Install CMake package config files with Meson

A Meson installation only provided the headers and a pkg-config file, so
consumers using find_package(nlohmann_json) could not find the library.
Meson now also installs nlohmann_jsonConfig.cmake and
nlohmann_jsonConfigVersion.cmake from the existing templates, plus a
static nlohmann_jsonTargets.cmake that defines the header-only imported
target with a relocatable include path.

A new CI job installs with Meson and builds tests/cmake_import/project
against the result.

Closes #3885

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/ubuntu.yml

@@ -31,6 +31,25 @@ jobs:
       - name: Build
         run: cmake --build build --target ci_test_gcc
 
+  ci_meson_install:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+      - name: Get latest CMake and ninja
+        uses: lukka/get-cmake@fffaaafeea488556c2c12dad60690008bc1caacb # v4.4.2
+      - name: Install Meson
+        run: pip install meson
+      - name: Install with Meson
+        run: |
+          meson setup build-meson --prefix=${{ github.workspace }}/install
+          meson install -C build-meson
+      - name: Use the installed package with find_package
+        run: |
+          cmake -S tests/cmake_import/project -B build-import -DCMAKE_PREFIX_PATH=${{ github.workspace }}/install
+          cmake --build build-import
+
   ci_infer:
     runs-on: ubuntu-latest
     steps:

FILES.md

@@ -262,7 +262,9 @@ The "Check amalgamation" workflow fails if the file is out of date.
 
 ### `meson.build`
 
-The build definition for the [Meson](https://mesonbuild.com) build system.
+The build definition for the [Meson](https://mesonbuild.com) build system. When installing, it also installs the CMake package config
+files so that `find_package(nlohmann_json)` works. As Meson cannot generate `nlohmann_jsonTargets.cmake` itself, it is
+created from the template `cmake/nlohmann_jsonTargets.cmake.in`, which is only used by Meson.
 
 ### `Package.swift`
 

cmake/nlohmann_jsonTargets.cmake.in

@@ -0,0 +1,28 @@
+# Imported target for installations made with Meson (see meson.build).
+#
+# CMake installations generate this file with install(EXPORT ...). Meson cannot
+# do that, but as the library is header-only, the target only needs an include
+# directory and the C++ standard. Paths are computed relative to this file so
+# that the installation can be relocated (e.g., into a sysroot).
+
+if(TARGET @PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@)
+    return()
+endif()
+
+get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_DIR}/@NLOHMANN_JSON_CONFIG_TO_PREFIX@" ABSOLUTE)
+
+add_library(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ INTERFACE IMPORTED)
+set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+    INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/@NLOHMANN_JSON_INCLUDE_INSTALL_DIR@"
+)
+if(CMAKE_VERSION VERSION_LESS 3.8)
+    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+        INTERFACE_COMPILE_FEATURES cxx_range_for
+    )
+else()
+    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+        INTERFACE_COMPILE_FEATURES cxx_std_11
+    )
+endif()
+
+unset(_IMPORT_PREFIX)

docs/mkdocs/docs/integration/package_managers.md

@@ -121,10 +121,15 @@ meson wrap install nlohmann_json
 Please see the Meson project for any issues regarding the packaging.
 
 The provided `meson.build` can also be used as an alternative to CMake for installing `nlohmann_json` system-wide in
-which case a pkg-config file is installed. To use it, have your build system require the `nlohmann_json`
-pkg-config dependency. In Meson, it is preferred to use the
-[`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a subproject fallback, rather than
-using the subproject directly.
+which case a pkg-config file and the CMake package config files are installed. To use it, have your build system require
+the `nlohmann_json` pkg-config dependency, or use [`find_package(nlohmann_json)`](cmake.md#external) in CMake. In Meson,
+it is preferred to use the [`dependency()`](https://mesonbuild.com/Reference-manual.html#dependency) object with a
+subproject fallback, rather than using the subproject directly.
+
+!!! note
+
+    The `meson.build` does not expose the library's [CMake options](cmake.md#cmake-options), so a Meson installation
+    always uses the library's default configuration.
 
 ??? example "Example: Wrap"
 

meson.build

@@ -21,4 +21,34 @@ pkgc.generate(name: 'nlohmann_json',
     version: meson.project_version(),
     description: 'JSON for Modern C++'
 )
+
+# CMake package config files, so that find_package(nlohmann_json) works
+cmake_install_dir = get_option('datadir') / 'cmake' / meson.project_name()
+cmake_to_prefix = []
+foreach component : cmake_install_dir.split('/')
+    cmake_to_prefix += '..'
+endforeach
+
+cmake_conf = configuration_data()
+cmake_conf.set('PROJECT_NAME', meson.project_name())
+cmake_conf.set('PROJECT_VERSION', meson.project_version())
+cmake_conf.set('PROJECT_VERSION_MAJOR', meson.project_version().split('.')[0])
+cmake_conf.set('NLOHMANN_JSON_TARGET_NAME', meson.project_name())
+cmake_conf.set('NLOHMANN_JSON_TARGETS_EXPORT_NAME', meson.project_name() + 'Targets')
+cmake_conf.set('NLOHMANN_JSON_INCLUDE_INSTALL_DIR', get_option('includedir'))
+cmake_conf.set('NLOHMANN_JSON_CONFIG_TO_PREFIX', '/'.join(cmake_to_prefix))
+
+foreach cmake_file : [
+    ['cmake/config.cmake.in', 'nlohmann_jsonConfig.cmake'],
+    ['cmake/nlohmann_jsonConfigVersion.cmake.in', 'nlohmann_jsonConfigVersion.cmake'],
+    ['cmake/nlohmann_jsonTargets.cmake.in', 'nlohmann_jsonTargets.cmake'],
+]
+    configure_file(
+        input: cmake_file[0],
+        output: cmake_file[1],
+        configuration: cmake_conf,
+        format: 'cmake@',
+        install_dir: cmake_install_dir,
+    )
+endforeach
 endif