Adding (very) preliminary support for C++ modules in Meson

One of the most common questions people ask about Meson is why does it not yet have support for building C++ modules. Up until now the answer has been simple: no compiler really supports it yet. However Visual Studio has added sufficient functionality in their latest 2019 developer preview that an implementation in Meson has become feasible. The actual code can be found in this merge request for those brave enough to try it out.

The basic problem with C++ modules is the same as with Fortran modules: you can no longer build source files in an arbitrary order. Instead you have to scan the contents of files, see what modules each source file generates and consumes and orchestrate the build so that all source files that produce modules are built before any source files that consume them. This requires dynamic dependency generation that has been added to Ninja only fairly recently.

The original idea was that compiler toolchain vendors would provide scanner binaries because parsing C++ code is highly unreliable due to C preprocessor macro shenanigans. It turns out that a "toolchain provided" dependency scanner can not obtain all necessary data reliably, because it requires higher level knowledge about the project setup. This can only be done reliably by the build system. An alternative would be to pass all this information to the compiler/scanner via compiler flags but that turns out to be a terrible thing to define and maintain stable over changes. It also has the downside that you'd need to spawn a single process for every file, which is fairly slow on Windows. Meson's approach is to write a custom dependency scanner. Yes, it is based on regexes, so it is not 100% reliable but on the other hand you only need to spawn one process per build target (exe, shared lib, static lib) as opposed to one per source file.

Still, the end result does work for simple projects. It does not handle things like module partitions but those can be added later. Even this simple project and test has brought about several notes and questions:

• Where should the generated module files be put? In the target private dir? In a global dir? If the latter, what happens if two unrelated parts in the code base specify the same module?
• Microsoft has not documented the module compiler flags and cl /? does not even list them. Because of this all module files get dumped to the build directory root.
• Only ixx files are supported. VS does not enforce file name extensions. I would really want to enforce module file name extensions to only one. We can't change change legacy code and force everyone to use a single extension for C++ source, but we totally should do that for new ones. Having to support many file name extensions for the same thing is madness.

Sadly I don't have any numbers on how much modules improve compilation speed. Feel free to try it out yourself, though. Bug reports and especially fixes are welcome.