Incorrect translation of post-modification of bitfield

[hvdijk]

Tested using c2rust 0.16.0

struct S {
  int i : 4;
} s;

int main(void) {
  int x = s.i++;
  int y = --s.i;
  return x + y;
}

This program returns zero. It is translated to a Rust program that returns -1.

The generated Rust code is

#![allow(dead_code, mutable_transmutes, non_camel_case_types, non_snake_case, non_upper_case_globals, unused_assignments, unused_mut)]
#![register_tool(c2rust)]
#![feature(register_tool)]
#[macro_use]
extern crate c2rust_bitfields;
use ::c2rust_out::*;
#[derive(Copy, Clone, BitfieldStruct)]
#[repr(C)]
pub struct S {
    #[bitfield(name = "i", ty = "libc::c_int", bits = "0..=3")]
    pub i: [u8; 1],
    #[bitfield(padding)]
    pub c2rust_padding: [u8; 3],
}
#[no_mangle]
pub static mut s: S = S {
    i: [0; 1],
    c2rust_padding: [0; 3],
};
unsafe fn main_0() -> libc::c_int {
    let ref mut fresh0 = s.i();
    let fresh1 = *fresh0;
    *fresh0 = *fresh0 + 1;
    let mut x: libc::c_int = fresh1;
    s.set_i(s.i() - 1);
    let mut y: libc::c_int = s.i();
    return x + y;
}
pub fn main() {
    unsafe { ::std::process::exit(main_0() as i32) }
}

Note here the *fresh0 = *fresh0 + 1;. This does not have the desired effect of modifying s.i. Instead, it modifies the temporary returned by s.i(), a temporary which is never again used after being assigned to.

The pre-decrement, on the other hand, is correctly translated into a call to s.set_i.

One possible corrected translation would be

unsafe fn main_0() -> libc::c_int {
    let fresh0 = s.i();
    s.set_i(fresh0 + 1);
    let mut x: libc::c_int = fresh0;
    s.set_i(s.i() - 1);
    let mut y: libc::c_int = s.i();
    return x + y;
}