example for injecting cmake variables (#3333)

* example for injecting cmake variables

* Update examples/tools/cmake/cmake_toolchain/inject_cmake_variables.rst

* Update examples/tools/cmake/cmake_toolchain/inject_cmake_variables.rst

* Add note about support for both user_toolchain and confs at the same time

* Update examples/tools/cmake/cmake_toolchain/inject_cmake_variables.rst

Co-authored-by: Carlos Zoido <mrgalleta@gmail.com>

---------

Co-authored-by: Carlos Zoido <mrgalleta@gmail.com>
Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

examples/tools/cmake/cmake.rst

@@ -10,4 +10,5 @@ tools.cmake
 
 
    cmake_toolchain/build_project_cmake_presets
-   cmake_toolchain/extend_own_cmake_presets
+   cmake_toolchain/extend_own_cmake_presets
+   cmake_toolchain/inject_cmake_variables

examples/tools/cmake/cmake_toolchain/inject_cmake_variables.rst

@@ -0,0 +1,87 @@
+.. _examples-tools-cmake-toolchain-inject-variables:
+
+CMakeToolchain: Inject arbitrary CMake variables into dependencies
+==================================================================
+
+You can find the sources to recreate this project in the `examples2.0 repository
+<https://github.com/conan-io/examples2>`_ in GitHub:
+
+.. code-block:: bash
+
+    $ git clone https://github.com/conan-io/examples2.git
+    $ cd examples2/examples/tools/cmake/cmake_toolchain/user_toolchain_profile
+
+
+In the general case, Conan package recipes provide the necessary abstractions via settings, confs, and options
+to control different aspects of the build. Many recipes define ``options`` to activate or deactivate features,
+optional dependencies, or binary characteristics. Configurations like ``tools.build:cxxflags`` can be used to
+inject arbitrary C++ compile flags.
+
+In some exceptional cases, it might be desired to inject CMake variables directly into dependencies doing CMake
+builds. This is possible when these dependencies use the ``CMakeToolchain`` integration. Let's check it in this
+simple example.
+
+If we have the following package recipe, with a simple ``conanfile.py`` and a ``CMakeLists.txt`` printing a variable:
+
+.. code-block:: python
+    :caption: conanfile.py
+
+    from conan import ConanFile
+    from conan.tools.cmake import CMake
+
+    class AppConan(ConanFile):
+        name = "foo"
+        version = "1.0"
+        settings = "os", "compiler", "build_type", "arch"
+        exports_sources = "CMakeLists.txt"
+
+        generators = "CMakeToolchain"
+
+        def build(self):
+            cmake = CMake(self)
+            cmake.configure()
+            cmake.build()
+
+
+.. code-block:: cmake
+    :caption: CMakeLists.txt
+
+    cmake_minimum_required(VERSION 3.15)
+    project(foo LANGUAGES NONE)
+    message(STATUS "MYVAR1 ${MY_USER_VAR1}!!")
+
+We can define a profile file and a ``myvars.cmake`` file (both in the same folder) like the following:
+
+.. code-block:: ini
+    :caption: myprofile
+
+    include(default)
+    [conf]
+    tools.cmake.cmaketoolchain:user_toolchain+={{profile_dir}}/myvars.cmake
+
+Note the ``{{profile_dir}}`` is a jinja template expression that evaluates to the current profile folder, allowing
+to compute the necessary path to ``myvars.cmake`` file. The ``tools.cmake.cmaketoolchain:user_toolchain`` is a **list**
+of files to inject to the generated ``conan_toolchain.cmake``, so the ``+=`` operator is used to append to it.
+
+The ``myvars.cmake`` can define as many variables as we want:
+
+.. code-block:: cmake
+    :caption: myvars.cmake
+
+    set(MY_USER_VAR1 "MYVALUE1")
+
+
+Applying this profile, we can see that the package CMake build effectively uses the variable provided in the 
+external ``myvars.cmake`` file:
+
+.. code-block:: bash
+
+    $ conan create . -pr=myprofile
+    ...
+    -- MYVAR1 MYVALUE1!!
+
+Note that using ``user_toolchain`` while defining values for confs like ``tools.cmake.cmaketoolchain:system_name`` is supported.
+
+The ``tools.cmake.cmaketoolchain:user_toolchain`` conf value might also be passed in the command line ``-c`` argument,
+but the location of the ``myvars.cmake`` needs to be absolute to be found, as jinja replacement doesn't happen in the
+command line.