[clangd] Add "Go to Declaration" functionality

[malaperle-ericsson]

Summary

This change allows to navigate to most identifiers' declarations in code. This is a first step towards implementing "Go to Definition". It reuses clangIndex in order to detect which occurrences corresponds to the position requested. The occurrences' Decls are then used to generate locations suitable for navigating to the declarations.

[ilya-biryukov]

Hi @malaperle-ericsson. Thanks for the patch.
IndexingAction has a very simple interface, exactly what clangd needs and nothing more.

BTW, we had a bug open for that: https://bugs.llvm.org/show_bug.cgi?id=33261, feel free to reassign to yourself.

clangd/ClangdServer.cpp

275

We must call runOnUnit here, calling runOnUnitWithoutReparse will result in an outdated AST.
(This can't happen if you use -run-synchronously, so tests don't catch it).

clangd/ClangdUnit.cpp

303

Minor: Maybe invert condition to reduce nesting, i.e. if (InputLocation != SourceLocationBeg || Pos.character == 0) return SourceLocationBeg;

PS. I'm perfectly fine if this comment is ignored, it's just a matter of preference.

306

Just wondering: is there a way to not do the lexing multiple times?

clangd/DeclarationLocationsFinder.cpp

48

Is it possible for DeclarationLocation to become large, so that quadratic behavior is observed?

If not, maybe add an assert that it's smaller than some threshold?
If yes, maybe use std::set instead of std::vector or use vector and later std::sort and std::unique in the end?

48

Maybe use std::find instead of std::none_of?

std::find(DeclarationLocations.begin(), DeclarationLocations.end(), L) == DeclarationLocations.end()

59

Could we implement handleMacroOccurence instead?
I suspect current code won't work if macro is undef'ed/redefined later:

#define FOO 123

int b = FO|O;

#define FOO 125
// or
// #undef FOO

Maybe also add this to tests?

clangd/DeclarationLocationsFinder.h

24

Maybe put this into ClangdUnit.cpp inside an anonymous namespace?
As it's just an implementation detail.

34

Maybe have a takeLocations() method that 'std::move's the vector instead of copying?

clangd/Protocol.cpp

57

Why this didn't require any changes to other code? This method hasn't been used before?

clangd/Protocol.h

42

Maybe add 'operator != ' for consistency?

98

Maybe add 'operator != ' for consistency?

test/clangd/definitions.test

1

Could we also add more readable tests specifically for that?
I propose to have a tool that could read files like:

int aaa;
int b = a/*{l1}*/aa;
int c = /*{l2}*/b;

And have it output the resulting goto results:

l1 -> {main.cpp:0:4} int |aaa;
l2 -> {main.cpp:1:4} int |b;

And we could also have a tool that prints expected clangd input/output based on that to check that action actually works.
It's not directly relevant to this patch, though. Just random thoughts of what we should do for easier testing.

[malaperle-ericsson]

Address comments

[malaperle-ericsson]

clangd/ClangdUnit.cpp

303

I like it better with the return

306

I was able to simplify this a bit. There's on only one call to getRawToken and one to GetBeginningOfToken.

clangd/DeclarationLocationsFinder.cpp

48

I went with std::sort/std::unique. I don't think this will ever be large but I don't think it would be good to have an arbitrary limit either. I think keeping the vector and cleaning it later is nice and simple.

59

You're right! It didn't work properly in this case. I added a few tests.
For handleMacroOccurence, it actually isn't even called so we'd have to improve the clangIndex code to do this. I think this is a good first step though.

clangd/Protocol.cpp

57

No it wasn't used before.

test/clangd/definitions.test

1

I think it's a good idea! Although I wonder if it's a bit too much work for something that very specific to "textDocument/definition". I fully agree that the tests need to be more readable and it would be worth a try!

[ilya-biryukov]

Looks good now, but I think we definitely need to change unique_ptr<vector<Location>> to vector<Location> before submitting it.

