[clang] Diagnose config_macros before building modules (#83641)

Before this patch, if a module fails to build because of a missing
config_macro, the user will never see the config macro warning. This
patch diagnoses this before building, and each subsequent time a module
is imported.

rdar://123921931

clang/lib/Frontend/CompilerInstance.cpp

@@ -1591,6 +1591,14 @@ static void checkConfigMacro(Preprocessor &PP, StringRef ConfigMacro,
   }
 }
 
+static void checkConfigMacros(Preprocessor &PP, Module *M,
+                              SourceLocation ImportLoc) {
+  clang::Module *TopModule = M->getTopLevelModule();
+  for (const StringRef ConMacro : TopModule->ConfigMacros) {
+    checkConfigMacro(PP, ConMacro, M, ImportLoc);
+  }
+}
+
 /// Write a new timestamp file with the given path.
 static void writeTimestampFile(StringRef TimestampFile) {
   std::error_code EC;
@@ -1829,6 +1837,13 @@ ModuleLoadResult CompilerInstance::findOrCompileModuleAndReadAST(
   Module *M =
       HS.lookupModule(ModuleName, ImportLoc, true, !IsInclusionDirective);
 
+  // Check for any configuration macros that have changed. This is done
+  // immediately before potentially building a module in case this module
+  // depends on having one of its configuration macros defined to successfully
+  // build. If this is not done the user will never see the warning.
+  if (M)
+    checkConfigMacros(getPreprocessor(), M, ImportLoc);
+
   // Select the source and filename for loading the named module.
   std::string ModuleFilename;
   ModuleSource Source =
@@ -2006,12 +2021,23 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
   if (auto MaybeModule = MM.getCachedModuleLoad(*Path[0].first)) {
     // Use the cached result, which may be nullptr.
     Module = *MaybeModule;
+    // Config macros are already checked before building a module, but they need
+    // to be checked at each import location in case any of the config macros
+    // have a new value at the current `ImportLoc`.
+    if (Module)
+      checkConfigMacros(getPreprocessor(), Module, ImportLoc);
   } else if (ModuleName == getLangOpts().CurrentModule) {
     // This is the module we're building.
     Module = PP->getHeaderSearchInfo().lookupModule(
         ModuleName, ImportLoc, /*AllowSearch*/ true,
         /*AllowExtraModuleMapSearch*/ !IsInclusionDirective);
 
+    // Config macros do not need to be checked here for two reasons.
+    // * This will always be textual inclusion, and thus the config macros
+    //   actually do impact the content of the header.
+    // * `Preprocessor::HandleHeaderIncludeOrImport` will never call this
+    //   function as the `#include` or `#import` is textual.
+
     MM.cacheModuleLoad(*Path[0].first, Module);
   } else {
     ModuleLoadResult Result = findOrCompileModuleAndReadAST(
@@ -2146,18 +2172,11 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
     TheASTReader->makeModuleVisible(Module, Visibility, ImportLoc);
   }
 
-  // Check for any configuration macros that have changed.
-  clang::Module *TopModule = Module->getTopLevelModule();
-  for (unsigned I = 0, N = TopModule->ConfigMacros.size(); I != N; ++I) {
-    checkConfigMacro(getPreprocessor(), TopModule->ConfigMacros[I],
-                     Module, ImportLoc);
-  }
-
   // Resolve any remaining module using export_as for this one.
   getPreprocessor()
       .getHeaderSearchInfo()
       .getModuleMap()
-      .resolveLinkAsDependencies(TopModule);
+      .resolveLinkAsDependencies(Module->getTopLevelModule());
 
   LastModuleImportLoc = ImportLoc;
   LastModuleImportResult = ModuleLoadResult(Module);

clang/test/Modules/Inputs/config.h

@@ -1,7 +0,0 @@
-#ifdef WANT_FOO
-int* foo(void);
-#endif
-
-#ifdef WANT_BAR
-char *bar(void);
-#endif

clang/test/Modules/Inputs/module.modulemap

@@ -260,11 +260,6 @@ module cxx_decls_merged {
   header "cxx-decls-merged.h"
 }
 
-module config {
-  header "config.h"
-  config_macros [exhaustive] WANT_FOO, WANT_BAR
-}
-
 module diag_flags {
   header "diag_flags.h"
 }

clang/test/Modules/config_macros.m

@@ -1,3 +1,50 @@
+// This test verifies that config macro warnings are emitted when it looks like
+// the user expected a `#define` to impact the import of a module.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// Prebuild the `config` module so it's in the module cache.
+// RUN: %clang_cc1 -std=c99 -fmodules -fimplicit-module-maps -x objective-c -fmodules-cache-path=%t -DWANT_FOO=1 -emit-module -fmodule-name=config %t/module.modulemap
+
+// Verify that each time the `config` module is imported the current macro state
+// is checked.
+// RUN: %clang_cc1 -std=c99 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %t -DWANT_FOO=1 %t/config.m -verify
+
+// Verify that warnings are emitted before building a module in case the command
+// line macro state causes the module to fail to build.
+// RUN: %clang_cc1 -std=c99 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %t %t/config_error.m -verify
+
+//--- module.modulemap
+
+module config {
+  header "config.h"
+  config_macros [exhaustive] WANT_FOO, WANT_BAR
+}
+
+module config_error {
+  header "config_error.h"
+  config_macros SOME_VALUE
+}
+
+//--- config.h
+
+#ifdef WANT_FOO
+int* foo(void);
+#endif
+
+#ifdef WANT_BAR
+char *bar(void);
+#endif
+
+//--- config_error.h
+
+struct my_thing {
+  char buf[SOME_VALUE];
+};
+
+//--- config.m
+
 @import config;
 
 int *test_foo(void) {
@@ -22,7 +69,8 @@ char *test_bar(void) {
 #define WANT_BAR 1 // expected-note{{macro was defined here}}
 @import config; // expected-warning{{definition of configuration macro 'WANT_BAR' has no effect on the import of 'config'; pass '-DWANT_BAR=...' on the command line to configure the module}}
 
-// RUN: rm -rf %t
-// RUN: %clang_cc1 -std=c99 -fmodules -fimplicit-module-maps -x objective-c -fmodules-cache-path=%t -DWANT_FOO=1 -emit-module -fmodule-name=config %S/Inputs/module.modulemap
-// RUN: %clang_cc1 -std=c99 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs -DWANT_FOO=1 %s -verify
+//--- config_error.m
 
+#define SOME_VALUE 5 // expected-note{{macro was defined here}}
+@import config_error; // expected-error{{could not build module}} \
+                      // expected-warning{{definition of configuration macro 'SOME_VALUE' has no effect on the import of 'config_error';}}