[Backend] Establish standalone build infrastructure

Add the thin monorepo CMake entry point and an independently configurable backend project. Enforce the Clang/C++23 toolchain, expose Make/Ninja-neutral tests and developer checks, export an installable RCCBackend package, and introduce the first Support error abstraction.

1 files changed, 93 insertions, 0 deletions

diff --git a/backend/cmake/modules/RCCBackendBuild.cmake b/backend/cmake/modules/RCCBackendBuild.cmake
new file mode 100644
index 0000000..79e7bb1
--- /dev/null
+++ b/backend/cmake/modules/RCCBackendBuild.cmake
@@ -0,0 +1,93 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+function(rcc_backend_initialize_toolchain)
+  string(REGEX MATCH "^[0-9]+" RCCBackendClangMajor "${CMAKE_CXX_COMPILER_VERSION}")
+
+  find_program(
+    RCC_BACKEND_CLANG_FORMAT
+    NAMES "clang-format-${RCCBackendClangMajor}" clang-format
+    REQUIRED
+  )
+  find_program(
+    RCC_BACKEND_CLANG_TIDY
+    NAMES "clang-tidy-${RCCBackendClangMajor}" clang-tidy
+    REQUIRED
+  )
+
+  foreach(Tool IN ITEMS RCC_BACKEND_CLANG_FORMAT RCC_BACKEND_CLANG_TIDY)
+    execute_process(
+      COMMAND "${${Tool}}" --version
+      OUTPUT_VARIABLE ToolVersionOutput
+      ERROR_VARIABLE ToolVersionError
+      RESULT_VARIABLE ToolVersionResult
+    )
+    if(NOT ToolVersionResult EQUAL 0)
+      message(
+        FATAL_ERROR
+          "Unable to query ${Tool}: ${ToolVersionOutput}${ToolVersionError}"
+      )
+    endif()
+
+    string(REGEX MATCH "version[ \t]+([0-9]+)" ToolVersionMatch "${ToolVersionOutput}")
+    if(NOT CMAKE_MATCH_1 STREQUAL RCCBackendClangMajor)
+      message(
+        FATAL_ERROR
+          "${Tool} must have Clang major version ${RCCBackendClangMajor}: "
+          "${ToolVersionOutput}"
+      )
+    endif()
+  endforeach()
+
+  set(RCC_BACKEND_CLANG_FORMAT "${RCC_BACKEND_CLANG_FORMAT}" PARENT_SCOPE)
+  set(RCC_BACKEND_CLANG_TIDY "${RCC_BACKEND_CLANG_TIDY}" PARENT_SCOPE)
+endfunction()
+
+function(rcc_backend_configure_target Target)
+  target_compile_features("${Target}" PUBLIC cxx_std_23)
+  set_target_properties(
+    "${Target}"
+    PROPERTIES
+      CXX_EXTENSIONS OFF
+      CXX_STANDARD 23
+      CXX_STANDARD_REQUIRED ON
+  )
+
+  target_compile_options(
+    "${Target}"
+    PRIVATE
+      -Wall
+      -Wextra
+      -Wpedantic
+      -Wconversion
+      -Wsign-conversion
+      -Wshadow
+      -Wnon-virtual-dtor
+      -Wold-style-cast
+      -Wcast-align
+      -Woverloaded-virtual
+      -Wnull-dereference
+      -Wdouble-promotion
+      -Wformat=2
+  )
+
+  if(RCC_BACKEND_WARNINGS_AS_ERRORS)
+    target_compile_options("${Target}" PRIVATE -Werror)
+  endif()
+
+  if(RCC_BACKEND_ENABLE_CLANG_TIDY)
+    set_target_properties(
+      "${Target}"
+      PROPERTIES
+        CXX_CLANG_TIDY
+          "${RCC_BACKEND_CLANG_TIDY};--warnings-as-errors=*"
+    )
+  endif()
+endfunction()
+
+function(rcc_backend_register_format_files)
+  set_property(GLOBAL APPEND PROPERTY RCC_BACKEND_FORMAT_FILES ${ARGN})
+endfunction()
+
+function(rcc_backend_register_tidy_files)
+  set_property(GLOBAL APPEND PROPERTY RCC_BACKEND_TIDY_FILES ${ARGN})
+endfunction()