Optimize indexing slices and strs with inclusive ranges

[Kmeakin]

Instead of separately checking for end == usize::MAX and end + 1 > slice.len(), we can check for end >= slice.len(). Also consolidate all the str indexing related panic functions into a single function which reports the correct error depending on the arguments, as the slice indexing code already does.

The downside of all this is that the panic message is slightly less specific when trying to index with [..=usize::MAX]: instead of saying "attempted to index str up to maximum usize" it just says "end byte index {end} out of bounds". But this is a rare enough case that I think it is acceptable

[rustbot]

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[scottmcm]

As you saw from the PR build failure, if you're touching things like slice_index_order_fail you'll need to look through every mention of those in the repo -- there are a bunch of codegen tests that look for their absence, and you need to make sure that you don't make those tests useless by renaming what they're looking for.

(You probably also need to re-bless some MIR tests that include indexing.)

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[oli-obk]

@bors try @rust-timer queue

[rust-bors]

⌛ Trying commit bd63d02 with merge 4249615…

To cancel the try build, run the command @bors try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: ddd9a12 (ddd9a12e711d4b329a39072cd3ed4cf138a43c4c, parent: b3cda168c8afd5c4240a9477f6a7f54e70e2589a)

[rust-timer]

Finished benchmarking commit (ddd9a12): comparison URL.

Overall result: ❌ regressions - 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)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

Results (primary 9.4%, secondary 5.2%)

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

	mean	range	count
Regressions ❌ (primary)	9.4%	[2.4%, 16.9%]	12
Regressions ❌ (secondary)	5.2%	[4.3%, 6.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	9.4%	[2.4%, 16.9%]	12

Binary size

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

Bootstrap: 472.506s -> 472.489s (-0.00%)
Artifact size: 383.60 MiB -> 383.59 MiB (-0.00%)

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[Kmeakin]

@rustbot ready

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

fmt: checked 6702 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [style (tests)]: /checkout/tests/codegen-llvm/str-range-indexing.rs:24: line longer than 100 chars
tidy [style (tests)]: /checkout/tests/codegen-llvm/str-range-indexing.rs:25: line longer than 100 chars
tidy [style (tests)]: /checkout/tests/codegen-llvm/str-range-indexing.rs:55: line longer than 100 chars
tidy [style (tests)]: /checkout/tests/codegen-llvm/str-range-indexing.rs:83: line longer than 100 chars
tidy [style (tests)]: FAIL
tidy: The following check failed: style (tests)
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/checkout --cargo-path=/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/checkout/obj/build --concurrency=4 --npm-path=yarn` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1365:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:00:49
  local time: Thu Feb  5 02:40:36 UTC 2026
  network time: Thu, 05 Feb 2026 02:40:36 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"