Macro redefinitions are not being respected

Input C/C++ Header

#define FOO 4
#define BAR (1 + FOO)
#undef FOO
#define FOO 5
#define BAZ (1 + FOO)

Bindgen Invocation

$ bindgen input.h

Actual Results

pub const FOO: u32 = 4;
pub const BAR: u32 = 5;
pub const BAZ: u32 = 6;

Expected Results

I think the actual results are inconsistent because BAZ was evaluated with the redefinition of FOO but FOO itself was not. For the sake of consistency I'd say that the expected behavior should be one of the following:

• The values of FOO, BAR and BAZ are computed using the first definition of FOO. Meaning that the second definition of FOO is ignored.

pub const FOO: u32 = 4;
pub const BAR: u32 = 5;
pub const BAZ: u32 = 5;

• Or even better, the value of BAR is computed the first definition of FOO and the values of FOO and BAZ are computed using the second definition of FOO.

pub const BAR: u32 = 5;
pub const FOO: u32 = 5;
pub const BAZ: u32 = 6;

• Alternatively, all the macros could be "expanded/evaluated" after reading all the file. Meaning that all the values are computed with the last definition of FOO:

pub const BAR: u32 = 6;
pub const FOO: u32 = 5;
pub const BAZ: u32 = 6;

I'd say that there is no "right" answer but I'd prefer the second or third option.

Reproduce the case with input.h and the bindgen input.h invocation, then compare the generated FOO, BAR, and BAZ values with the three proposed outcomes. Review merged pull request #2543 before starting, since it is linked to this report. Done means the project has an agreed macro-redefinition behavior and the generated Rust constants follow it.