[clang][modules-driver] Add scanner to detect C++20 module presence

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -577,6 +577,16 @@ def err_drv_reduced_module_output_overrided : Warning<
   "please consider use '-fmodule-output=' to specify the output file for reduced BMI explicitly">,
   InGroup<DiagGroup<"reduced-bmi-output-overrided">>;
 
+def remark_found_cxx20_module_usage : Remark<
+  "found C++20 module usage in file '%0'">,
+  InGroup<ModulesDriver>;
+def remark_performing_explicit_module_build : Remark<
+  "performing explicit module build">,
+  InGroup<ModulesDriver>;
+def warn_modules_driver_unsupported_standard : Warning<
+  "'-fmodules-driver' is not supported before C++20">,
+  InGroup<ModulesDriver>;
+
 def warn_drv_delayed_template_parsing_after_cxx20 : Warning<
   "-fdelayed-template-parsing is deprecated after C++20">,
   InGroup<DiagGroup<"delayed-template-parsing-in-cxx20">>;

clang/include/clang/Basic/DiagnosticGroups.td

@@ -625,6 +625,7 @@ def ModuleConflict : DiagGroup<"module-conflict">;
 def ModuleFileExtension : DiagGroup<"module-file-extension">;
 def ModuleIncludeDirectiveTranslation : DiagGroup<"module-include-translation">;
 def ModuleMap : DiagGroup<"module-map">;
+def ModulesDriver : DiagGroup<"modules-driver">;
 def RoundTripCC1Args : DiagGroup<"round-trip-cc1-args">;
 def NewlineEOF : DiagGroup<"newline-eof">;
 def Nullability : DiagGroup<"nullability">;

clang/include/clang/Driver/Driver.h

@@ -505,6 +505,8 @@ class Driver {
 
   /// BuildActions - Construct the list of actions to perform for the
   /// given arguments, which are only done for a single architecture.
+  /// If the compilation is an explicit module build, delegates to
+  /// BuildExplicitModuleBuildActions. Otherwise, uses BuildDefaultActions.
   ///
   /// \param C - The compilation that is being built.
   /// \param Args - The input arguments.
@@ -790,6 +792,35 @@ class Driver {
   /// compilation based on which -f(no-)?lto(=.*)? option occurs last.
   void setLTOMode(const llvm::opt::ArgList &Args);
 
+  /// BuildDefaultActions - Constructs the list of actions to perform
+  /// for the provided arguments, which are only done for a single architecture.
+  ///
+  /// \param C - The compilation that is being built.
+  /// \param Args - The input arguments.
+  /// \param Actions - The list to store the resulting actions onto.
+  void BuildDefaultActions(Compilation &C, llvm::opt::DerivedArgList &Args,
+                           const InputList &Inputs, ActionList &Actions) const;
+
+  /// BuildExplicitModuleBuildActions - Performs a dependency scan and
+  /// constructs the list of actions to perform for dependency order and
+  /// the provided arguments. This is only done for a single a architecture.
+  ///
+  /// \param C - The compilation that is being built.
+  /// \param Args - The input arguments.
+  /// \param Actions - The list to store the resulting actions onto.
+  void BuildExplicitModuleBuildActions(Compilation &C,
+                                       llvm::opt::DerivedArgList &Args,
+                                       const InputList &Inputs,
+                                       ActionList &Actions) const;
+
+  /// Scans the leading lines of the C++ source inputs to detect C++20 module
+  /// usage.
+  ///
+  /// \returns True if module usage is detected, false otherwise, or an error on
+  /// read failure.
+  llvm::ErrorOr<bool>
+  ScanInputsForCXXModuleUsage(const InputList &Inputs) const;
+
   /// Retrieves a ToolChain for a particular \p Target triple.
   ///
   /// Will cache ToolChains for the life of the driver object, and create them

clang/include/clang/Driver/Options.td

@@ -3240,6 +3240,13 @@ def modules_reduced_bmi : Flag<["-"], "fmodules-reduced-bmi">,
   HelpText<"Generate the reduced BMI">,
   MarshallingInfoFlag<FrontendOpts<"GenReducedBMI">>;
 
