Add substring API for OsStr

[blyxxyz]

This adds a method for taking a substring of an OsStr, which in combination with OsStr::as_encoded_bytes() makes it possible to implement most string operations in safe code.

API:

impl OsStr {
    pub fn slice_encoded_bytes<R: ops::RangeBounds<usize>>(&self, range: R) -> &Self;
}

Motivation, examples and research at rust-lang/libs-team#306.

Tracking issue: #118485

cc @epage
r? libs-api

[epage]

Open question: Currently this enforces the same requirements on all platforms, but on Unix (and some other platforms) the internal encoding of OsStr is already fully specified to be arbitrary bytes by means of the OsStrExt trait. Should we:

Should we defer this discussion to the tracking issue?

[BurntSushi]

LGTM. r=me after adding some messages to the asserts. Maybe copy what we do for str slice panicking?

[blyxxyz]

str gives messages like byte index 1 is not a char boundary; it is inside '∞' (bytes 0..3) of "∞". It's difficult to be that helpful when we don't have any known-valid context, so I made it byte index {index} is not an OsStr boundary.

I've started work on an implementation with platform logic but maybe that could become a followup PR.

[BurntSushi]

I buy it. @bors r+

[bors]

📌 Commit 729851e has been approved by BurntSushi

It is now in the queue for this repository.

[bors]

⌛ Testing commit 729851e with merge d5fab33...

[bors]

☀️ Test successful - checks-actions
Approved by: BurntSushi
Pushing d5fab33 to master...

[rust-timer]

Finished benchmarking commit (d5fab33): 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)

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

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-3.9%, -3.6%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 672.666s -> 671.733s (-0.14%)
Artifact size: 313.96 MiB -> 313.93 MiB (-0.01%)