Moving SciPy to the Meson build system
This is a pretty exciting milestone, and good news for SciPy maintainers and contributors - they can look forward to much faster builds and a more pleasant development experience. So how fast is it? Currently the build takes about 1min 50s (a ~4x improvement) on my 3 year old 12-core Intel CPU (i9-7920X @ 2.90GHz):
As you can see from the tracing results, building a single C++ file (bsr.cxx, which is one of SciPy's sparse matrix formats) takes over 90 seconds. So the 1min 50 sec build time is close to optimal - the only ways to improve it are major surgery on that C++ code, or buying a faster CPU.
In Python 3.10 distutils will be deprecated and in Python 3.12 it will be removed (see PEP 632). SciPy uses numpy.distutils and setuptools as its build system, so the removal of distutils from the Python standard library would have a major impact on SciPy - and even more on numpy.distutils, which directly extends distutils. That PEP was written almost a year ago, and at the time my first instinct was to wait until setuptools integrated distutils (which the setuptools maintainers still plan to do, with a cleaned up API) and then update numpy.distutils for that change. That would also require moving parts of numpy.distutils into setuptools, like Fortran support (see setuptools/issues/2372 for more details). It has become clear though that this will be a really slow and painful process - after almost a year, the vendored distutils still hasn't been re-enabled inside setuptools.
The other driver was that SciPy development has become more painful over time. SciPy contains C, C++, Fortran and Cython code, exports C and Cython APIs, has complex dependencies (see diagram below) and does a lot of code generation. The growing amount of C++ and Cython code has increased build times. CI systems frequently time out because build plus test suite run takes close to 60 minutes (the limit on several free-for-open-source CI offerings). Working on compiled code is cumbersome and slow, to the point where a couple of other maintainers have told me it's a blocker for them to work on new features. Finally, debugging build issues is quite difficult - distutils doesn't really have a design, so every other extension just monkey patches things as needed. Earlier in the year I spent half a day hunting a build problem through four (!) different projects that all modified how extensions are built.
