Signedness mismatch between core::ffi::c_char and libc::c_char

[jothan]

I tried this code:

unsafe { libc::printf(c"%s".as_ptr(), line.as_ptr()) };

I expected to be able to compile the above code, but the compiler had a different opinion:

error[E0308]: mismatched types
  --> src/logging.rs:90:27
   |
90 |     unsafe { libc::printf(c"%s".as_ptr(), line.as_ptr()) };
   |              ------------ ^^^^^^^^^^^^^^ expected `*const u8`, found `*const i8`
   |              |
   |              arguments to this function are incorrect
   |
   = note: expected raw pointer `*const u8`
              found raw pointer `*const i8`
note: function defined here
  --> index.crates.io-6f17d22bba15001f/libc-0.2.169/src/unix/mod.rs:694:12

It would seem that the latest libc 0.2.169 flipped its signedness of c_char from signed to unsigned for ESP-IDF:
rust-lang/libc@c66faeb

rustc should probably match this signedness change on ESP-IDF and update the definition somewhere around here: https://doc.rust-lang.org/beta/src/core/ffi/mod.rs.html#52

Doing a bit of searching around, I am about 80% sure that char is unsigned on the C side on ESP-IDF.

Meta

rustc --version --verbose:

rustc 1.83.0-nightly (103cde010 2024-11-26) (1.83.0.0)
binary: rustc
commit-hash: 103cde01096863220765fd2e54519d011a5740ac
commit-date: 2024-11-26
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 18.1.2