CMake: Run configure tests lazily

qt_config_compile_test doesn't run the compile test immediately anymore
but just defines the test. Compile tests defined with this function are
run only if their value TEST_${name} is requested in a feature's
condition variable (EMIT_IF, CONDITION, ENABLE, ...).

If you need the TEST_${name} variable regardless of any feature
conditions, call qt_run_config_compile_test right after
qt_config_compile_test.

This avoids having e.g. Windows-specific compile tests on Linux (and
vice versa) and speeds configuration up a bit.

Fixes: QTBUG-115705
Task-number: QTBUG-107248
Change-Id: I38c4d2a48b1e5dc2f5133e0ad507ede0520ca3b8
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtBuildInformation.cmake

@@ -27,6 +27,26 @@ function(qt_print_feature_summary)
         endif()
     endif()
 
+    # Print debug information about which tests were not run.
+    get_property(known_compile_tests GLOBAL PROPERTY _qtfeature_known_compile_tests)
+    list(REMOVE_DUPLICATES known_compile_tests)
+    set(tests_not_run "")
+    foreach(test_name IN LISTS known_compile_tests)
+        if(NOT DEFINED "TEST_${test_name}")
+            list(APPEND tests_not_run "${test_name}")
+        endif()
+    endforeach()
+    if(tests_not_run STREQUAL "")
+        message(DEBUG "All known compile tests were run.")
+    else()
+        message(DEBUG
+            "The following compile tests were not run, because their values were not requested."
+        )
+        foreach(test_name IN LISTS tests_not_run)
+            message(DEBUG "The compile test '${test_name}' was not run.")
+        endforeach()
+    endif()
+
     # Show which packages were found.
     feature_summary(INCLUDE_QUIET_PACKAGES
                     WHAT PACKAGES_FOUND

cmake/QtFeature.cmake

@@ -127,6 +127,7 @@ function(qt_internal_evaluate_config_expression resultVar outIdx startIdx)
     set(result "")
     set(expression "${ARGN}")
     list(LENGTH expression length)
+    get_property(known_compile_tests GLOBAL PROPERTY _qtfeature_known_compile_tests)
 
     math(EXPR memberIdx "${startIdx} - 1")
     math(EXPR length "${length}-1")
@@ -177,8 +178,13 @@ function(qt_internal_evaluate_config_expression resultVar outIdx startIdx)
             string(COMPARE EQUAL "${lhs}" "${rhs}" stringCompareResult)
             list(APPEND result ${stringCompareResult})
         else()
-            string(FIND "${member}" "QT_FEATURE_" idx)
-            if(idx EQUAL 0)
+            if(member MATCHES "^TEST_")
+                # Remove the TEST_ prefix
+                string(SUBSTRING "${member}" 5 -1 test_name)
+                if(test_name IN_LIST known_compile_tests)
+                    qt_run_config_compile_test("${test_name}")
+                endif()
+            elseif(member MATCHES "^QT_FEATURE_")
                 # Remove the QT_FEATURE_ prefix
                 string(SUBSTRING "${member}" 11 -1 feature)
                 qt_evaluate_feature(${feature})
@@ -949,13 +955,33 @@ endfunction()
 #
 # Sets a TEST_${name}_OUTPUT variable with the build output, to the scope of the calling function.
 # Sets a TEST_${name} cache variable to either TRUE or FALSE if the build is successful or not.
-function(qt_config_compile_test name)
+#
+# The test is only run if a feature condition needs to evaluate the TEST_${name} variable. If you
+# need the test result regardless of any feature conditions, call
+# qt_run_config_compile_test right after qt_config_compile_test.
+macro(qt_config_compile_test name)
+    set_property(GLOBAL APPEND PROPERTY _qtfeature_known_compile_tests ${name})
+    set_property(GLOBAL PROPERTY _qtfeature_compile_test_args_${name} ${ARGN})
+    if(QT_RUN_COMPILE_TESTS_IMMEDIATELY)
+        qt_run_config_compile_test(${name})
+    endif()
+endmacro()
+
+# Runs a compile test that was defined with qt_config_compile_test.
+function(qt_run_config_compile_test name)
     if(DEFINED "TEST_${name}")
         return()
     endif()
 
+    get_property(test_args GLOBAL PROPERTY _qtfeature_compile_test_args_${name})
+    if("${test_args}" STREQUAL "")
+        message(FATAL_ERROR
+            "Can't find definition for compile test '${name}'. "
+            "The test probably wasn't defined with qt_config_compile_test."
+        )
+    endif()
     cmake_parse_arguments(arg "" "LABEL;PROJECT_PATH;C_STANDARD;CXX_STANDARD"
-        "COMPILE_OPTIONS;LIBRARIES;CODE;PACKAGES;CMAKE_FLAGS" ${ARGN})
+        "COMPILE_OPTIONS;LIBRARIES;CODE;PACKAGES;CMAKE_FLAGS" ${test_args})
 
     if(arg_PROJECT_PATH)
         message(STATUS "Performing Test ${arg_LABEL}")

cmake/QtProcessConfigureArgs.cmake

@@ -241,6 +241,7 @@ macro(defstub name)
 endmacro()
 
 defstub(qt_add_qmake_lib_dependency)
+defstub(qt_run_config_compile_test)
 defstub(qt_config_compile_test)
 defstub(qt_config_compile_test_armintrin)
 defstub(qt_config_compile_test_machine_tuple)