Add a `write_char` method to `std::fmt::Write`

[SimonSapin]

as accepted in RFC 526.

Note that this brand new method is marked as stable. I judged this safe enough: it’s simple enough that it’s very unlikely to change. Still, I can mark it unstable instead if you prefer.

r? @alexcrichton

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[aturon]

I'm fine with this coming in as #[stable] (as you say, it's very unlikely to change).

That said, I don't think we'll cherry-pick this for 1.0, so can you please put since = "1.1.0"?

[SimonSapin]

Done. Is it still feature = "rust1"?

[aturon]

@bors: r+

[bors]

📌 Commit 16181e6 has been approved by aturon

[Valloric]

No tests? :(

[ncm]

I am kind of out of the loop, here. Is Rust perpetuating C's confusion of octets with characters? Or does write_char write out enough octets to form a code point (which we can, informally, call a character because it's close enough for most purposes)? If it just writes a byte, that seems like a worthy goal, but then the name is wrong.

[SimonSapin]

@ncm: I don’t think there is such a confusion. A char is a Unicode scalar value. The point of std::fmt::Write (as opposed to std::io::Write) is to only accept well-formed Unicode data, not arbitrary bytes.

My goal here is to use this trait not just for formatting, but as a general-purpose Unicode-based stream. The default implementation of write_char is to write up to four bytes with write_str, but it could be the reverse (write_char is fundamental and write_str uses it) in an implementation that’s not based on UTF-8.

[SimonSapin]

@Valloric My bad. #24688 adds a test.