Rationale

Meson doesn't support user defined functions as one of its core principles. Why would muon relax this limitation? Partially because I believe that careful implementation of user defined functionality would not be harmful. We have seen meson scripts can get plenty complex anyways. The main reason however, is that requiring every last bit of functionality to be implemented in the build system itself is fundamentally untenable, especially if the build system will support multiple implementations.

• Many meson modules are effectively reusable chunks of meson code (calls to dependency, custom_target, etc.), so it is not necessary to access things that a native module has access to like the interpreter state.
• Lowering the bar for module contributions (this is important because they often involve specific tools I have no experience with/interest in).
• Potential to some day share module code with other meson implementations.

That last point is a bit pie in the sky, but at least this makes it technically possible from muon's point of view and proves that it has value.