const struct member becomes mut if type not defined

Input C/C++ Header

// Comment out this to turn *mut b -> *const b in generated Rust code.
#define BUG
#if !defined(BUG)
struct b;
#endif
struct a {
	const struct b *x;
};

Bindgen Invocation

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

Actual Results

/* automatically generated by rust-bindgen */

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct a {
    pub x: *mut b,
}
#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct b {
    pub _address: u8,
}

Expected Results

The struct member becomes const/mut depending on if it's defined or not.

Start by running bindgen input.h --no-layout-tests with the provided header, both with BUG defined and with struct b declared. Compare the generated Rust pointer mutability; the fix is complete when the member is const or mut according to whether struct b is defined, without changing the other generated output.