doc: cmake: add cmake style guidelines

Add cmake style guidelines.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

doc/build/cmake/index.rst

@@ -43,6 +43,9 @@ source file :file:`src/main.c`, behavior that we surely do not want. The
 ``PUBLIC`` keyword could however be useful when modifying the include
 paths of a target library.
 
+When introducing build system code using CMake or adding new CMake files,
+please follow the style guidlines outline :ref:`here <cmake-style>`.
+
 
 Build and Configuration Phases
 ==============================

doc/build/cmake/style.rst

@@ -0,0 +1,139 @@
+:orphan:
+
+.. _cmake-style:
+
+CMake Style Guidelines
+======================
+
+General Formatting
+------------------
+
+- **Indentation**: Use **2 spaces** for indentation. Avoid tabs to ensure
+  consistency across different environments.
+- **Line Length**: Limit line length to **100 characters** where possible.
+- **Empty Lines**: Use empty lines to separate logically distinct sections
+  within a CMake file.
+- **No Space Before Opening Brackets**: Do not add a space between a command
+  and the opening parenthesis.
+  Use ``if(...)`` instead of ``if (...)``.
+
+  .. code-block:: cmake
+
+     # Good:
+     if(ENABLE_TESTS)
+       add_subdirectory(tests)
+     endif()
+
+     # Bad:
+     if (ENABLE_TESTS)
+       add_subdirectory(tests)
+     endif()
+
+Commands and Syntax
+-------------------
+
+- **Lowercase Commands**: Always use **lowercase** CMake commands (e.g.,
+  ``add_executable``, ``find_package``). This improves readability and
+  consistency.
+
+  .. code-block:: cmake
+
+     # Good:
+     add_library(my_lib STATIC src/my_lib.cpp)
+
+     # Bad:
+     ADD_LIBRARY(my_lib STATIC src/my_lib.cpp)
+
+- **One File Argument per Line**: Break the file arguments across multiple
+  lines to make it easier to scan and identify each source file or item.
+
+  .. code-block:: cmake
+
+     # Good:
+     target_sources(my_target PRIVATE
+       src/file1.cpp
+       src/file2.cpp
+     )
+
+      # Bad:
+     target_sources(my_target PRIVATE src/file1.cpp src/file2.cpp)
+
+Variable Naming
+---------------
+
+- **Use Uppercase for Cache Variables or variables shared across CMake files**:
+  When defining cache variables using ``option`` or ``set(... CACHE ...)``, use
+  **UPPERCASE names**.
+
+  .. code-block:: cmake
+
+     option(ENABLE_TESTS "Enable test suite" ON)
+     set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard version")
+
+- **Use Lowercase for Local Variables**: For local variables within CMake
+  files, use **lowercase** or **snake_case**.
+
+  .. code-block:: cmake
+
+     set(output_dir "${CMAKE_BINARY_DIR}/output")
+
+- **Consistent Prefixing**: Use consistent prefixes for variables to avoid name
+  conflicts, especially in large projects.
+
+  .. code-block:: cmake
+
+     set(MYPROJECT_SRC_DIR "${CMAKE_SOURCE_DIR}/src")
+
+Quoting
+-------
+
+- **Quote Strings and Variables**: Always quote string literals and variables
+  to prevent unintended behavior, especially when dealing with paths or
+  arguments that may contain spaces.
+
+  .. code-block:: cmake
+
+     # Good:
+     set(my_path "${CMAKE_SOURCE_DIR}/include")
+
+     # Bad:
+     set(my_path ${CMAKE_SOURCE_DIR}/include)
+
+- **Do Not Quote Boolean Values**: For boolean values (``ON``, ``OFF``,
+  ``TRUE``, ``FALSE``), avoid quoting them.
+
+  .. code-block:: cmake
+
+     option(BUILD_SHARED_LIBS "Build shared libraries" OFF)
+
+Avoid Hardcoding Paths
+----------------------
+
+- Use CMake variables (``CMAKE_SOURCE_DIR``, ``CMAKE_BINARY_DIR``,
+  ``CMAKE_CURRENT_SOURCE_DIR``) instead of hardcoding paths.
+
+  .. code-block:: cmake
+
+     set(output_dir "${CMAKE_BINARY_DIR}/bin")
+
+Conditional Logic
+-----------------
+
+- Use ``if``, ``elseif``, and ``else`` with proper indentation, and close with
+  ``endif()``.
+
+  .. code-block:: cmake
+
+     if(ENABLE_TESTS)
+       add_subdirectory(tests)
+     endif()
+
+Documentation
+-------------
+
+- Use comments to document complex logic in the CMake files.
+
+  .. code-block:: cmake
+
+     # Find LlvmLld components required for building with llvm
+     find_package(LlvmLld 14.0.0 REQUIRED)