I won't be available until next Wednesday, so feel free to submit without my final approval.

clangd/ClangdUnit.cpp

273

Why do we need to use unique_ptr<vector<Location>> here and in other places instead of vector<Location>?

clangd/DeclarationLocationsFinder.cpp

48

Totally agree, nice and simple.

59

Awesome!
clangIndex indeed never calls handleMacroOccurence, I wonder why it's there in the first place.

test/clangd/definitions.test

1

We could also reuse most of the code for all caret-based action, so it's actually useful for other things as well.

[malaperle-ericsson]

clangd/ClangdUnit.cpp

273

It was to implement "takeLocations". Calling it claims ownship of the vector. Did you have something else in mind when you suggested to implement takeLocations with a std::move? Disclaimer: this c++11 stuff is all new to me so I wouldn't be surprised if I'm doing it in a terrible way.

[malaperle-ericsson]

clangd/ClangdUnit.cpp

273

I guess I can make takeLocations return a vector&& and the other places will return a normal vector RVO will take care of not making copies?

[ilya-biryukov]

clangd/ClangdUnit.cpp

273

Just return std::vector and std::move the original field. It will have the same behaviour (i.e. no copies of the vector data, ownership transferred to the caller) without extra heap allocs:

  std::vector<Location> takeLocations() {
    // .... Do postprocessing here, i.e. sort+unique+erase
    return std::move(DeclarationLocations);
  }
// ...
private:
  std::vector<Location> DeclarationLocations;

[malaperle-ericsson]

Remove use of unique_ptr

[ilya-biryukov]

LGTM

clangd/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS
   Support
   )
 
 add_clang_library(clangDaemon
   ClangdLSPServer.cpp
   ClangdServer.cpp
   ClangdUnit.cpp
   ClangdUnitStore.cpp
+  DeclarationLocationsFinder.cpp
   DraftStore.cpp
   GlobalCompilationDatabase.cpp
   JSONRPCDispatcher.cpp
   Protocol.cpp
   ProtocolHandlers.cpp
 
   LINK_LIBS
   clangAST
   clangBasic
   clangFormat
   clangFrontend
+  clangIndex
   clangSema
   clangTooling
   clangToolingCore
   ${LLVM_PTHREAD_LIB}
   )
 
 add_subdirectory(tool)

clangd/ClangdLSPServer.cpp

public:
   void onDocumentRangeFormatting(DocumentRangeFormattingParams Params,
                                  StringRef ID, JSONOutput &Out) override;
   void onDocumentFormatting(DocumentFormattingParams Params, StringRef ID,
                             JSONOutput &Out) override;
   void onCodeAction(CodeActionParams Params, StringRef ID,
                     JSONOutput &Out) override;
   void onCompletion(TextDocumentPositionParams Params, StringRef ID,
                     JSONOutput &Out) override;
+  void onGoToDefinition(TextDocumentPositionParams Params, StringRef ID,
+                            JSONOutput &Out) override;
 
 private:
   ClangdLSPServer &LangServer;
 };
 
 void ClangdLSPServer::LSPProtocolCallbacks::onInitialize(StringRef ID,
                                                          JSONOutput &Out) {
   Out.writeMessage(
       R"({"jsonrpc":"2.0","id":)" + ID +
       R"(,"result":{"capabilities":{
           "textDocumentSync": 1,
           "documentFormattingProvider": true,
           "documentRangeFormattingProvider": true,
           "documentOnTypeFormattingProvider": {"firstTriggerCharacter":"}","moreTriggerCharacter":[]},
           "codeActionProvider": true,
-          "completionProvider": {"resolveProvider": false, "triggerCharacters": [".",">"]}
+          "completionProvider": {"resolveProvider": false, "triggerCharacters": [".",">"]},
+          "definitionProvider": true
         }}})");
 }
 
 void ClangdLSPServer::LSPProtocolCallbacks::onShutdown(JSONOutput &Out) {
   LangServer.IsDone = true;
 }
 
 void ClangdLSPServer::LSPProtocolCallbacks::onDocumentDidOpen(
void ClangdLSPServer::LSPProtocolCallbacks::onCompletion(
   }
   if (!Completions.empty())
     Completions.pop_back();
   Out.writeMessage(
       R"({"jsonrpc":"2.0","id":)" + ID.str() +
       R"(,"result":[)" + Completions + R"(]})");
 }
 
