To compile Cython sources code both `cython` and `c` need to be passed to `project()`

[dnicolodi]

I don't know if this is expected because Cython is a a transpiler to C or C++, but using Cython requires passing both cython and c to project() otherwise Meson complains with no toolchain found. The strange thing is that this happens only on some platforms (some Linux distributions but not others, for example).

[eli-schwartz]

This error string is only ever emitted by dependency() when it is using a method: 'system' lookup that tries to access a self.clib_compiler in order to internally run find_library or has_header or similar. You should therefore be able to get that error message regardless of whether the project uses cython at all.

[dnicolodi]

This is the whole meson.build:

project('complex', 'c', 'cython', version: '1.0.0')

py = import('python').find_installation()

install_subdir('complex', install_dir: py.get_install_dir(pure: false))

py.extension_module('test', 'test.pyx', install: true, subdir: 'complex')

If I remove c from the project() call, I get the no toolchain found error. Example: https://cirrus-ci.com/task/5918684011560960

[eli-schwartz]

Aha, yes. That happens because:

Did not find pkg-config by name 'pkg-config'
Found Pkg-config: NO
../../../../cirrus-ci-build/tests/packages/complex/meson.build:11:3: ERROR: no toolchain found

So pkg-config was not found, and the python module's find_installation + dependency handling attempted to find libpython using sysconfig + the clib_compiler.

[dnicolodi]

I see. It is a bit unexpected.

[eli-schwartz]

Yes, but we used to simply crash instead. :D Now the dependencies are just not found, which in some cases are mandatory dependencies.

The question is, how this plays together with cython... do we not need to specify both cython and c just to ensure the transpiled files can then be compiled?

[dnicolodi]

Dunno, on some platforms it worked without specifying c. Maybe the less surprising thing would be to error out if someone specifies cython without adding c or cpp, if it is reasonable to implement.

[rgommers]

A user ran into this in mesonbuild/meson-python#558.

Maybe the less surprising thing would be to error out if someone specifies cython without adding c or cpp, if it is reasonable to implement.

This seems like a good solution indeed. Automatically adding 'c' would be a bit more questionable, since it's very well possible that the user only wants 'cpp'.