c_char on AIX should be u8

[taiki-e]

Currently, core::ffi::c_char on AIX is i8, but AFAIK, the default of this platform is unsigned (u8).

XL C for AIX:

Defaults
-qchars=unsigned

Clang:

$ clang --version
Homebrew clang version 17.0.6

$ clang -E -dM -x c /dev/null -target powerpc64-ibm-aix | grep __CHAR_        
#define __CHAR_BIT__ 8
#define __CHAR_UNSIGNED__ 1

Rust:

$ tail -2 src/lib.rs 

const C: core::ffi::c_char = 0u8;

$ cargo check -Zbuild-std=core --target powerpc64-ibm-aix
error[E0308]: mismatched types
    --> src/lib.rs:3972:30
     |
3972 | const C: core::ffi::c_char = 0u8;
     |                              ^^^ expected `i8`, found `u8`
     |
help: change the type of the numeric literal from `u8` to `i8`
     |
3972 | const C: core::ffi::c_char = 0i8;
     |                               ~~

cc @ecnelises @bzEq (target maintainers)

Meta

rustc --version --verbose:

rustc 1.79.0-nightly (2f090c30d 2024-03-23)
binary: rustc
commit-hash: 2f090c30ddd6b3bbe5c81c087579a5166e7c7278
commit-date: 2024-03-23
host: aarch64-apple-darwin
release: 1.79.0-nightly
LLVM version: 18.1.2

[taiki-e]

I opened rust-lang/libc#3631 and #122986 as drafts to fix this, but I want feedback from the target maintainers before undrafting/merging them.

[taiki-e]

Ideally, we could detect such a problem in CI by checking the Clang's default. However, AFAIK, most tier3 targets are not checked at all in CI at this time (#109099), so that would need to be fixed first.

[bzEq]

Yes, we should follow clang on AIX's behavior.