[clangd] Mark code action as "preferred" if it's the sole quickfix action

clang-tools-extra/clangd/ClangdLSPServer.cpp

auto ConsumeActions =
         if (!Tweaks)
           return Reply(Tweaks.takeError());
 
         std::vector<CodeAction> Actions = std::move(FixIts);
         Actions.reserve(Actions.size() + Tweaks->size());
         for (const auto &T : *Tweaks)
           Actions.push_back(toCodeAction(T, File, Selection));
 
+        // If there's exactly one quick-fix, call it "preferred".
+        // We never consider refactorings etc as preferred.
+        CodeAction *OnlyFix = nullptr;
+        for (auto &Action : Actions) {
+          if (Action.kind && *Action.kind == CodeAction::QUICKFIX_KIND) {
+            if (OnlyFix) {
+              OnlyFix->isPreferred = false;
+              break;
+            }
+            Action.isPreferred = true;
+            OnlyFix = &Action;
+          }
+        }
+
         if (SupportsCodeAction)
           return Reply(llvm::json::Array(Actions));
         std::vector<Command> Commands;
         for (const auto &Action : Actions) {
           if (auto Command = asCommand(Action))
             Commands.push_back(std::move(*Command));
         }
         return Reply(llvm::json::Array(Commands));

clang-tools-extra/clangd/Diagnostics.cpp

case Diag::ClangTidy:
     break;
   case Diag::Unknown:
     break;
   }
   if (Opts.EmbedFixesInDiagnostics) {
     Main.codeActions.emplace();
     for (const auto &Fix : D.Fixes)
       Main.codeActions->push_back(toCodeAction(Fix, File));
+    if (Main.codeActions->size() == 1)
+      Main.codeActions->front().isPreferred = true;
   }
   if (Opts.SendDiagnosticCategory && !D.Category.empty())
     Main.category = D.Category;
 
   Main.message = mainMessage(D, Opts);
   if (Opts.EmitRelatedLocations) {
     Main.relatedInformation.emplace();
     for (auto &Note : D.Notes) {

clang-tools-extra/clangd/Protocol.h

struct CodeAction {
   llvm::Optional<std::string> kind;
   const static llvm::StringLiteral QUICKFIX_KIND;
   const static llvm::StringLiteral REFACTOR_KIND;
   const static llvm::StringLiteral INFO_KIND;
 
   /// The diagnostics that this code action resolves.
   llvm::Optional<std::vector<Diagnostic>> diagnostics;
 
+  /// Marks this as a preferred action. Preferred actions are used by the
+  /// `auto fix` command and can be targeted by keybindings.
+  /// A quick fix should be marked preferred if it properly addresses the
+  /// underlying error. A refactoring should be marked preferred if it is the
+  /// most reasonable choice of actions to take.
+  bool isPreferred = false;
+
   /// The workspace edit this code action performs.
   llvm::Optional<WorkspaceEdit> edit;
 
   /// A command this code action executes. If a code action provides an edit
   /// and a command, first the edit is executed and then the command.
   llvm::Optional<Command> command;
 };
 llvm::json::Value toJSON(const CodeAction &);

clang-tools-extra/clangd/Protocol.cpp

 const llvm::StringLiteral CodeAction::INFO_KIND = "info";
 
 llvm::json::Value toJSON(const CodeAction &CA) {
   auto CodeAction = llvm::json::Object{{"title", CA.title}};
   if (CA.kind)
     CodeAction["kind"] = *CA.kind;
   if (CA.diagnostics)
     CodeAction["diagnostics"] = llvm::json::Array(*CA.diagnostics);
+  if (CA.isPreferred)
+    CodeAction["isPreferred"] = true;
   if (CA.edit)
     CodeAction["edit"] = *CA.edit;
   if (CA.command)
     CodeAction["command"] = *CA.command;
   return std::move(CodeAction);
 }
 
 llvm::raw_ostream &operator<<(llvm::raw_ostream &O, const DocumentSymbol &S) {

clang-tools-extra/clangd/test/fixits-embed-in-diagnostic.test

 # CHECK-NEXT:                        "character": 17,
 # CHECK-NEXT:                        "line": 0
 # CHECK-NEXT:                      }
 # CHECK-NEXT:                    }
 # CHECK-NEXT:                  }
 # CHECK-NEXT:                ]
 # CHECK-NEXT:              }
 # CHECK-NEXT:            },
+# CHECK-NEXT:            "isPreferred": true,
 # CHECK-NEXT:            "kind": "quickfix",
 # CHECK-NEXT:            "title": "change 'union' to 'struct'"
 # CHECK-NEXT:          }
 # CHECK-NEXT:        ],
 # CHECK-NEXT:        "message": "Use of 'Point' with tag type that does not match previous declaration (fix available)\n\nfoo.c:1:8: note: previous use is here",
 # CHECK-NEXT:        "range": {
 # CHECK-NEXT:          "end": {
 # CHECK-NEXT:            "character": 22,