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

[Chuanqi Xu via cfe-commits]

================
@@ -0,0 +1,81 @@
+//===---------------- 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"
+#include "support/Logger.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> ScanningResult =
+      ScanningTool.getP1689ModuleDependencyFile(*Cmd, Cmd->Directory);
+
+  if (auto E = ScanningResult.takeError()) {
+    log("Scanning modules dependencies for {0} failed: {1}", FilePath,
+        llvm::toString(std::move(E)));
+    return std::nullopt;
+  }
+
+  ModuleDependencyInfo Result;
+
+  if (ScanningResult->Provides) {
+    ModuleNameToSource[ScanningResult->Provides->ModuleName] = FilePath;
----------------
ChuanqiXu9 wrote:

We can't make `scan()` const, since we will pass the member variable `Service` as a non-const reference to construct `DependencyScanningTool`.

And we can't (or we don't want) assume the `getRequiredModules()` will be executed after `globalScan()`. We require to run `getSourceForModuleName()` after `globalScan()` since we have to.

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