+void ClangdLSPServer::LSPProtocolCallbacks::onGoToDefinition(
+    TextDocumentPositionParams Params, StringRef ID, JSONOutput &Out) {
+
+  auto Items = LangServer.Server.findDefinitions(
+      Params.textDocument.uri.file,
+      Position{Params.position.line, Params.position.character}).Value;
+
+  std::string Locations;
+  for (const auto &Item : Items) {
+    Locations += Location::unparse(Item);
+    Locations += ",";
+  }
+  if (!Locations.empty())
+    Locations.pop_back();
+  Out.writeMessage(
+      R"({"jsonrpc":"2.0","id":)" + ID.str() +
+      R"(,"result":[)" + Locations + R"(]})");
+}
+
 ClangdLSPServer::ClangdLSPServer(JSONOutput &Out, bool RunSynchronously)
     : Out(Out), DiagConsumer(*this),
       Server(CDB, DiagConsumer, FSProvider, RunSynchronously) {}
 
 void ClangdLSPServer::run(std::istream &In) {
   assert(!IsDone && "Run was called before");
 
   // Set up JSONRPCDispatcher.

clangd/ClangdServer.h

public:
   /// None, they will used only for code completion, i.e. no diagnostics update
   /// will be scheduled and a draft for \p File will not be updated.
   /// If \p OverridenContents is None, contents of the current draft for \p File
   /// will be used.
   /// This method should only be called for currently tracked files.
   Tagged<std::vector<CompletionItem>>
   codeComplete(PathRef File, Position Pos,
                llvm::Optional<StringRef> OverridenContents = llvm::None);
+  /// Get definition of symbol at a specified \p Line and \p Column in \p File.
+  Tagged<std::vector<Location>> findDefinitions(PathRef File, Position Pos);
 
   /// Run formatting for \p Rng inside \p File.
   std::vector<tooling::Replacement> formatRange(PathRef File, Range Rng);
   /// Run formatting for the whole \p File.
   std::vector<tooling::Replacement> formatFile(PathRef File);
   /// Run formatting after a character was typed at \p Pos in \p File.
   std::vector<tooling::Replacement> formatOnType(PathRef File, Position Pos);
 

clangd/ClangdServer.cpp

Units.runOnExistingUnit(File, [&DumpPromise](ClangdUnit &Unit) {
       Unit.dumpAST(ResultOS);
       ResultOS.flush();
 
       DumpPromise.set_value(std::move(Result));
     });
   });
   return DumpFuture.get();
 }
+
+Tagged<std::vector<Location>>
+ClangdServer::findDefinitions(PathRef File, Position Pos) {
+  auto FileContents = DraftMgr.getDraft(File);
+  assert(FileContents.Draft && "findDefinitions is called for non-added document");
+
+  std::vector<Location> Result;
+  auto TaggedFS = FSProvider.getTaggedFileSystem(File);
+  Units.runOnUnitWithoutReparse(
+      File, *FileContents.Draft, CDB, PCHs, TaggedFS.Value, [&](ClangdUnit &Unit) {
+        Result = Unit.findDefinitions(Pos);
+      });
+  return make_tagged(std::move(Result), TaggedFS.Tag);
+}

clangd/ClangdUnit.h

public:
 