+def fmodules_driver : Flag<["-"], "fmodules-driver">,
+  Group<f_Group>, Visibility<[ClangOption]>,
+  HelpText<"Enable support for explicit module builds from the driver (experimental)">;
+def fno_modules_driver : Flag<["-"], "fno-modules-driver">,
+  Group<f_Group>, Visibility<[ClangOption]>,
+  HelpText<"Disable support for explicit module builds from the driver (experimental)">;
+
 def experimental_modules_reduced_bmi : Flag<["-"], "fexperimental-modules-reduced-bmi">,
   Group<f_Group>, Visibility<[ClangOption, CC1Option]>, Alias<modules_reduced_bmi>;
 

clang/lib/Driver/Driver.cpp

@@ -53,6 +53,7 @@
 #include "ToolChains/WebAssembly.h"
 #include "ToolChains/XCore.h"
 #include "ToolChains/ZOS.h"
+#include "clang/Basic/CharInfo.h"
 #include "clang/Basic/DiagnosticDriver.h"
 #include "clang/Basic/TargetID.h"
 #include "clang/Basic/Version.h"
@@ -4285,6 +4286,13 @@ void Driver::handleArguments(Compilation &C, DerivedArgList &Args,
     YcArg = nullptr;
   }
 
+  if (Args.hasArgNoClaim(options::OPT_fmodules_driver))
+    // TODO: Check against all incompatible -fmodules-driver arguments
+    if (!ModulesModeCXX20) {
+      Diag(diag::warn_modules_driver_unsupported_standard);
+      Args.eraseArg(options::OPT_fmodules_driver);
+    }
+
   Arg *FinalPhaseArg;
   phases::ID FinalPhase = getFinalPhase(Args, &FinalPhaseArg);
 
@@ -4403,6 +4411,174 @@ void Driver::handleArguments(Compilation &C, DerivedArgList &Args,
   }
 }
 
+static void skipWhitespace(const char *&Ptr) {
+  while (isWhitespace(*Ptr))
+    ++Ptr;
+}
+
+// Returns the length of EOL, either 0 (no end-of-line), 1 (\n) or 2 (\r\n).
+static unsigned isEOL(const char *Ptr) {
+  if (*Ptr == '\0')
+    return 0;
+  if (*(Ptr + 1) != '\0' && isVerticalWhitespace(Ptr[0]) &&
+      isVerticalWhitespace(Ptr[1]) && Ptr[0] != Ptr[1])
+    return 2;
+  return !!isVerticalWhitespace(Ptr[0]);
+}
+
+static void skipLine(const char *&Ptr) {
+  for (;;) {
+    char LastNonWhitespace = ' ';
+    while (!isVerticalWhitespace(*Ptr) && *Ptr != '\0') {
+      if (!isHorizontalWhitespace(*Ptr))
+        LastNonWhitespace = *Ptr;
+      ++Ptr;
+    }
+
+    const unsigned Len = isEOL(Ptr);
+    if (!Len)
+      return;
+
+    Ptr += Len;
+    if (LastNonWhitespace != '\\')
+      break;
+  }
+}
+
+// Returns the length of a line splice sequence (including trailing
+// whitespace), or 0 if no line splice is found.
+static unsigned isLineSplice(const char *Start) {
+  if (*Start != '\\')
+    return 0;
+
+  const char *Ptr = Start + 1;
+  while (isHorizontalWhitespace(*Ptr))
+    ++Ptr;
+
+  if (unsigned Len = isEOL(Ptr))
+    return Ptr - Start + Len;
+  return 0;
+}
+
+static bool trySkipLineSplice(const char *&Ptr) {
+  if (unsigned Len = isLineSplice(Ptr); Len) {
+    Ptr += Len;
+    return true;
+  }
+  return false;
+}
+
+static bool trySkipDirective(const char *&Ptr) {
+  if (*Ptr != '#')
+    return false;
+
+  ++Ptr;
+  skipLine(Ptr);
+  return true;
+}
+
+static bool trySkipLineComment(const char *&Ptr) {
+  if (Ptr[0] != '/' || Ptr[1] != '/')
+    return false;
+
+  Ptr += 2;
+  skipLine(Ptr);
+  return true;
+}
+
+static bool trySkipBlockComment(const char *&Ptr) {
+  if (Ptr[0] != '/' || Ptr[1] != '*')
+    return false;
+
+  Ptr += 2;
+  while (*Ptr != '\0') {
+    if (Ptr[0] == '*' && Ptr[1] == '/') {
+      Ptr += 2; // '*/'
+      return true;
+    }
+    ++Ptr;
+  }
+  return true;
+}
+
+static bool trySkipComment(const char *&Ptr) {
+  return trySkipLineComment(Ptr) || trySkipBlockComment(Ptr);
+}
+
+// Skipps over comments and (non-module) directives
+static void skipToRelevantCXXModuleText(const char *&Ptr) {
+  while (*Ptr != '\0') {
+    skipWhitespace(Ptr);
+    if (trySkipComment(Ptr) || trySkipDirective(Ptr) || trySkipLineSplice(Ptr))
+      continue;
+    break; // Found relevant text!
+  }
+}
+
+static bool scanBufferForCXXModuleUsage(const llvm::MemoryBuffer &Buffer) {
+  const char *Ptr = Buffer.getBufferStart();
+  skipToRelevantCXXModuleText(Ptr);
+
+  // Check if the buffer has enough remaining bytes left for any of the
+  // module-related declaration fragments we are checking for, without making
+  // the potentially memory-mapped buffer load unnecessary pages.
+  constexpr int MinKeywordLength = 6;
+  const char *Begin = Ptr;
+  for (int i = 0; i < MinKeywordLength; ++i) {
+    if (*Ptr == '\0')
+      return false;
+    ++Ptr;
+  }
+  StringRef Text(Begin, MinKeywordLength);
+
+  const bool IsGlobalModule = Text.starts_with("module");
+  if (!IsGlobalModule && !Text.starts_with("import") &&
+      !Text.starts_with("export"))
+    return false;
+
+  // Ensure the keyword has a proper ending and isn't part of a identifier
+  // or namespace. For this we might have to skip comments and line
+  // continuations.
+  while (*Ptr != '\0') {
+    if (isWhitespace(*Ptr) || (IsGlobalModule && *Ptr == ';'))
+      return true;
+    if (trySkipBlockComment(Ptr) || trySkipLineSplice(Ptr))
+      continue;
+    return false;
+  }
+
+  return false;
+}
+
+static bool hasCXXModuleInputType(const Driver::InputList &Inputs) {
+  const auto IsTypeCXXModule = [](const auto &Input) -> bool {
+    const auto TypeID = Input.first;
+    return (TypeID == types::TY_CXXModule);
+  };
+  return llvm::any_of(Inputs, IsTypeCXXModule);
+}
+
+llvm::ErrorOr<bool>
+Driver::ScanInputsForCXXModuleUsage(const InputList &Inputs) const {
+  const auto CXXInputs = llvm::make_filter_range(
+      Inputs, [](const auto &Input) { return types::isCXX(Input.first); });
+
+  for (const auto &Input : CXXInputs) {
+    StringRef Filename = Input.second->getSpelling();
+    auto ErrOrBuffer = VFS->getBufferForFile(Filename);
+    if (!ErrOrBuffer)
+      return ErrOrBuffer.getError();
+    const auto Buffer = std::move(*ErrOrBuffer);
+
+    if (scanBufferForCXXModuleUsage(*Buffer)) {
+      Diags.Report(diag::remark_found_cxx20_module_usage) << Filename;
+      return true;
+    }
+  }
+
+  return false;
+}
+
 void Driver::BuildActions(Compilation &C, DerivedArgList &Args,
                           const InputList &Inputs, ActionList &Actions) const {
   llvm::PrettyStackTraceString CrashInfo("Building compilation actions");
@@ -4414,6 +4590,33 @@ void Driver::BuildActions(Compilation &C, DerivedArgList &Args,
 
