Stabilize const char convert

[est31]

Split out const_char_from_u32_unchecked from const_char_convert and stabilize the rest, i.e. stabilize the following functions:

impl char {
    pub const fn from_u32(self, i: u32) -> Option<char>;
    pub const fn from_digit(self, num: u32, radix: u32) -> Option<char>;
    pub const fn to_digit(self, radix: u32) -> Option<u32>;
}

// Available through core::char and std::char
mod char {
    pub const fn from_u32(i: u32) -> Option<char>;
    pub const fn from_digit(num: u32, radix: u32) -> Option<char>;
}

And put the following under the from_u32_unchecked const stability gate as it needs Option::unwrap which isn't const-stable (yet):

impl char {
    pub const unsafe fn from_u32_unchecked(i: u32) -> char;
}

// Available through core::char and std::char
mod char {
    pub const unsafe fn from_u32_unchecked(i: u32) -> char;
}

cc the tracking issue #89259 (which I'd like to keep open for const_char_from_u32_unchecked).

[rustbot]

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 @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. 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
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[rust-highfive]

r? @scottmcm

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[est31]

I'd like to have an FCP for the stabilization, ideally here because I'd like to re-use the tracking issue. I've opened #89258 12 months ago, and it's been merged 10 months ago, so the feature had plenty of time in nightly.

@rustbot label +T-libs-api -T-libs

[est31]

r? rust-lang/libs-api

[scottmcm]

There's no group for libs-api, so I (got ddg to) flip a coin, and it came up
r? @joshtriplett

[joshtriplett]

Seems reasonable to me!

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @yaahc

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[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[est31]

r? @joshtriplett it's ready for approval now that the FCP has passed :)

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[joshtriplett]

@bors r+ rollup

[bors]

📌 Commit 176c44c has been approved by joshtriplett

It is now in the queue for this repository.