cmake: pass QT_MODULES from CMake to Rust and set link libs

We need to set the link libs onto the static target and not
the resultant crate target otherwise the order of the linking
can be incorrect which then fails with ld.bfd

Author: ahayzen-kdab

cmake/CxxQt.cmake

@@ -17,7 +17,7 @@ if(NOT Corrosion_FOUND)
 endif()
 
 function(cxx_qt_import_crate)
-  cmake_parse_arguments(IMPORT_CRATE "" "CXX_QT_EXPORT_DIR;QMAKE" "" ${ARGN})
+  cmake_parse_arguments(IMPORT_CRATE "" "CXX_QT_EXPORT_DIR;QMAKE" "QT_MODULES" ${ARGN})
 
   corrosion_import_crate(IMPORTED_CRATES __cxx_qt_imported_crates ${IMPORT_CRATE_UNPARSED_ARGUMENTS})
 
@@ -37,12 +37,27 @@ function(cxx_qt_import_crate)
     endif()
   endif()
 
+  if (NOT DEFINED IMPORT_CRATE_QT_MODULES)
+    message(FATAL_ERROR "Missing QT_MODULES argument! You must specify at least one Qt module to link to.")
+  else()
+    message(VERBOSE "CXX_QT_QT_MODULES: ${IMPORT_CRATE_QT_MODULES}")
+  endif()
+
   foreach(CRATE ${__cxx_qt_imported_crates})
+    # Join modules by a comma so that we can pass easily via an env variable
+    #
+    # TODO: can we instead read the module from _qt_config_module_name or
+    # _qt_public_module_interface_name of the target, but need to consider
+    # private modules too
+    list(JOIN IMPORT_CRATE_QT_MODULES "," IMPORT_CRATE_QT_MODULES_STR)
+
     corrosion_set_env_vars(${CRATE}
       # Tell cxx-qt-build where to export the data
       "CXX_QT_EXPORT_DIR=${IMPORT_CRATE_CXX_QT_EXPORT_DIR}"
       # Tell cxx-qt-build which crate to export
       "CXX_QT_EXPORT_CRATE_${CRATE}=1"
+      # Tell cxx-qt-build which Qt modules we are using
+      "CXX_QT_QT_MODULES=${IMPORT_CRATE_QT_MODULES_STR}"
       "QMAKE=${IMPORT_CRATE_QMAKE}"
       $<$<BOOL:${CMAKE_RUSTC_WRAPPER}>:RUSTC_WRAPPER=${CMAKE_RUSTC_WRAPPER}>)
 
@@ -75,6 +90,14 @@ function(cxx_qt_import_crate)
     # See also the "Linking Object Libraries" and "Linking Object Libraries via $<TARGET_OBJECTS>" sections:
     # https://cmake.org/cmake/help/latest/command/target_link_libraries.html
     target_link_libraries(${CRATE} INTERFACE ${CRATE}_initializers $<TARGET_OBJECTS:${CRATE}_initializers>)
+
+    # Link the static library to Qt
+    # Note that we cannot do this on the final CRATE target as this is an interface
+    # which depends on the static library. If we do target_link_libraries on the ${CRATE} target,
+    # the static library will not actually depend on the Qt modules, but be a kind of "sibling dependency", which CMake may reorder.
+    # This can cause CMake to emit the wrong link order, with Qt before the static library, which then fails to build with ld.bfd
+    # https://stackoverflow.com/questions/51333069/how-do-the-library-selection-rules-differ-between-gold-and-the-standard-bfd-li
+    target_link_libraries(${CRATE}-static INTERFACE ${IMPORT_CRATE_QT_MODULES})
   endforeach()
 
 endfunction()