   handleArguments(C, Args, Inputs, Actions);
 
+  if (Args.hasFlag(options::OPT_fmodules_driver,
+                   options::OPT_fno_modules_driver, false)) {
+    // TODO: Move the logic for implicitly enabling explicit-module-builds out
+    // of -fmodules-driver once it is no longer experimental.
+    // Currently, this serves diagnostic purposes only.
+    bool UsesCXXModules = hasCXXModuleInputType(Inputs);
+    if (!UsesCXXModules) {
+      const auto ErrOrScanResult = ScanInputsForCXXModuleUsage(Inputs);
+      if (!ErrOrScanResult) {
+        Diags.Report(diag::err_cannot_open_file)
+            << ErrOrScanResult.getError().message();
+        return;
+      }
+      UsesCXXModules = *ErrOrScanResult;
+    }
+    if (UsesCXXModules)
+      BuildExplicitModuleBuildActions(C, Args, Inputs, Actions);
+    return;
+  }
+
+  BuildDefaultActions(C, Args, Inputs, Actions);
+}
+
+void Driver::BuildDefaultActions(Compilation &C, DerivedArgList &Args,
+                                 const InputList &Inputs,
+                                 ActionList &Actions) const {
+
   bool UseNewOffloadingDriver =
       C.isOffloadingHostKind(Action::OFK_OpenMP) ||
       C.isOffloadingHostKind(Action::OFK_SYCL) ||
@@ -4693,6 +4896,14 @@ void Driver::BuildActions(Compilation &C, DerivedArgList &Args,
   Args.ClaimAllArgs(options::OPT_cl_ignored_Group);
 }
 
+void Driver::BuildExplicitModuleBuildActions(Compilation &C,
+                                             llvm::opt::DerivedArgList &Args,
+                                             const InputList &Inputs,
+                                             ActionList &Actions) const {
+  Diags.Report(diag::remark_performing_explicit_module_build);
+  return;
+}
+
 /// Returns the canonical name for the offloading architecture when using a HIP
 /// or CUDA architecture.
 static StringRef getCanonicalArchString(Compilation &C,