   /// Get code completions at a specified \p Line and \p Column in \p File.
   ///
   /// This function is thread-safe and returns completion items that own the
   /// data they contain.
   std::vector<CompletionItem>
   codeComplete(StringRef Contents, Position Pos,
                IntrusiveRefCntPtr<vfs::FileSystem> VFS);
+  /// Get definition of symbol at a specified \p Line and \p Column in \p File.
+  std::vector<Location> findDefinitions(Position Pos);
   /// Returns diagnostics and corresponding FixIts for each diagnostic that are
   /// located in the current file.
   std::vector<DiagWithFixIts> getLocalDiagnostics() const;
 
   /// For testing/debugging purposes. Note that this method deserializes all
   /// unserialized Decls, so use with care.
   void dumpAST(llvm::raw_ostream &OS) const;
 
 private:
   Path FileName;
   std::unique_ptr<ASTUnit> Unit;
   std::shared_ptr<PCHContainerOperations> PCHs;
+
+  SourceLocation getBeginningOfIdentifier(const Position& Pos, const FileEntry* FE) const;
 };
 
 } // namespace clangd
 } // namespace clang
 #endif

clangd/ClangdUnit.cpp

 //===--- ClangdUnit.cpp -----------------------------------------*- C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===---------------------------------------------------------------------===//
 
 #include "ClangdUnit.h"
+
+#include "DeclarationLocationsFinder.h"
+
 #include "clang/Frontend/ASTUnit.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Frontend/Utils.h"
+#include "clang/Index/IndexingAction.h"
+#include <clang/Lex/Lexer.h>
 #include "clang/Tooling/CompilationDatabase.h"
 #include "llvm/Support/Format.h"
 
 using namespace clang::clangd;
 using namespace clang;
 
 ClangdUnit::ClangdUnit(PathRef FileName, StringRef Contents,
                        std::shared_ptr<PCHContainerOperations> PCHs,
for (ASTUnit::stored_diag_iterator D = Unit->stored_diag_begin(),
     Result.push_back({Diag, std::move(FixItsForDiagnostic)});
   }
   return Result;
 }
 
 void ClangdUnit::dumpAST(llvm::raw_ostream &OS) const {
   Unit->getASTContext().getTranslationUnitDecl()->dump(OS, true);
 }
+
+std::vector<Location> ClangdUnit::findDefinitions(Position Pos) {
+  const FileEntry *FE = Unit->getFileManager().getFile(Unit->getMainFileName());
+  if (!FE)
+    return {};
+
+  SourceLocation SourceLocationBeg = getBeginningOfIdentifier(Pos, FE);
+
+  auto DeclLocationsFinder = std::make_shared<DeclarationLocationsFinder>(llvm::errs(),
+      SourceLocationBeg, *Unit);
+  index::IndexingOptions IndexOpts;
+  IndexOpts.SystemSymbolFilter =
+      index::IndexingOptions::SystemSymbolFilterKind::All;
+  IndexOpts.IndexFunctionLocals = true;
+  index::indexASTUnit(*Unit, DeclLocationsFinder, IndexOpts);
+
+  return DeclLocationsFinder->getLocations();
+}
+
+SourceLocation ClangdUnit::getBeginningOfIdentifier(const Position &Pos,
+    const FileEntry *FE) const {
+  // The language server protocol uses zero-based line and column numbers.
+  // Clang uses one-based numbers.
+  SourceLocation InputLocation = Unit->getLocation(FE, Pos.line + 1,
+      Pos.character + 1);
+  SourceManager &SourceMgr = Unit->getSourceManager();
+  SourceLocation SourceLocationBeg = Lexer::GetBeginningOfToken(InputLocation,
+      SourceMgr, Unit->getASTContext().getLangOpts());
+  // Handle when we're right after the end of a token because
+  // GetBeginningOfToken doesn't return the beginning then.
+  // i.e. foo|
+  // Try to go back by one and see if the end of that token matches the input
+  // location.
+  // fo|o -> foo|  good!
+  if (InputLocation == SourceLocationBeg && Pos.character > 0) {
+    SourceLocation PeekBeforeLocation = Unit->getLocation(FE, Pos.line + 1,
+        Pos.character);
+    SourceLocation PeekBeforeLocationEnd = Lexer::getLocForEndOfToken(
+        PeekBeforeLocation, 0, SourceMgr, Unit->getASTContext().getLangOpts());
+    if (InputLocation == PeekBeforeLocationEnd) {
+      Token Result;
+      // Make sure the token we peeked is an identifier, otherwise,
+      // this could match parenthesis, dots, etc.
+      if (!Lexer::getRawToken(PeekBeforeLocation, Result, SourceMgr,
+          Unit->getASTContext().getLangOpts(), false)) {
+        if (Result.is(tok::raw_identifier)) {
+          SourceLocationBeg = Lexer::GetBeginningOfToken(PeekBeforeLocation,
+              SourceMgr, Unit->getASTContext().getLangOpts());
+        }
+      }
+    }
+  }
+  return SourceLocationBeg;
+}

