Layout of char

[gnzlbg]

Currently we only say that char has size 4. Is there anything we can say about its alignment beyond "implementation-defined" ?

Also @ubsan mentioned:

I'd argue it'd be useful to be ABI compatible with char32_t) (note: that type only exists in C++).

People have argued that they shouldn't be ABI compatible, since char32_t doesn't have the correctness guarantees Rust's char does; I would argue that it's the same idea as C-like enums in Rust vs enums in C++.

This question is still unresolved.

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We probably also want to mention whether on all currently supported platforms the ABI of char is INTEGER or AGGREGATE.

[joshtriplett]

An interesting thought that came up today: there's an alternative representation that might make sense. Since any valid Unicode character can be represented as at most 4 bytes of UTF-8, we could represent char as a [u8; 4] containing UTF-8.

That would have the interesting property of allowing a function on char to return a &str without allocating memory (rather than the current char::encode_utf8). And &str or String could return a char much more quickly, without having to translate UTF-8 first.

This would have tradeoffs, as well. In exchange for fast character -> UTF-8 and UTF-8 string -> character operations, we'd have to change the implementations of functions that do Unicode character classification and similar, to either convert to a character number first, or provide an optimized mapping from UTF-8 directly to the needed classification (which may be possible in some cases).

I don't know if we'd want to consider this tradeoff, but it seems worth considering. If code can't currently assume the representation is UCS-4 (e.g. transmuting to/from u32), then we can consider such an alternative representation.

[Diggsey]

@joshtriplett that sounds like a useful type, but IMO the extra complexity combined with the extra cost of converting to the corresponding scalar make it not worthwhile.

However, it might be a cool idea as the basis for a "fixed size string" type similar to fixed-size arrays. For example with const generics: FixedStr<1> could be equivalent to the type you are describing and have From/To implementations for char, whilst also being able to be borrowed as a &str.