[clangd] Add an option to specify a workspece-level config file

[daiyousei-qz]

Summary

Add an option "--workspace-config=<path>" to specify a workspace config file, which stands between the in-tree .clangd and the global user config file and offers per-workspace customization.

[sammccall]

I have some concerns about adding an undefined "workspace" concept, and in general extending the config system in a way that's hard to use (because of the need to customize flags per workspace). Maybe best to discuss on the bug? https://github.com/clangd/clangd/issues/1336

clang-tools-extra/clangd/tool/ClangdMain.cpp

opt<bool> PrettyPrint{
     desc("Pretty-print JSON output"),
     init(false),
 };
 
 opt<bool> EnableConfig{
     "enable-config",
     cat(Misc),
     desc(
-        "Read user and project configuration from YAML files.\n"
+        "Read user/workspace/project configuration from YAML files.\n"
         "Project config is from a .clangd file in the project directory.\n"
+        "Workspace config is specified by the --workspace-config option.\n"
         "User config is from clangd/config.yaml in the following directories:\n"
         "\tWindows: %USERPROFILE%\\AppData\\Local\n"
         "\tMac OS: ~/Library/Preferences/\n"
         "\tOthers: $XDG_CONFIG_HOME, usually ~/.config\n"
         "Configuration is documented at https://clangd.llvm.org/config.html"),
     init(true),
 };
 
+opt<std::string> WorkspaceConfig{
+    "workspace-config",
+    cat(Misc),
+    desc("Path to a workspace configuration file."),
+    init(""),
+};
+
 opt<bool> UseDirtyHeaders{"use-dirty-headers", cat(Misc),
                           desc("Use files open in the editor when parsing "
                                "headers instead of reading from the disk"),
                           Hidden,
                           init(ClangdServer::Options().UseDirtyHeaders)};
 
 opt<bool> PreambleParseForwardingFunctions{
     "parse-forwarding-functions",
#endif
   Opts.CodeComplete.RankingModel = RankingModel;
 
   RealThreadsafeFS TFS;
   std::vector<std::unique_ptr<config::Provider>> ProviderStack;
   std::unique_ptr<config::Provider> Config;
   if (EnableConfig) {
     ProviderStack.push_back(
         config::Provider::fromAncestorRelativeYAMLFiles(".clangd", TFS));
+    if (!WorkspaceConfig.empty() && llvm::sys::fs::exists(WorkspaceConfig)) {
+      llvm::SmallString<256> AbsWorkspaceConfigPath =
+          StringRef{WorkspaceConfig.getValue()};
+      llvm::sys::fs::make_absolute("", AbsWorkspaceConfigPath);
+      ProviderStack.push_back(config::Provider::fromYAMLFile(
+          AbsWorkspaceConfigPath, /*Directory=*/"", TFS, /*Trusted=*/true));
+    } else {
+      elog("Couldn't find workspace config file, not loading");
+    }
     llvm::SmallString<256> UserConfig;
     if (llvm::sys::path::user_config_directory(UserConfig)) {
       llvm::sys::path::append(UserConfig, "clangd", "config.yaml");
       vlog("User config file is {0}", UserConfig);
       ProviderStack.push_back(config::Provider::fromYAMLFile(
           UserConfig, /*Directory=*/"", TFS, /*Trusted=*/true));
     } else {
       elog("Couldn't determine user config file, not loading");