🚨 Suppress clang module scanner unused arguments warning

README.md

@@ -62,6 +62,17 @@ What it does:
 - Checks for Ninja/Visual Studio generator (required for modules)
 - Sets up clang-tidy if enabled
 - Adds compile_commands.json copy target
+- Globally injects `-Qunused-arguments` into `CMAKE_CXX_FLAGS` for Clang/AppleClang
+
+> [!NOTE]
+>
+> `-Qunused-arguments` is appended to `CMAKE_CXX_FLAGS` (not
+> `add_compile_options`) so it is visible to CMake's C++20 module dependency
+> scanning commands for imported targets. During scanning, `clang-scan-deps`
+> passes both `--precompile` and `-c` to the compiler, making `-c` unused and
+> producing noisy `clang++: warning: argument unused during compilation: '-c'`
+> diagnostics. `-Qunused-arguments` suppresses this at the Clang driver level,
+> where `-Wno-unused-command-line-argument` does not.
 
 ```cmake
 libhal_project_init()

v5/cmake/LibhalCMakeUtilConfig.cmake

@@ -38,6 +38,17 @@ function(libhal_project_init)
         message(FATAL_ERROR "C++20 modules require Ninja or Visual Studio generator")
     endif()
 
+    # Suppress clang's "argument unused during compilation: '-c'" warning that
+    # fires during CMake's C++20 module dependency scanning phase.
+    # clang-scan-deps passes both --precompile and -c to the compiler, making
+    # -c unused. Using -Qunused-arguments via CMAKE_CXX_FLAGS (not
+    # add_compile_options) ensures the flag is visible to module scanning
+    # commands for imported targets.
+    if(CMAKE_CXX_COMPILER_ID MATCHES "Clang|AppleClang")
+        string(APPEND CMAKE_CXX_FLAGS " -Qunused-arguments")
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}" PARENT_SCOPE)
+    endif()
+
     # Set up clang-tidy if enabled
     libhal_setup_clang_tidy()