Replace str::sbuf with *ctypes::c_char

[brson]

sbuf is mostly used for C interop. Interestingly, we define sbuf as *u8 and usually talk about strings as arrays of u8, but C char is a signed value. It probably doesn't make any difference.

[marijnh]

See also #1672

[graydon]

C char is actually a signed-or-unsigned value depending on platform. C int and unsigned are the signed/unsigned pair, likewise short and unsigned short. But char is the weird one. There are three of them: char, unsigned char and signed char. Really:

ISO/IEC 9899:1999, 6.2.5.15 (p. 49)
The three types char, signed char, and unsigned char are collectively called
the character types. The implementation shall define char to have the same range,
representation, and behavior as either signed char or unsigned char.

[graydon]

Needless to say, I think this is such utter nonsense that it has no place re-infecting our type system, and I'm happy to encapsulate the madness in the ctypes module!

[brson]

graydon, I can't tell where you fall on getting rid of sbuf. I now think we have two use cases - treating a string as a byte buffer, and interop with C. So I think we should get rid of str::sbuf, make str::as_buf use a *u8 buffer, and add a new str::as_cstr that does the exact same thing, but typed as a *ctypes::c_char buffer. Also str::from_cstr/from_cstr_len take *c_char and we get two new str::from_buf/from_buf_len functions for *u8.

[marijnh]

Can someone show me any (sane) code where it is important for the signedness of characters in Rust aligns with that of C? It seems that just being able to assume unsignedness (on the Rust side) makes thing easier. You don't want to conditionalize all code that works with >127 char values on some obscure flag.

[brson]

I'm in favor of defining c_char as u8.

[brson]

This is what i intend to do:

1. Define c_char correctly for the platform
2. Eliminate sbuf and replace it with *u8 everywhere
3. Add str::as_c_str that does the same as as_buf but casts to *c_char
4. Convert callers to the new function as appropriate

[brson]

Also, modifying str::from_cstr, etc. to use *c_char and adding companion from_buf methods

[brson]

Done.