ACP: impl TryFrom<char> for u16

[lukas-code]

Proposal

Problem statement

Implement TryFrom<char> for u16, similar to the existing TryFrom<char> for u8.

Motivation, use-cases

I have project, which (unfortunately) has to deal with UCS-2 encoded strings (UCS-2 is like UTF-16, but no surrogates). I am currently in the process of replacing as casts with safer alternatives and found that this impl was missing.

One example of using this feature for collecting an iterator of chars into a preallocated UCS-2 string: (playground)

use core::mem::MaybeUninit;
use core::slice;

#[derive(Debug)]
struct Error;

fn collect_ucs2(iter: impl IntoIterator<Item = char>, buffer: &mut [MaybeUninit<u16>]) -> Result<&[u16], Error> {
    let mut index = 0;
    for ch in iter {
        if index >= buffer.len() {
            return Err(Error);
        }
        let ucs2 = ch.try_into().map_err(|_| Error)?; // <- doesn't work :(
        buffer[index] = MaybeUninit::new(ucs2);
        index += 1;
    }
    Ok(unsafe { slice::from_raw_parts(buffer.as_ptr().cast::<u16>(), index) })
}

fn main() {
    let mut buffer = [MaybeUninit::uninit(); 16];
    println!("{:X?}", collect_ucs2("hello".chars(), &mut buffer));
}

Solution sketches

Copy-paste the impl from u8, but replace u8 with u16:

impl TryFrom<char> for u16 {
    type Error = TryFromCharError;

    #[inline]
    fn try_from(c: char) -> Result<u16, Self::Error> {
        u16::try_from(u32::from(c)).map_err(|_| TryFromCharError(()))
    }
}

Links and related work

• impl TryFrom<char> for u8: Implement TryFrom<char> for u8 rust#84640

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.

[scottmcm]

My instinct is that this is too uncommon and situational to be a trait impl. I don't know that we want anything in traits that's UCS-2, since I don't think we even have UTF-16 in traits.

Overall, it just doesn't seem so bad to do

-let ucs2 = ch.try_into().map_err(|_| Error)?;
+let ucs2 = u32::from(ch).try_into().map_err(|_| Error)?;

so it's hard to pass the "we can't experiment with it because it's insta-stable" bar.

Is there a version of this that could be a method instead? The bar for fn encode_ucs2(self) -> Option<u16> or similar is much lower, though I'm still unsure whether it's something we want. And having the encoding in the name emphasizes that most people probably don't want it.

[m-ou-se]

We discussed this in the libs-api meeting just now. There was some discussion on whether this would wrongly be used in situations where utf-16 should be used instead, but we mostly felt positive about the conversion. We didn't reach full consensus yet, but this change would be insta-stable (since it's a trait implementation), so can you send a PR to rust-lang/rust for this? Then we can propose an FCP and see if we can get consensus on it. Thanks!