clangd/DeclarationLocationsFinder.h

+//===--- DeclarationLocationsFinder.h - Finds declarations locations-------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_DECLARATIONLOCATIONSFINDER_H_
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_DECLARATIONLOCATIONSFINDER_H_
+
+#include "Protocol.h"
+
+#include "clang/Basic/SourceLocation.h"
+#include "clang/Frontend/ASTUnit.h"
+#include "clang/Index/IndexDataConsumer.h"
+
+namespace clang {
+
+namespace clangd {
+
+/// Finds declarations locations that a given source location refers to.
+class DeclarationLocationsFinder : public index::IndexDataConsumer {
+  std::vector<Location> DeclarationLocations;
+  const SourceLocation &SearchedLocation;
+  ASTUnit &Unit;
+public:
+  DeclarationLocationsFinder(raw_ostream &OS,
+      const SourceLocation &SearchedLocation, ASTUnit &Unit) :
+      SearchedLocation(SearchedLocation), Unit(Unit) {
+  }
+
+  std::vector<Location> getLocations() {
+    return DeclarationLocations;
+  }
+
+  bool handleDeclOccurence(const Decl* D, index::SymbolRoleSet Roles,
+      ArrayRef<index::SymbolRelation> Relations, FileID FID, unsigned Offset,
+      index::IndexDataConsumer::ASTNodeInfo ASTNode) override;
+
+private:
+  bool IsSearchedLocation(FileID FID, unsigned Offset) const {
+    const SourceManager &SourceMgr = Unit.getSourceManager();
+    return SourceMgr.getFileOffset(SearchedLocation) == Offset
+        && SourceMgr.getFileID(SearchedLocation) == FID;
+  }
+
+  void AddDeclarationLocation(const SourceRange& ValSourceRange);
+
+  void finish() override;
+};
+
+} // namespace clangd
+} // namespace clang
+
+#endif

