Add flag for Makefile dependency rules generation

[robertwb]

Add flag that will make cython output a rule suitable for make describing the dependencies of the main source file.

like this:

$ cython -M foo.pxd

foo.c: bar.pxd

Migrated from http://trac.cython.org/ticket/655

[robertwb]

scoder changed component from Code Generation to Build System
commented

[robertwb]

scoder changed priority from major to minor
commented

[gerion0]

This would be useful for meson, too. For example, the pxd file is not tracked in this test case.

[scoder]

Since the cython tool does not deal with dependencies (it's a one-shot command), a better place to implement this seems to be the cythonize frontend.

[dcbaker]

cython still needs to deal with C headers though, correct? and the pxd file as was pointed out above.

[scoder]

Cython's dependency collection in Cython.Build.Dependencies does not look at transitive header files but only those that are mentioned directly via cdef extern from clauses. It wouldn't be able to decide which of them will be included in a platform/configuration specific way by the C preprocessor anyway.

It transitively collects .pxd, and .pxi files, as well as include files from cdef extern from sections (.h, .c, whatever people use there).

I'm pretty sure it also currently does not look into verbatim C code sections (docstrings of cdef extern from), so if they include anything, it won't be noticed. That's probably ok – if users want to include something here, they can just name it as the input file of that section. And if they include different files in a platform dependent way, then a) Cython can't decide it anyway and b) we are probably dealing with system provided header files which don't normally change.

[dcbaker]

I guess for cython the header files don't matter as long as they shouldn't trigger cython to re-compiler/transpile (at least from Meson's point of view) as the C/C++ compiler will generate its own depfile that would cover the inclusion of C language headers, so for Meson I guess that just means pxd files?

[scoder]

The feature was only added to the cythonize command for now, in #4563.

Whether it should be added to the cython command as well is less obvious since it does not currently deal with dependencies at all. Only cythonize has this intention.

[scoder]

Backported to 0.29.27 in #4576