[M1] add util::Error, Result<T>, format()

CMakeLists.txt

@@ -13,7 +13,7 @@ project(cargoxx LANGUAGES CXX VERSION 0.1.0)
 
 set(CMAKE_CXX_STANDARD 23)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_CXX_EXTENSIONS ON)
 set(CMAKE_CXX_SCAN_FOR_MODULES ON)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
@@ -29,9 +29,11 @@ if(CARGOXX_WERROR)
     add_compile_options(-Werror)
 endif()
 
-# ----- cargoxx library: all module units -----
+# ----- cargoxx library: module units + implementation units -----
 add_library(cargoxx STATIC)
 target_sources(cargoxx
+    PRIVATE
+        src/util/error.cpp
     PUBLIC
         FILE_SET CXX_MODULES FILES
             src/lib.cppm
@@ -50,3 +52,10 @@ target_sources(cargoxx
 add_executable(cargoxx_bin src/main.cpp)
 set_target_properties(cargoxx_bin PROPERTIES OUTPUT_NAME cargoxx)
 target_link_libraries(cargoxx_bin PRIVATE cargoxx)
+
+# ----- tests -----
+option(CARGOXX_BUILD_TESTS "Build cargoxx tests" ON)
+if(CARGOXX_BUILD_TESTS)
+    enable_testing()
+    add_subdirectory(tests)
+endif()