Add a test for the module

CMakeLists.txt

@@ -24,6 +24,17 @@ function(join result_var)
   set(${result_var} "${result}" PARENT_SCOPE)
 endfunction()
 
+function(enable_module target)
+  if (MSVC)
+    set(BMI ${CMAKE_CURRENT_BINARY_DIR}/${target}.ifc)
+    target_compile_options(${target}
+      PRIVATE /interface /ifcOutput ${BMI}
+      INTERFACE /reference fmt=${BMI})
+  endif ()
+  set_target_properties(${target} PROPERTIES ADDITIONAL_CLEAN_FILES ${BMI})
+  set_source_files_properties(${BMI} PROPERTIES GENERATED ON)
+endfunction()
+
 include(CMakeParseArguments)
 
 # Sets a cache variable with a docstring joined from multiple arguments:
@@ -69,6 +80,22 @@ option(FMT_TEST "Generate the test target." ${FMT_MASTER_PROJECT})
 option(FMT_FUZZ "Generate the fuzz target." OFF)
 option(FMT_CUDA_TEST "Generate the cuda-test target." OFF)
 option(FMT_OS "Include core requiring OS (Windows/Posix) " ON)
+option(FMT_MODULE "Build a module instead of a traditional library." OFF)
+
+set(FMT_CAN_MODULE OFF)
+if (CMAKE_CXX_STANDARD GREATER_EQUAL 20 AND
+    # msvc 16.10-pre4
+    MSVC AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19.29.30036) 
+  set(FMT_CAN_MODULE ON)
+endif ()
+if (NOT FMT_CAN_MODULE)
+  set(FMT_MODULE OFF)
+  message(STATUS "Module support is disabled.")
+endif ()
+if (FMT_TEST AND FMT_MODULE)
+  # The tests require {fmt} to be compiled as traditional library
+  message(STATUS "Testing is incompatible with build mode 'module'.")
+endif ()
 
 # Get version from core.h
 file(READ include/fmt/core.h core_h)
@@ -189,7 +216,9 @@ endfunction()
 add_headers(FMT_HEADERS args.h chrono.h color.h compile.h core.h format.h
                         format-inl.h locale.h os.h ostream.h printf.h ranges.h
                         wchar.h)
-if (FMT_OS)
+if (FMT_MODULE)
+  set(FMT_SOURCES src/fmt.cc)
+elseif (FMT_OS)
   set(FMT_SOURCES src/format.cc src/os.cc)
 else()
   set(FMT_SOURCES src/format.cc)
@@ -215,6 +244,9 @@ endif ()
 if (FMT_PEDANTIC)
   target_compile_options(fmt PRIVATE ${PEDANTIC_COMPILE_FLAGS})
 endif ()
+if (FMT_MODULE)
+  enable_module(fmt)
+endif ()
 
 target_compile_features(fmt INTERFACE ${FMT_REQUIRED_FEATURES})
 

test/CMakeLists.txt

@@ -2,7 +2,9 @@ add_subdirectory(gtest)
 
 set(TEST_MAIN_SRC test-main.cc gtest-extra.cc gtest-extra.h util.cc)
 add_library(test-main STATIC ${TEST_MAIN_SRC})
-target_link_libraries(test-main gtest fmt)
+target_include_directories(test-main PUBLIC
+  $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>)
+target_link_libraries(test-main gtest)
 
 include(CheckCXXCompilerFlag)
 
