Tracking issue: rust-lang#112811 ~~Use unchecked sub-slicing operations for `<T>::split_once` and `<T>::rsplit_once`.~~ Note that unchecked sub-slicing operations are equivalent to the current checked sub-slicing operations.

Rollup of 8 pull requests Successful merges: - #157960 (delegation: add support for infers in generics) - #158105 (Extract all instance shim variants into new `ShimKind` enum) - #158207 (Resolver: local/external split of `resolve_ident_in_module_non_globs_unadjusted` ) - #158222 (format: ignore println newline in foreign format hints) - #158252 (Use `cfg_select` in `std::os`) - #158257 ( fix escaping placeholder check in next solver normalization folder) - #158274 (triagebot: Stop pinging myself) - #158282 (slice_split_once: bounds check optimization note)

… r=tgross35,jhpratt slice_split_once: bounds check optimization note Tracking issue: rust-lang#112811 ~~Use unchecked sub-slicing operations for `<T>::split_once` and `<T>::rsplit_once`.~~ Note that unchecked sub-slicing operations are equivalent to the current checked sub-slicing operations.

…uwer Rollup of 16 pull requests Successful merges: - #156885 (Fix misattributed type inference error span for index expressions) - #157271 (simplify some `proc_macro` things) - #157883 (Remove strict invariant node_type on hir_type during ty privacy visit) - #157921 (trait solver: Resolve region vars in max universe) - #157960 (delegation: add support for infers in generics) - #158105 (Extract all instance shim variants into new `ShimKind` enum) - #158207 (Resolver: local/external split of `resolve_ident_in_module_non_globs_unadjusted` ) - #158279 (Follow goto and drop when linting unreachable code) - #157807 (don't ice on non-lifetime binders under `-Zassumptions-on-binders`) - #158020 (Update mingw-w64 C toolchain) - #158222 (format: ignore println newline in foreign format hints) - #158223 (Move target checking for #[lang] to the attribute parser) - #158252 (Use `cfg_select` in `std::os`) - #158257 ( fix escaping placeholder check in next solver normalization folder) - #158274 (triagebot: Stop pinging myself) - #158282 (slice_split_once: bounds check optimization note) Failed merges: - #158256 (Avoid parser panics bubbling out to proc macros)

… r=tgross35,jhpratt slice_split_once: bounds check optimization note Tracking issue: rust-lang#112811 ~~Use unchecked sub-slicing operations for `<T>::split_once` and `<T>::rsplit_once`.~~ Note that unchecked sub-slicing operations are equivalent to the current checked sub-slicing operations.

…uwer Rollup of 23 pull requests Successful merges: - #158315 (`rust-analyzer` subtree update) - #155739 (Add temporary scope to assert_eq and assert_ne) - #156885 (Fix misattributed type inference error span for index expressions) - #157271 (simplify some `proc_macro` things) - #157883 (Remove strict invariant node_type on hir_type during ty privacy visit) - #157921 (trait solver: Resolve region vars in max universe) - #157960 (delegation: add support for infers in generics) - #157983 (Lift the same-signature restriction for `extern "tail"`) - #158105 (Extract all instance shim variants into new `ShimKind` enum) - #158207 (Resolver: local/external split of `resolve_ident_in_module_non_globs_unadjusted` ) - #158279 (Follow goto and drop when linting unreachable code) - #157527 (Move derive tests into their dedicated folder) - #157807 (don't ice on non-lifetime binders under `-Zassumptions-on-binders`) - #158020 (Update mingw-w64 C toolchain) - #158222 (format: ignore println newline in foreign format hints) - #158223 (Move target checking for #[lang] to the attribute parser) - #158252 (Use `cfg_select` in `std::os`) - #158257 ( fix escaping placeholder check in next solver normalization folder) - #158263 (Only load the feature list once in the entire resolver) - #158274 (triagebot: Stop pinging myself) - #158282 (slice_split_once: bounds check optimization note) - #158300 (Improve unknown crate_type diagnostic suggestions) - #158304 (mailmap: update mu001999) Failed merges: - #158256 (Avoid parser panics bubbling out to proc macros)

… r=tgross35,jhpratt slice_split_once: bounds check optimization note Tracking issue: rust-lang#112811 ~~Use unchecked sub-slicing operations for `<T>::split_once` and `<T>::rsplit_once`.~~ Note that unchecked sub-slicing operations are equivalent to the current checked sub-slicing operations.

Rollup of 27 pull requests Successful merges: - #158315 (`rust-analyzer` subtree update) - #155739 (Add temporary scope to assert_eq and assert_ne) - #156885 (Fix misattributed type inference error span for index expressions) - #157271 (simplify some `proc_macro` things) - #157883 (Remove strict invariant node_type on hir_type during ty privacy visit) - #157921 (trait solver: Resolve region vars in max universe) - #157960 (delegation: add support for infers in generics) - #157983 (Lift the same-signature restriction for `extern "tail"`) - #158053 (Optimize network address parser) - #158105 (Extract all instance shim variants into new `ShimKind` enum) - #158207 (Resolver: local/external split of `resolve_ident_in_module_non_globs_unadjusted` ) - #158279 (Follow goto and drop when linting unreachable code) - #157527 (Move derive tests into their dedicated folder) - #157807 (don't ice on non-lifetime binders under `-Zassumptions-on-binders`) - #158020 (Update mingw-w64 C toolchain) - #158039 (c-variadic: test that we use equality up to free lifetimes) - #158222 (format: ignore println newline in foreign format hints) - #158223 (Move target checking for #[lang] to the attribute parser) - #158252 (Use `cfg_select` in `std::os`) - #158257 ( fix escaping placeholder check in next solver normalization folder) - #158263 (Only load the feature list once in the entire resolver) - #158267 (FromUtf8Error::into_utf8_lossy better example and suggest use) - #158274 (triagebot: Stop pinging myself) - #158282 (slice_split_once: bounds check optimization note) - #158300 (Improve unknown crate_type diagnostic suggestions) - #158304 (mailmap: update mu001999) - #158309 (Update `rustc-literal-escaper` version to `0.0.8`) Failed merges: - #158256 (Avoid parser panics bubbling out to proc macros)

… r=tgross35,jhpratt slice_split_once: bounds check optimization note Tracking issue: rust-lang#112811 ~~Use unchecked sub-slicing operations for `<T>::split_once` and `<T>::rsplit_once`.~~ Note that unchecked sub-slicing operations are equivalent to the current checked sub-slicing operations.

rustbot assigned jhpratt Jun 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 Jun 23, 2026

ArhanChaudhary mentioned this pull request Jun 23, 2026

Tracking Issue for slice_split_once #112811

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slice_split_once: bounds check optimization note

5d07ed5

rust-bors Bot force-pushed the unchecked-subslice branch from 879e4c6 to 5d07ed5 Compare June 23, 2026 05:10

ArhanChaudhary changed the title ~~slice_split_once: use unchecked subslicing operations~~ slice_split_once: bounds check optimization note Jun 23, 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 Jun 23, 2026

jhpratt mentioned this pull request Jun 23, 2026

Rollup of 2 pull requests #158287

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Rollup of 8 pull requests #158288

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Rollup of 16 pull requests #158301

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Rollup of 23 pull requests #158316

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Rollup of 23 pull requests #158324

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Rollup of 27 pull requests #158335

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Rollup of 34 pull requests #158337

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Rollup of 35 pull requests #158342

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