Support char::encode_utf8 in const scenarios.

[bjoernager]

This PR implements rust-lang/rfcs#3696.

This assumes const_slice_from_raw_parts_mut.

[rustbot]

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

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[bjoernager]

Checks complain about missing stability attribute but I'm not sure what version I should point it to.

[clubby789]

#[rustc_const_unstable(feature = "const_char_encode_utf8", issue = "<tracking issue>")] should be used. You should create a tracking issue (like #91850) and use the issue number from that.

[tgross35]

r? libs-api

[bjoernager]

Should len_utf8 not have the must_use attribute? Reading "When to add #[must_use]" would suggest yes.

[clubby789]

Should len_utf8 not have the must_use attribute?

I wouldn't say so - methods like len don't. The attribute should be used when an issue would be caused by the value not being used - e.g. an un-handled error, expecting a value to be mutated when a function is actually pure, an iterator not being run. It's not an error to call a len function and not use the result (probably a mistake but not one that matters)

Also (once your code passes tests) you should squash your commits into one

[dtolnay]

Thank you!

[dtolnay]

@bors r+

[bors]

📌 Commit fb475e4 has been approved by dtolnay

It is now in the queue for this repository.

[bors]

⌛ Testing commit fb475e4 with merge f8192ba...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing f8192ba to master...

[rust-timer]

Finished benchmarking commit (f8192ba): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.3%, 0.4%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.5%, -0.3%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.5%, 0.4%]	5

Max RSS (memory usage)

Results (primary 0.9%, secondary -1.9%)

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)	10.7%	[6.3%, 15.2%]	2
Regressions ❌ (secondary)	1.7%	[1.7%, 1.7%]	1
Improvements ✅ (primary)	-3.0%	[-4.0%, -1.3%]	5
Improvements ✅ (secondary)	-2.8%	[-3.5%, -2.4%]	4
All ❌✅ (primary)	0.9%	[-4.0%, 15.2%]	7

Cycles

Results (secondary 3.9%)

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)	3.9%	[2.1%, 4.6%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%, secondary 0.3%)

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)	0.2%	[0.0%, 0.5%]	21
Regressions ❌ (secondary)	0.3%	[0.0%, 0.5%]	67
Improvements ✅ (primary)	-0.1%	[-0.4%, -0.0%]	30
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.4%, 0.5%]	51

Bootstrap: 767.849s -> 769.07s (0.16%)
Artifact size: 341.34 MiB -> 341.31 MiB (-0.01%)