Preprocessing meta-issue

[pvdrz]

This issue keeps track of all the bindgen issues related to preprocessing directives. The two most common reasons for these issues are the following:

• libclang doesn't have a "good enough" API to implement all the desired features.
• rust-cexpr cannot evaluate certain expressions.

However, this is not an exhaustive list and there might be more issues blocking these.

• Failure to evaluate #define directives if they use values from defines passed through clang args #2394
• Interaction between wrapped statics and redefined function #2420
• Bindgen skips macros with token concatenation. #2402
• Add flag to parse matching #define * into enums instead of constants #2174
• Float constants with exponents are not parsed in macros #1928
• Define macro type aliases / Custom macro parsing in special use cases #1754
• Treat #define similarly to templates when used for types #1044
• Functional C macros are not expanded. #753
• Improve macro defined constants to recognize #define CONSTANT ((int) 1) #316
• #define + sizeof(...) fails to emit constant #666
• macros defining integer constants using expressions containing enum values don't work #258
• bad type recognition, despite explicit suffix #1875
• Error when using unicode character literals #2475
• Avoid assigning (wrong) types to constants using #define #2120
• Macro redefinitions are not being respected #2472
• Support #define old_function new_function #2544
• MSVC C Integer constants are not supported #2600

[pvdrz]

tagging @reitermarkus

[carbotaniuman]

I've finally had the time to iron out bugs and work on my C preprocessor, which is derivative of but largely a rewrite of the earlier Saltwater attempt. #1782

It looks like cexpr appears to handle macro parsing and expression parsing according to the code I see, but the issue obviously means it doesn't. I'd like to trial an attempt to at least integrate the preprocessor, while keeping cexpr for expression parsing. Do you have any pointers for this?

[pvdrz]

@reitermarkus is already working on something similar here #2369.