The CMake module doesn't allow for native dependencies when cross compiling

[robbert-vdh]

I'm not sure whether this counts as a bug or as a feature request.

Describe the bug
When cross compiling (i.e. with --cross-file), it is not possible to use include('cmake').subproject('foo').dependency('bar') in a native build target. This is possible when using regular subprojects and with dependency(). <cmake_subproject>.dependency() also does not support the native option.

To Reproduce

1. Create a project with a meson.build file like this:

   # meson.build
   project('projectname', 'cpp', version : '1.0.0')
   
   cmake = import('cmake')
   vst3_sdk_options = cmake.subproject_options()
   vst3_sdk_options.add_cmake_defines({
     'SMTG_ADD_VST3_HOSTING_SAMPLES': 'OFF',
     'SMTG_ADD_VST3_PLUGINS_SAMPLES': 'OFF',
     'SMTG_ADD_VSTGUI': 'OFF',
   })
   vst3_sdk = cmake.subproject('vst3', options : vst3_sdk_options)
   vst3_pluginterfaces_dep = vst3_sdk.dependency('pluginterfaces')
   
   shared_library(
     'some-library',
     ['src/some-library.cpp'],
     native : true,
     dependencies : [vst3_pluginterfaces_dep],
   )

   With some arbitrary cross.conf file, for instance:

   # cross.conf
   [binaries]
   c = 'gcc'
   cpp = 'g++'
   ar = 'ar'
   strip = 'strip'
   pkgconfig = 'pkg-config'
   

   And the following wrap file in subprojects/vst3.wrap (any CMake project will work, this is just how I ran into this issue):

   # subprojects/vst3.wrap
   [wrap-git]
   url = https://github.com/robbert-vdh/vst3sdk.git
   revision = 2a1a230d45766532360a2f432083f0795f2b0d93
   clone-recursive = true
   depth = 1
   

2. Create an empty src/some-library.cpp file.

3. Now try to initialize the project with meson setup build --cross-file cross.conf.

Expected behavior
The project gets created without any errors. Running ninja -C build should compile the empty file without any issues.

Observed behavior
Meson errors out during meson setup with the following error:

meson.build:14:3: ERROR: Tried to mix libraries for machines 0 and 1 in target 'some-library' This is not possible in a cross build.

system parameters

• Cross build: yes
• Operating system: Manjaro Linux
• Python version: 3.8.6
• Meson version: 0.56.0
• Ninja version: 1.10.1

[mensinda]

As far as I know, CMake does not have the equivalent of the meson native system. So a CMake project is either completely cross, or completely native.

Do you want to compile the entire subproject as native?

[robbert-vdh]

I would want to have the subproject both as native and as non-native in the same meson.build file, so having both a vst3_sdk_native and a vst3_sdk_cross in that example meson.build I posted above would be perfect. I'm building an application that consists of both a native library and a cross compiled binary that both share most of the same dependencies.

[mensinda]

This should be fairly easy to implement with the cmake.subproject_options() to specifically set the native kwarg for all/some targets.

Assuming the project itself does not specify platform-specific defines/compiler options that mess up the final build...

[robbert-vdh]

Ouch yeah side effects like that sound like it could make having both a native and a non-native copy of the same CMake subproject a bit tricky. I didn't want to reinvent the wheel so I tried making it work with the CMake module first, but I'll just write my own meson.build file for the subproject for the time being. Cross-builds are always a bit tricky, so I'm interested to see how if a good solution to this issue is possible!

[robbert-vdh]

I came to the conclusion that using native library dependencies from subprojects also doesn't work when cross compiling, is this correct or am I missing something? So in a subproject (using wrap-git) I have a Meson build definition that defines a static library and declares a dependency for it. I couldn't figure out a way to use this dependency with both my native and my non-native targets. For the time being I've just resorted to exporting all of the sources, include directories and compiler options needed to build those libraries and then I just assemble the dependencies myself in my own project's meson.build file, but that of course is one giant hack.

[dcbaker]

Ya, its a know deficiency, it's on my list is things to fix, but it's non trivial