Inconsistent representation of char (constants ignore type)

On my platform char is signed, thus trying to assign a char-string constant (which becomes &[u8, _] in Rust) to a char * field in a struct results in a mismatched types error.

This one was a little surprising, but is understandable given C's fuzzyness around what a char is.
I could understand closing this as WONTFIX due to the likely breakage such a change would cause (as well as the annoyance of working with std::os::raw::c_char), but would at least be useful to add to the FAQ.

Alternatively there could be some sort of flag to bindgen control this behaviour.

Input C/C++ Header

struct my_struct {
    const char *str;
    const unsigned char *ustr;
    const signed char *sstr;
};

static const char STR[] = "Hi!";
static const signed char USTR[] = "Hi!";
static const unsigned char SSTR[] = "Hi!";

Bindgen Invocation

$ bindgen input.h --no-layout-tests

Actual Results

/* automatically generated by rust-bindgen */

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct my_struct {
    pub str: *const ::std::os::raw::c_char,
    pub ustr: *const ::std::os::raw::c_uchar,
    pub sstr: *const ::std::os::raw::c_schar,
}
pub const STR: &'static [u8; 4usize] = b"Hi!\0";
pub const USTR: &'static [u8; 4usize] = b"Hi!\0";
pub const SSTR: &'static [u8; 4usize] = b"Hi!\0";

Expected Results

This one kinda sucks because char can be either signed or unsigned (and technically doesn't even have to be 8-bits). Here's a version that strictly preserves the types:

/* automatically generated by rust-bindgen */

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct my_struct {
    pub str: *const ::std::os::raw::c_char,
    pub ustr: *const ::std::os::raw::c_uchar,
    pub sstr: *const ::std::os::raw::c_schar,
}
const STR: &'static [::std::os::raw::c_char; 4] = &[72, 105, 33, 0];
const USTR: &'static [::std::os::raw::c_uchar; 4] = &[72, 105, 33, 0];
const SSTR: &'static [::std::os::raw::c_schar; 4] = &[72, 105, 33, 0];

Start by reproducing the issue with input.h and bindgen input.h --no-layout-tests, then inspect how the generated constants represent char, signed char, and unsigned char. Compare the output with the type-preserving expected results. Done means either preserving the declared character types or documenting the limitation in the FAQ, with any proposed option clearly defined.