Avoid assigning (wrong) types to constants using #define

Input C/C++ Header

#define MYCONST 2

Bindgen Invocation

$ bindgen input.h

Actual Results

/* automatically generated by rust-bindgen 0.59.1 */

pub const MYCONST: u32 = 2;

Expected Results

It would be better to generate something like

macro_rules! MYCONST { () => { 2 }; }

and to expect users to use MYCONST!() instead of MYCONST.

Reasoning

Constants using #define in a C header file do not have a type. They are inserted by the preprocessor as a literal where needed. Translating them to a Rust const with a type (u32, i32, or others) may lead to unexpected results. For example, i32 can be interchaged with std::os::raw::c_int on most platforms. But code that relies on that will not be portable. See discussion on Rust Users Forum "Interfacing C code with bindgen: #define and types".

Migration

Maybe it can be made possible to enable this new described behavior with an option that is not enabled by default; or the generated macro_rules!s could be created in addition to the consts (unless being switched off by an option).

Start with the input.h example and run the bindgen invocation shown to compare the current handling of #define constants with the expected output. Review the constant-generation entry point and decide how an opt-in or default behavior should avoid assigning a fixed Rust type. Done means the migration behavior is settled and the generated result matches the agreed macro_rules! form.