[clangd] Add a switch to specify a default clangd configuration file

[voertler]

Summary

This patch allows it to set a default configuration as discussed in issue https://github.com/clangd/clangd/issues/845 by adding a startup option --default-config.

[voertler]

Ping

[voertler]

Ping!

[sammccall]

Sorry about the delay, I think this is OK to add.

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

500

why this change? user config is still loaded from those directories

Maybe after the user config text, add "Extra config may be specified by --extra-config-file"

509

"default" doesn't seem like the right term for this, as it's clearly not the default - the user is providing it!

Also, there's also been desire to provide config inline in a flag (as opposed to a file) as this is more self-contained. Calling this config when it's actually a config filename is a bit confusing.

maybe --extra-config-file?

512

I think mentioning --enable-config is more confusing than helpful, since it's on by default.

959

none of this path checking/resolution should be necessary, the fromYAMLFile provider already handles the case where the path does not exist.

We don't expand tildes in paths clangd (or clang, generally) processes.

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

 RetiredFlag<bool> RecoveryASTType("recovery-ast-type");
 RetiredFlag<bool> AsyncPreamble("async-preamble");
 RetiredFlag<bool> CollectMainFileRefs("collect-main-file-refs");
 RetiredFlag<bool> CrossFileRename("cross-file-rename");
 RetiredFlag<std::string> ClangTidyChecks("clang-tidy-checks");
 RetiredFlag<bool> InlayHints("inlay-hints");
 RetiredFlag<bool> FoldingRanges("folding-ranges");
 
-
 opt<int> LimitResults{
     "limit-results",
     cat(Features),
     desc("Limit the number of results returned by clangd. "
          "0 means no limit (default=100)"),
     init(100),
 };
 

 };
 
 opt<bool> EnableConfig{
     "enable-config",
     cat(Misc),
     desc(
         "Read user and project configuration from YAML files.\n"
         "Project config is from a .clangd file in the project directory.\n"
-        "User config is from clangd/config.yaml in the following directories:\n"
+        "User config is from clangd/config.yaml in the following directories \n"
+        "(unless specified with --default-config):\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> DefaultConfig{
+    "default-config",
+    cat(Misc),
+    desc("Path to a default clangd configuration file. A clangd user and "
+         "project configuration has a higher priority (requires "
+         "--enable-config) "),
+    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",
if (EnableConfig) {
     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");
     }
+    if (DefaultConfig.getNumOccurrences()) {
+      llvm::SmallString<256> DefaultConfigPath;
+      if (auto Error = llvm::sys::fs::real_path(
+              DefaultConfig, DefaultConfigPath, /*expand_tilde=*/true)) {
+        elog("Couldn't determine default config file {0}: {1} , not loading",
+             DefaultConfig, Error.message());
+      } else {
+        vlog("Default config file is {0}", DefaultConfigPath);
+        // Give lowest priority to default config 
+        ProviderStack.insert(
+            ProviderStack.begin(),
+            config::Provider::fromYAMLFile(DefaultConfigPath, /*Directory=*/"",
+                                           TFS, /*Trusted=*/true));
+      }
+    }
   }
   ProviderStack.push_back(std::make_unique<FlagsConfigProvider>());
   std::vector<const config::Provider *> ProviderPointers;
   for (const auto &P : ProviderStack)
     ProviderPointers.push_back(P.get());
   Config = config::Provider::combine(std::move(ProviderPointers));
   Opts.ConfigProvider = Config.get();