[C++20] [Modules] Introduce ForceCheckCXX20ModulesInputFiles options for

C++20 modules

Previously, we banned the check for input files from C++20 modules since
we thought the BMI from C++20 modules should be a standalone artifact.

However, during the recent experiment with clangd for modules, I find
it is necessary to tell whether or not a BMI is out-of-date by checking the
input files especially for language servers.

So this patch brings a header search option
ForceCheckCXX20ModulesInputFiles to allow the tools (concretly, clangd)
to check the input files from BMI.

Author: ChuanqiXu9

clang/include/clang/Driver/Options.td

@@ -2906,6 +2906,11 @@ def fvalidate_ast_input_files_content:
            " Files with mismatching mtime's are considered valid"
            " if both contents is identical">,
   MarshallingInfoFlag<HeaderSearchOpts<"ValidateASTInputFilesContent">>;
+def fforce_check_cxx20_modules_input_files:
+  Flag <["-"], "fforce-check-cxx20-modules-input-files">,
+  Group<f_Group>, Visibility<[ClangOption, CC1Option]>,
+  HelpText<"Check the input source files from C++20 modules explicitly">,
+  MarshallingInfoFlag<HeaderSearchOpts<"ForceCheckCXX20ModulesInputFiles">>;
 def fmodules_validate_input_files_content:
   Flag <["-"], "fmodules-validate-input-files-content">,
   Group<f_Group>, Flags<[NoXarchOption]>,

clang/include/clang/Lex/HeaderSearchOptions.h

@@ -211,6 +211,9 @@ class HeaderSearchOptions {
   // validate consistency.
   unsigned ValidateASTInputFilesContent : 1;
 
+  // Whether the input files from C++20 Modules should be checked.
+  unsigned ForceCheckCXX20ModulesInputFiles : 1;
+
   /// Whether the module includes debug information (-gmodules).
   unsigned UseDebugInfo : 1;
 
@@ -233,7 +236,8 @@ class HeaderSearchOptions {
         UseStandardCXXIncludes(true), UseLibcxx(false), Verbose(false),
         ModulesValidateOncePerBuildSession(false),
         ModulesValidateSystemHeaders(false),
-        ValidateASTInputFilesContent(false), UseDebugInfo(false),
+        ValidateASTInputFilesContent(false),
+        ForceCheckCXX20ModulesInputFiles(false), UseDebugInfo(false),
         ModulesValidateDiagnosticOptions(true), ModulesHashContent(false),
         ModulesStrictContextHash(false) {}
 

clang/lib/Frontend/ASTUnit.cpp

@@ -550,12 +550,16 @@ class ASTInfoCollector : public ASTReaderListener {
   bool ReadHeaderSearchOptions(const HeaderSearchOptions &HSOpts,
                                StringRef SpecificModuleCachePath,
                                bool Complain) override {
-    // Preserve previously set header search paths.
+    // llvm::SaveAndRestore doesn't support bit field.
+    auto ForceCheckCXX20ModulesInputFiles =
+        this->HSOpts.ForceCheckCXX20ModulesInputFiles;
     llvm::SaveAndRestore X(this->HSOpts.UserEntries);
     llvm::SaveAndRestore Y(this->HSOpts.SystemHeaderPrefixes);
     llvm::SaveAndRestore Z(this->HSOpts.VFSOverlayFiles);
 
     this->HSOpts = HSOpts;
+    this->HSOpts.ForceCheckCXX20ModulesInputFiles =
+        ForceCheckCXX20ModulesInputFiles;
 
     return false;
   }

clang/lib/Serialization/ASTReader.cpp

@@ -2428,6 +2428,16 @@ InputFile ASTReader::getInputFile(ModuleFile &F, unsigned ID, bool Complain) {
   // For standard C++ modules, we don't need to check the inputs.
   bool SkipChecks = F.StandardCXXModule;
 
+  const HeaderSearchOptions &HSOpts =
+      PP.getHeaderSearchInfo().getHeaderSearchOpts();
+
+  // The option ForceCheckCXX20ModulesInputFiles is only meaningful for C++20
+  // modules.
+  if (F.StandardCXXModule && HSOpts.ForceCheckCXX20ModulesInputFiles) {
+    SkipChecks = false;
+    Overridden = false;
+  }
+
   OptionalFileEntryRefDegradesToFileEntryPtr File = OptionalFileEntryRef(
       expectedToOptional(FileMgr.getFileRef(Filename, /*OpenFile=*/false)));
 

clang/test/Modules/cxx20-force-check-input.cppm

@@ -0,0 +1,33 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple \
+// RUN:     %t/a.cppm -emit-module-interface -o %t/a.pcm
+//
+// RUN: echo "inline int bar = 46;" >> %t/foo.h
+// RUNX: %clang_cc1 -std=c++20 -triple %itanium_abi_triple \
+// RUNX:     %t/use.cpp -fmodule-file=a=%t/a.pcm -verify -fsyntax-only
+// RUNX: %clang_cc1 -std=c++20 -triple %itanium_abi_triple \
+// RUNX:     %t/a.pcm -emit-llvm -o - | FileCheck %t/a.ll
+//
+// RUN: echo "export int var = 43;" >> %t/a.cppm
+// RUNX: %clang_cc1 -std=c++20 -triple %itanium_abi_triple \
+// RUNX:     %t/use.cpp -fmodule-file=a=%t/a.pcm -verify -fsyntax-only
+// RUNX: %clang_cc1 -std=c++20 -triple %itanium_abi_triple \
+// RUNX:     %t/a.pcm -emit-llvm -o - | FileCheck %t/a.ll
+//
+// RUN: not %clang_cc1 -std=c++20 -triple %itanium_abi_triple \
+// RUN:     -fforce-check-cxx20-modules-input-files %t/a.pcm \
+// RUN:     -emit-llvm -o -  2>&1 | FileCheck %t/a.cppm -check-prefix=CHECK-FAILURE
+
+//--- foo.h
+inline int foo = 43;
+
+//--- a.cppm
+// expected-no-diagnostics
+module;
+#include "foo.h"
+export module a;
+export using ::foo;
+
+// CHECK-FAILURE: fatal error:{{.*}}a.cppm' has been modified since the AST file {{.*}}was built