[clang-tools-extra] [clangd] [C++20] [Modules] Introduce initial support for C++20 Modules (PR #66462)

[Sam McCall via cfe-commits]

================
@@ -0,0 +1,78 @@
+//===---------------- ModuleDependencyScanner.cpp ----------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "ModuleDependencyScanner.h"
+
+namespace clang {
+namespace clangd {
+
+std::optional<ModuleDependencyScanner::ModuleDependencyInfo>
+ModuleDependencyScanner::scan(PathRef FilePath) {
+  std::optional<tooling::CompileCommand> Cmd = CDB.getCompileCommand(FilePath);
+
+  if (!Cmd)
+    return std::nullopt;
+
+  using namespace clang::tooling::dependencies;
+
+  llvm::SmallString<128> FilePathDir(FilePath);
+  llvm::sys::path::remove_filename(FilePathDir);
+  DependencyScanningTool ScanningTool(Service, TFS.view(FilePathDir));
+
+  llvm::Expected<P1689Rule> P1689Result =
+      ScanningTool.getP1689ModuleDependencyFile(*Cmd, Cmd->Directory);
+
+  if (auto E = P1689Result.takeError()) {
+    // Ignore any error.
+    llvm::consumeError(std::move(E));
+    return std::nullopt;
+  }
+
+  ModuleDependencyInfo Result;
+
+  if (P1689Result->Provides) {
+    ModuleNameToSource[P1689Result->Provides->ModuleName] = FilePath;
+    Result.ModuleName = P1689Result->Provides->ModuleName;
+  }
+
+  for (auto &Required : P1689Result->Requires)
+    Result.RequiredModules.push_back(Required.ModuleName);
+
+  return Result;
+}
+
+void ModuleDependencyScanner::globalScan(
+    const std::vector<std::string> &AllFiles) {
+  for (auto &File : AllFiles)
+    scan(File);
+
+  GlobalScanned = true;
+}
+
+PathRef ModuleDependencyScanner::getSourceForModuleName(StringRef ModuleName) const {
+  assert(
+      GlobalScanned &&
+      "We should only call getSourceForModuleName after calling globalScan()");
+
+  if (!ModuleNameToSource.count(ModuleName))
----------------
sam-mccall wrote:

nit: rather `if (auto It = find(); It != end())` to avoid the double-lookup?

https://github.com/llvm/llvm-project/pull/66462