Don't panic in ceil_char_boundary

[clarfonthey]

Implementing the alternative mentioned in this comment: #93743 (comment)

Since floor_char_boundary will always work (rounding down to the length of the string is possible), it feels best for ceil_char_boundary to not panic either. However, the semantics of "rounding up" past the length of the string aren't very great, which is why the method originally panicked in these cases.

Taking into account how people are using this method, it feels best to simply return the end of the string in these cases, so that the result is still a valid char boundary.

[rustbot]

r? @cuviper

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[cuviper]

I think this needs to be an API question, even though it's not stable yet. It seems surprising to me that it would return something less than index when it is stated up front, "not below index", so panicking is the only choice. But I also kind of think that floor should not paper over out-of-bounds index either.

@bors label -T-libs +T-libs-api
r? libs-api

[clarfonthey]

For a more direct link to what I was searching: https://github.com/search?utf8=%E2%9C%93&q=ceil_char_boundary+language%3ARust&type=code&l=Rust

Essentially, I saw a few uses of ceil_char_boundary(index + 1) or something to that effect, and while I didn't scrutinise the exact usages to make sure they were using it properly, that alongside the combination of people asking for a non-panicking version made me feel that it would make the most sense for this to not panic.

My (flimsy) logic is to think of it like climbing a ladder, and if you climb past the last rung on the ladder you just stay at the last rung.

[m-ou-se]

We discussed this in the libs-api meeting last week. While we felt it was a bit odd for this method to return a value lower than its input, we agree that one of its main use cases is basically to sanitize input to e.g. s.split_at(x), s[..x], etc. The guarantee that any return value from floor_char_boundary and ceil_char_boundary is always a valid index to give to split_at is a very useful one.

Since this is still unstable, this PR doesn't require an FCP. (The behavior of this method will be part of the FCP that will happen when stabilizing it.)

@bors r+

[bors]

📌 Commit 2f75dd4 has been approved by m-ou-se

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2f75dd4 with merge 4f4dae0...

[bors]

☀️ Test successful - checks-actions
Approved by: m-ou-se
Pushing 4f4dae0 to master...

[rust-timer]

Finished benchmarking commit (4f4dae0): 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)

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)	2.5%	[2.1%, 2.8%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[2.1%, 2.8%]	3

Cycles

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

Binary size

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

Bootstrap: 633.856s -> 632.472s (-0.22%)
Artifact size: 346.73 MiB -> 346.76 MiB (0.01%)