`--clang-macro-fallback` interaction with `-Wp,-MMD,file.d`

[ojeda]

--clang-macro-fallback, when asking the preprocessor to generate a dependencies file, changes behavior.

For instance, given a header that would benefit from --clang-macro-fallback, such as:

#define F(n) n
#define N F(4)

Then

bindgen x.h -- -Wp,-MMD,file.d

would generate a dependencies file like:

x.o: x.h

However, if we pass --clang-macro-fallback, e.g.

bindgen x.h --clang-macro-fallback -- -Wp,-MMD,file.d

then it generates something like:

macro_eval.o: ...abspath.../x-precompile.h ...abspath.../x.h .macro_eval.c

This was reduced from trying to use the --clang-macro-fallback feature in the Linux kernel (mailing list post including a kernel patch to test/debug the feature).

A possible workaround may be to run bindgen twice.

Cc @jbaublitz

[jbaublitz]

@ojeda Please see the linked PR that fixes this issue.

@pvdrz When you initially recommend passing all of the CFLAGS to the FallbackTranslationUnit, this is kind of what I was afraid of. It seems like we're going to have to keep an eye on this and either take an allow or deny policy with CFLAGS. Either we have to filter out the ones that are desirable or only pass forward ones we know are safe.