Meson's policy on mixing multiple build systems in one build directory
Meson has been designed with the principle that all dependencies are either provided by "the platform" via a mechanism such as Pkg-Config or that they are built as Meson subprojects under the main project. There are several projects that would like to mix build systems, that is, build dependencies in the same build directory as the other build system by having one build system call the other. The build directories do not necessarily need to be inside each other, but that is the common case.
This page lists the Meson project's stance on mixing build systems. The tl/dr version is that while we do provide some functionality for this use case, it only works for simple cases. Anything more complex can not be made reliable and trying to do that would burden Meson developers with an effectively infinite maintenance burden. Thus these use cases are not guaranteed to work, and even if a project using them works today there are no guarantees that it will work in any future version.
The definition of "build system mixing"
For the purposes of this page, mixing build systems means any and all mechanisms where one build system uses build artifacts from a different build system's build directory in any way.
Note that this definition does not specify what the dependencies are and how they are built, only how they are consumed. For example suppose you have a standalone dependency library that builds with build system X. In this case having Meson call the build system to build the dependency at build time would be interpreted as mixing build systems. On the other hand a "Flatpak-like" approach of building and installing the library with an external mechanism and consuming it via a standard build-system agnostic method such as Pkg-Config would not be considered build system mixing. Use of uninstalled-pkgconfig files is considered mixing, though.
What does this mean for support and compatibility?
The Meson project will not take on any maintenance burden to ensure anything other than the simple builds setups as discussed above will work. Nor will we make changes to support these use cases that would worsen the user experience of users of plain Meson. This includes, but is not limited to, the following:
Any changes in other build systems that cause mixed project breakage will not be considered a bug in Meson.
Breakages in mixed build projects will not be considered regressions and such problems will never be considered release blockers, regardless of what the underlying issue is.
Any use case that would require major changes in Meson to work around missing or broken functionality in the other build system is not supported. These issues must be fixed upstream.
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