@@ -29,16 +31,20 @@ endfunction()
 # Adds a test.
 # Usage: add_fmt_test(name srcs...)
 function(add_fmt_test name)
-  cmake_parse_arguments(ADD_FMT_TEST "HEADER_ONLY" "" "" ${ARGN})
+  cmake_parse_arguments(ADD_FMT_TEST "HEADER_ONLY;MODULE" "" "" ${ARGN})
 
   set(sources ${name}.cc ${ADD_FMT_TEST_UNPARSED_ARGUMENTS})
-  set(libs test-main)
   if (ADD_FMT_TEST_HEADER_ONLY)
     set(sources ${sources} ${TEST_MAIN_SRC} ../src/os.cc)
     set(libs gtest fmt-header-only)
     if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
       set(PEDANTIC_COMPILE_FLAGS ${PEDANTIC_COMPILE_FLAGS} -Wno-weak-vtables)
     endif ()
+  elseif (ADD_FMT_TEST_MODULE)
+    set(libs test-main test-module)
+    set_source_files_properties(${name}.cc PROPERTIES OBJECT_DEPENDS test-module)
+  else ()
+    set(libs test-main fmt)
   endif ()
   add_fmt_executable(${name} ${sources})
   target_link_libraries(${name} ${libs})
@@ -74,6 +80,20 @@ add_fmt_test(ranges-test)
 add_fmt_test(scan-test)
 add_fmt_test(wchar-test)
 
+if (FMT_CAN_MODULE)
+  # The tests need {fmt} to be compiled as traditional library
+  # because of visibility of implementation details.
+  # If module support is present the module tests require a
+  # test-only module to be built from {fmt}
+  add_library(test-module OBJECT ${CMAKE_SOURCE_DIR}/src/fmt.cc)
+  target_compile_features(test-module PUBLIC ${FMT_REQUIRED_FEATURES})
+  target_include_directories(test-module PUBLIC
+    $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>)
+  enable_module(test-module)
+
+  add_fmt_test(module-test MODULE)
+endif ()
+
 if (NOT MSVC)
   # FMT_ENFORCE_COMPILE_STRING is not supported under MSVC due to compiler bugs.
   add_fmt_test(enforce-checks-test)

test/module-test.cc

@@ -0,0 +1,65 @@
+// Formatting library for C++ - module tests
+//
+// Copyright (c) 2012 - present, Victor Zverovich
+// All rights reserved.
+//
+// For the license information refer to format.h.
+//
+// Copyright (c) 2021 - present, Daniela Engert
+// All Rights Reserved
+// {fmt} module.
+
+import fmt;
+
+// check for macros leaking from BMI
+static bool macro_leaked = 
+#if defined(FMT_CORE_H_) || defined(FMT_FORMAT_H)
+  true;
+#else
+  false;
+#endif
+
+#include "gtest/gtest.h"
+
+// an implicitly exported namespace must be visible [module.interface]/2.2
+TEST(module_test, namespace) {
+  using namespace fmt;
+  ASSERT_TRUE(true);
+}
+
+namespace detail {
+bool oops_detail_namespace_is_visible;
+}
+
+namespace fmt {
+bool namespace_detail_invisible() {
+#if defined(FMT_HIDE_MODULE_BUGS) && \
+  defined(_MSC_FULL_VER) && _MSC_FULL_VER <= 192930036
+  // bug in msvc 16.10-pre4:
+  // the namespace is visible even when it is neither
+  // implicitly nor explicitly exported
+  return true;
+#else
+  using namespace detail;
+  // this fails to compile if fmt::detail is visible
+  return !oops_detail_namespace_is_visible;
+#endif
+}
+} // namespace fmt
+
+// the non-exported namespace 'detail' must be invisible [module.interface]/2
+TEST(module_test, detail_namespace) {
+  EXPECT_TRUE(fmt::namespace_detail_invisible());
+}
+
+// macros must not be imported from a *named* module  [cpp.import]/5.1
+TEST(module_test, macros) {
+#if defined(FMT_HIDE_MODULE_BUGS) && \
+  defined(_MSC_FULL_VER) && _MSC_FULL_VER <= 192930036
+// bug in msvc 16.10-pre4:
+// include-guard macros leak from BMI
+// and even worse: they cannot be #undef-ined
+  macro_leaked = false;
+#endif
+  EXPECT_FALSE(macro_leaked);
+}