Reland [clang][modules-driver] Add scanner to detect C++20 module presence (#147630)

This patch is part of a series to natively support C++20 module usage
from the Clang driver (without requiring an external build system). This
introduces a new scanner that detects C++20 module usage in source files
without using the preprocessor or lexer.

For now, it is enabled only with the `-fmodules-driver` flag and serves
solely diagnostic purposes. In the future, the scanner will be enabled
for any (modules-driver compatible) compilation with two or more inputs,
and will help the driver determine whether to implicitly enable the
modules driver.

Since the scanner adds very little overhead, we are also exploring
enabling it for compilations with only a single input. This approach
could allow us to detect `import std` usage in a single-file
compilation, which would then activate the modules driver. For
performance measurements on this, see
https://github.com/naveen-seth/llvm-dev-cxx-modules-check-benchmark.

RFC:

https://discourse.llvm.org/t/rfc-modules-support-simple-c-20-modules-use-from-the-clang-driver-without-a-build-system

This patch relands commit ded1426. The CI failure is resolved by
removing the compatibility warning for using the `-fmodules-driver` flag
with pre-C++20 standards, which also better aligns its behavior with
other features/flags supported only in newer standards.

Author: naveen-seth

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -577,6 +577,13 @@ 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_driver_managed_module_build : Remark<
+  "performing driver managed module build">,
+  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

@@ -628,6 +628,7 @@ 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

@@ -504,6 +504,9 @@ 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
+  /// BuildDriverManagedModuleBuildActions. Otherwise, BuildDefaultActions is
+  /// used.
   ///
   /// \param C - The compilation that is being built.
   /// \param Args - The input arguments.
@@ -789,6 +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;
+
+  /// BuildDriverManagedModuleBuildActions - 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 BuildDriverManagedModuleBuildActions(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>
+  ScanInputsForCXX20ModulesUsage(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

@@ -3263,6 +3263,13 @@ defm modules_reduced_bmi : BoolOption<"f", "modules-reduced-bmi",
   PosFlag<SetTrue, [], [ClangOption, CC1Option],
           "Generate the reduced BMI">>;
 
+def fmodules_driver : Flag<["-"], "fmodules-driver">,
+  Group<f_Group>, Visibility<[ClangOption]>,
+  HelpText<"Enable support for driver managed module builds (experimental)">;
+def fno_modules_driver : Flag<["-"], "fno-modules-driver">,
+  Group<f_Group>, Visibility<[ClangOption]>,
+  HelpText<"Disable support for driver managed module builds (experimental)">;
+
 def experimental_modules_reduced_bmi : Flag<["-"], "fexperimental-modules-reduced-bmi">,
   Group<f_Group>, Visibility<[ClangOption, CC1Option]>, Alias<fmodules_reduced_bmi>;
 

clang/include/clang/Lex/DependencyDirectivesScanner.h

@@ -136,6 +136,13 @@ void printDependencyDirectivesAsSource(
     ArrayRef<dependency_directives_scan::Directive> Directives,
     llvm::raw_ostream &OS);
 
+/// Scan an input source buffer for C++20 named module usage.
+///
+/// \param Source The input source buffer.
+///
+/// \returns true if any C++20 named modules related directive was found.
+bool scanInputForCXX20ModulesUsage(StringRef Source);
+
 /// Functor that returns the dependency directives for a given file.
 class DependencyDirectivesGetter {
 public:

clang/lib/Driver/Driver.cpp

@@ -66,6 +66,7 @@
 #include "clang/Driver/Tool.h"
 #include "clang/Driver/ToolChain.h"
 #include "clang/Driver/Types.h"
+#include "clang/Lex/DependencyDirectivesScanner.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringExtras.h"
@@ -4290,6 +4291,12 @@ 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) {
+      Args.eraseArg(options::OPT_fmodules_driver);
+    }
+
   Arg *FinalPhaseArg;
   phases::ID FinalPhase = getFinalPhase(Args, &FinalPhaseArg);
 
@@ -4416,6 +4423,35 @@ void Driver::handleArguments(Compilation &C, DerivedArgList &Args,
   }
 }
 
+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::ScanInputsForCXX20ModulesUsage(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 (scanInputForCXX20ModulesUsage(Buffer->getBuffer())) {
+      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");
@@ -4427,6 +4463,33 @@ 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 = ScanInputsForCXX20ModulesUsage(Inputs);
+      if (!ErrOrScanResult) {
+        Diags.Report(diag::err_cannot_open_file)
+            << ErrOrScanResult.getError().message();
+        return;
+      }
+      UsesCXXModules = *ErrOrScanResult;
+    }
+    if (UsesCXXModules)
+      BuildDriverManagedModuleBuildActions(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) ||
@@ -4710,6 +4773,13 @@ void Driver::BuildActions(Compilation &C, DerivedArgList &Args,
   Args.ClaimAllArgs(options::OPT_cl_ignored_Group);
 }
 
+void Driver::BuildDriverManagedModuleBuildActions(
+    Compilation &C, llvm::opt::DerivedArgList &Args, const InputList &Inputs,
+    ActionList &Actions) const {
+  Diags.Report(diag::remark_performing_driver_managed_module_build);
+  return;
+}
+
 /// Returns the canonical name for the offloading architecture when using a HIP
 /// or CUDA architecture.
 static StringRef getCanonicalArchString(Compilation &C,

clang/lib/Lex/DependencyDirectivesScanner.cpp

@@ -83,6 +83,8 @@ struct Scanner {
   /// \returns True on error.
   bool scan(SmallVectorImpl<Directive> &Directives);
 
+  friend bool clang::scanInputForCXX20ModulesUsage(StringRef Source);
+
 private:
   /// Lexes next token and advances \p First and the \p Lexer.
   [[nodiscard]] dependency_directives_scan::Token &
@@ -1075,3 +1077,50 @@ void clang::printDependencyDirectivesAsSource(
     }
   }
 }
+
+static void skipUntilMaybeCXX20ModuleDirective(const char *&First,
+                                               const char *const End) {
+  assert(First <= End);
+  while (First != End) {
+    if (*First == '#') {
+      ++First;
+      skipToNewlineRaw(First, End);
+    }
+    skipWhitespace(First, End);
+    if (const auto Len = isEOL(First, End)) {
+      First += Len;
+      continue;
+    }
+    break;
+  }
+}
+
+bool clang::scanInputForCXX20ModulesUsage(StringRef Source) {
+  const char *First = Source.begin();
+  const char *const End = Source.end();
+  skipUntilMaybeCXX20ModuleDirective(First, End);
+  if (First == End)
+    return false;
+
+  // Check if the next token can even be a module directive before creating a
+  // full lexer.
+  if (!(*First == 'i' || *First == 'e' || *First == 'm'))
+    return false;
+
+  llvm::SmallVector<dependency_directives_scan::Token> Tokens;
+  Scanner S(StringRef(First, End - First), Tokens, nullptr, SourceLocation());
+  if (S.lexModule(First, End))
+    return false;
+  auto IsCXXNamedModuleDirective = [](const DirectiveWithTokens &D) {
+    switch (D.Kind) {
+    case dependency_directives_scan::cxx_module_decl:
+    case dependency_directives_scan::cxx_import_decl:
+    case dependency_directives_scan::cxx_export_module_decl:
+    case dependency_directives_scan::cxx_export_import_decl:
+      return true;
+    default:
+      return false;
+    }
+  };
+  return llvm::any_of(S.DirsWithToks, IsCXXNamedModuleDirective);
+}