Meson has the ability to generate code and process data files. Why do you need to communicate with REST? Meson support that however by allowing you to break out using the `run_command()` function.
Uploading build output somewhere for instance. However this may overlap with CI tasks (but there, usually YAML is used to run shell commands, which is also a bit of a crutch).
That's just your opinion though ;) Why should the process of producing a build artifact be different from putting the artifact into the right place? E.g. Makefiles usually have a 'make install' step which isn't all that different from uploading the build result somewhere, and make definitely counts as a build system.
