[clang][modules] Print library module manifest path.

This implements a way for the compiler to find the modules.json
associated with the C++23 Standard library modules.

This is based on a discussion in SG15. At the moment no Standard library
installs this manifest. llvm#75741 adds this feature in libc++.

Author: mordante

clang/include/clang/Driver/Driver.h

@@ -602,6 +602,16 @@ class Driver {
   // FIXME: This should be in CompilationInfo.
   std::string GetProgramPath(StringRef Name, const ToolChain &TC) const;
 
+  /// GetModuleManifestPath - Lookup the name of the Standard library manifest.
+  ///
+  /// \param C - The compilation.
+  /// \param TC - The tool chain for additional information on
+  /// directories to search.
+  //
+  // FIXME: This should be in CompilationInfo.
+  std::string GetModuleManifestPath(const Compilation &C,
+                                    const ToolChain &TC) const;
+
   /// HandleAutocompletions - Handle --autocomplete by searching and printing
   /// possible flags, descriptions, and its arguments.
   void HandleAutocompletions(StringRef PassedFlags) const;

clang/include/clang/Driver/Options.td

@@ -5280,6 +5280,9 @@ def print_resource_dir : Flag<["-", "--"], "print-resource-dir">,
 def print_search_dirs : Flag<["-", "--"], "print-search-dirs">,
   HelpText<"Print the paths used for finding libraries and programs">,
   Visibility<[ClangOption, CLOption]>;
+def print_library_module_manifest_path : Flag<["-", "--"], "print-library-module-manifest-path">,
+  HelpText<"Print the path for the C++ Standard library module manifest">,
+  Visibility<[ClangOption, CLOption]>;
 def print_targets : Flag<["-", "--"], "print-targets">,
   HelpText<"Print the registered targets">,
   Visibility<[ClangOption, CLOption]>;

clang/lib/Driver/Driver.cpp

@@ -2164,6 +2164,12 @@ bool Driver::HandleImmediateArgs(const Compilation &C) {
     return false;
   }
 
+  if (C.getArgs().hasArg(options::OPT_print_library_module_manifest_path)) {
+    llvm::outs() << "module: ="
+                 << GetModuleManifestPath(C, C.getDefaultToolChain()) << '\n';
+    return false;
+  }
+
   if (C.getArgs().hasArg(options::OPT_print_runtime_dir)) {
     if (std::optional<std::string> RuntimePath = TC.getRuntimePath())
       llvm::outs() << *RuntimePath << '\n';
@@ -6135,6 +6141,40 @@ std::string Driver::GetProgramPath(StringRef Name, const ToolChain &TC) const {
   return std::string(Name);
 }
 
+std::string Driver::GetModuleManifestPath(const Compilation &C,
+                                          const ToolChain &TC) const {
+
+  switch (TC.GetCXXStdlibType(C.getArgs())) {
+  case ToolChain::CST_Libcxx: {
+    std::string lib = "libc++.so";
+    std::string path = GetFilePath(lib, TC);
+
+    // Note when there are multiple flavours of libc++ the module json needs to
+    // look at the command-line arguments for the proper json.
+
+    // For example
+    /*
+        const SanitizerArgs &Sanitize = TC.getSanitizerArgs(C.getArgs());
+        if (Sanitize.needsAsanRt())
+          return path.replace(path.size() - lib.size(), lib.size(),
+                              "modules-asan.json");
+    */
+
+    path = path.replace(path.size() - lib.size(), lib.size(), "modules.json");
+    if (TC.getVFS().exists(path))
+      return path;
+
+    return "";
+  }
+
+  case ToolChain::CST_Libstdcxx:
+    // libstdc++ does not provide Standard library modules yet.
+    return "";
+  }
+
+  return "";
+}
+
 std::string Driver::GetTemporaryPath(StringRef Prefix, StringRef Suffix) const {
   SmallString<128> Path;
   std::error_code EC = llvm::sys::fs::createTemporaryFile(Prefix, Suffix, Path);

clang/test/Driver/Inputs/cxx23_modules/usr/lib/x86_64-linux-gnu/libc++.so

@@ -0,0 +1,15 @@
+// Test that -print-library-module-manifest-path finds the correct file.
+
+// RUN: %clang -print-library-module-manifest-path \
+// RUN:     -stdlib=libc++ \
+// RUN:     --sysroot=%S/Inputs/cxx23_modules \
+// RUN:     --target=x86_64-linux-gnu 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-LIBCXX %s
+// CHECK-LIBCXX: module: ={{.*}}/Inputs/cxx23_modules/usr/lib/x86_64-linux-gnu/modules.json
+
+// RUN: %clang -print-library-module-manifest-path \
+// RUN:     -stdlib=libstdc++ \
+// RUN:     --sysroot=%S/Inputs/cxx23_modules \
+// RUN:     --target=x86_64-linux-gnu 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-LIBSTDCXX %s
+// CHECK-LIBSTDCXX: module: =

clang/test/Driver/Inputs/cxx23_modules/usr/lib/x86_64-linux-gnu/modules.json

@@ -0,0 +1,9 @@
+// Test that -print-library-module-manifest-path finds the correct file.
+//
+// Note this file is currently not available on Apple platforms
+
+// RUN: %clang -print-library-module-manifest-path \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     --target=x86_64-unknown-linux-gnu 2>&1 \
+// RUN:   | FileCheck %s
+// CHECK: module: =