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

[Chuanqi Xu via cfe-commits]

================
@@ -0,0 +1,78 @@
+//===-------------- ModuleDependencyScanner.h --------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_MODULEDEPENDENCYSCANNER_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_MODULEDEPENDENCYSCANNER_H
+
+#include "GlobalCompilationDatabase.h"
+#include "support/Path.h"
+#include "support/ThreadsafeFS.h"
+
+#include "clang/Tooling/DependencyScanning/DependencyScanningService.h"
+#include "clang/Tooling/DependencyScanning/DependencyScanningTool.h"
+
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringMap.h"
+
+namespace clang {
+namespace clangd {
+
+/// A scanner to produce P1689 format for C++20 Modules.
+///
+/// The scanner can scan a single file with `scan(PathRef)` member function
+/// or scan the whole project with `globalScan(PathRef)` member function. See
+/// the comments of `globalScan` to see the details.
+class ModuleDependencyScanner {
+public:
+  ModuleDependencyScanner(const GlobalCompilationDatabase &CDB,
+                          const ThreadsafeFS *TFS)
+      : CDB(CDB), TFS(TFS),
+        Service(tooling::dependencies::ScanningMode::CanonicalPreprocessing,
+                tooling::dependencies::ScanningOutputFormat::P1689) {}
+
+  /// Scanning the single file specified by \param FilePath.
+  std::optional<clang::tooling::dependencies::P1689Rule> scan(PathRef FilePath);
----------------
ChuanqiXu9 wrote:

Yeah, I added `ModuleDependencyInfo` struct.

```
struct ModuleDependencyInfo {
    // The name of the module if the file is a module unit.
    std::optional<std::string> ModuleName;
    // A list of names for the modules that the file directly depends.
    std::vector<std::string> RequiredModules;
  };
```

The complete P1689 information may be redundant for us.

I didn't add the suggested interfaces. Since I found the following two interfaces are sufficient and more straight forward in practice (from my mind. I feel like we don't need to look at the definition of `ModuleNode` now when we use the scanner):

```
PathRef getSourceForModuleName(StringRef ModuleName);
std::vector<std::string> getRequiredModules(PathRef File);
```



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