Stabilize checked slice->str conversion functions

[WaffleLapkin]

This PR stabilizes the following APIs as const functions in Rust 1.63:

// core::str

pub const fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error>;

impl Utf8Error {
    pub const fn valid_up_to(&self) -> usize;
    pub const fn error_len(&self) -> Option<usize>;
}

Note that the from_utf8_mut function is not stabilized as unique references (&mut _) are unstable in const context.

FCP: #91006 (comment)

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-highfive]

r? @thomcc

(rust-highfive has picked a reviewer for you, use r? to override)

[WaffleLapkin]

r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs

[dtolnay]

@bors r+

[bors]

📌 Commit 8929535 has been approved by dtolnay

[bors]

⌛ Testing commit 8929535 with merge 5fb8a39...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 5fb8a39 to master...

[rust-timer]

Finished benchmarking commit (5fb8a39): comparison url.

Instruction count

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

Max RSS (memory usage)

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	4.4%	4.7%	2
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-1.7%	-1.7%	1
All 😿🎉 (primary)	N/A	N/A	0

Cycles

Results

• Primary benchmarks: 😿 relevant regression found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions 😿 (primary)	4.3%	4.3%	1
Regressions 😿 (secondary)	2.6%	2.9%	2
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-4.1%	-4.1%	1
All 😿🎉 (primary)	4.3%	4.3%	1

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. the arithmetic mean of the percent change ↩ ↩²

2. number of relevant changes ↩ ↩²