Tracking issue for const char conversions

[est31]

This is a tracking issue for const char conversions. List of functions belonging to !#[feature(const_char_from_u32_unchecked)]:

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;
}

List of functions belonging to stable #![feature(const_char_convert)]:

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>;
}

Steps:

• Introduced by Make char conversion functions unstably const #89258
• Partially stabilized by Stabilize const char convert #102470 , const_char_from_u32_unchecked is still unstable.
• Stabilization is probably blocked by const panicing. Edit: const panic is stable now!
• Stabilization is blocked by Option::unwrap const stability, which is blocked by Tracking issue for #![feature(const_precise_live_drops)] #73255.
• Stabilize const unchecked conversion from u32 to char #126958

[Nugine]

Hi! Is there any plan of stabilization?

[est31]

@Nugine I've made a stabilization PR, let's see where this goes: #102470

[ChrisDenton]

Stabilization is blocked by #67441 const stability, which is blocked by #73255.

Could we not manually write out a match statement instead of using unwrap?

[est31]

I think the idea was that there should be no extra hacks done just to get things compatible with const fn, but I don't remember it fully any more.

[ChrisDenton]

I've opened #118979 to see what T-libs thinks about it.

[ChrisDenton]

#118979 is now merged so I think there's no blocker for from_u32_unchecked being stabilized as const.

[dtolnay]

@rust-lang/libs-api:
@rfcbot fcp merge

I propose stabilizing const for the existing core::char::from_u32_unchecked function (stable since Rust 1.5) and <char>::from_u32_unchecked associated function (stable since Rust 1.52).

These functions were left out of the initial set of feature(const_char_convert) stabilizations in #102470, but have since been unblocked by #118979.

If unsafe { from_u32_unchecked(u) } is called in const with a value for which from_u32(u) returns None, we get the following compile error.

fn main() {
    let _ = const { unsafe { char::from_u32_unchecked(0xd800) } };
}

error[E0080]: it is undefined behavior to use this value
 --> src/main.rs:2:19
  |
2 |     let _ = const { unsafe { char::from_u32_unchecked(0xd800) } };
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered 0x0000d800, but expected a valid unicode scalar value (in `0..=0x10FFFF` but not in `0xD800..=0xDFFF`)
  |
  = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
  = note: the raw bytes of the constant (size: 4, align: 4) {
              00 d8 00 00                                     │ ....
          }

note: erroneous constant encountered
 --> src/main.rs:2:13
  |
2 |     let _ = const { unsafe { char::from_u32_unchecked(0xd800) } };
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[rfcbot]

Team member @dtolnay 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

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

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

[RalfJung]

If unsafe { from_u32_unchecked(u) } is called in const with a value for which from_u32(u) returns None, we get the following compile error.

Note that this is the error you get when the final value of the const is an invalid char.

If you ignore the return value of from_u32_unchecked, there is no error. It's still UB, we just don't catch the UB.

[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.