clangd/DeclarationLocationsFinder.cpp

+//===--- DeclarationLocationsFinder.cpp -------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===---------------------------------------------------------------------===//
+
+#include "DeclarationLocationsFinder.h"
+
+#include "clang/Lex/MacroInfo.h"
+#include "clang/Lex/Preprocessor.h"
+
+namespace clang {
+namespace clangd {
+
+bool DeclarationLocationsFinder::handleDeclOccurence(const Decl* D,
+    index::SymbolRoleSet Roles, ArrayRef<index::SymbolRelation> Relations,
+    FileID FID, unsigned Offset,
+    index::IndexDataConsumer::ASTNodeInfo ASTNode) {
+  if (IsSearchedLocation(FID, Offset)) {
+    AddDeclarationLocation(D->getSourceRange());
+  }
+  return true;
+}
+
+void DeclarationLocationsFinder::AddDeclarationLocation(
+    const SourceRange& ValSourceRange) {
+  const SourceManager& SourceMgr = Unit.getSourceManager();
+  const LangOptions& LangOpts = Unit.getLangOpts();
+  SourceLocation LocStart = ValSourceRange.getBegin();
+  SourceLocation LocEnd = Lexer::getLocForEndOfToken(ValSourceRange.getEnd(), 0,
+      SourceMgr, LangOpts);
+  Position P1;
+  P1.line = SourceMgr.getSpellingLineNumber(LocStart) - 1;
+  P1.character = SourceMgr.getSpellingColumnNumber(LocStart) - 1;
+  Position P2;
+  P2.line = SourceMgr.getSpellingLineNumber(LocEnd) - 1;
+  P2.character = SourceMgr.getSpellingColumnNumber(LocEnd) - 1;
+  Range R = { P1, P2 };
+  Location L;
+  L.uri = URI::fromFile(
+      SourceMgr.getFilename(SourceMgr.getSpellingLoc(LocStart)));
+  L.range = R;
+  // Don't add the same location multiple times.
+  // This can happen when nodes in the AST are visited twice.
+  if (std::none_of(DeclarationLocations.begin(), DeclarationLocations.end(),
+      [&L](const Location& Loc) {
+        return L == Loc;
+      })) {
+    DeclarationLocations.push_back(L);
+  }
+}
+
+void DeclarationLocationsFinder::finish() {
+  // Also handle possible macro at the searched location.
+  Token Result;
+  if (!Lexer::getRawToken(SearchedLocation, Result, Unit.getSourceManager(),
+      Unit.getASTContext().getLangOpts(), false)) {
+    if (Result.is(tok::raw_identifier)) {
+      Unit.getPreprocessor().LookUpIdentifierInfo(Result);
+    }
+    IdentifierInfo* IdentifierInfo = Result.getIdentifierInfo();
+    if (IdentifierInfo && IdentifierInfo->hadMacroDefinition()) {
+      MacroDefinition MacroDef = Unit.getPreprocessor().getMacroDefinition(
+          IdentifierInfo);
+      MacroInfo* MacroInf = MacroDef.getMacroInfo();
+      if (MacroInf) {
+        AddDeclarationLocation(
+            SourceRange(MacroInf->getDefinitionLoc(),
+                MacroInf->getDefinitionEndLoc()));
+      }
+    }
+  }
+}
+
+} // namespace clangd
+} // namespace clang

clangd/Protocol.h

struct URI {
   std::string uri;
   std::string file;
 
   static URI fromUri(llvm::StringRef uri);
   static URI fromFile(llvm::StringRef file);
 
   static URI parse(llvm::yaml::ScalarNode *Param);
   static std::string unparse(const URI &U);
+
+  friend bool operator==(const URI &LHS, const URI &RHS) {
+    return LHS.uri == RHS.uri;
+  }
 };
 
 struct TextDocumentIdentifier {
   /// The text document's URI.
   URI uri;
 
   static llvm::Optional<TextDocumentIdentifier>
   parse(llvm::yaml::MappingNode *Params);
struct Range {
   friend bool operator<(const Range &LHS, const Range &RHS) {
     return std::tie(LHS.start, LHS.end) < std::tie(RHS.start, RHS.end);
   }
 
   static llvm::Optional<Range> parse(llvm::yaml::MappingNode *Params);
   static std::string unparse(const Range &P);
 };
 
+struct Location {
+  /// The text document's URI.
+  URI uri;
+  Range range;
+
+  friend bool operator==(const Location &LHS, const Location &RHS) {
+    return LHS.uri == RHS.uri && LHS.range == RHS.range;
+  }
+
+  static std::string unparse(const Location &P);
+};
+
 struct TextEdit {
   /// The range of the text document to be manipulated. To insert
   /// text into a document create a range where start === end.
   Range range;
 
   /// The string to be inserted. For delete operations use an
   /// empty string.
   std::string newText;

clangd/Protocol.cpp

 }
 
 URI URI::parse(llvm::yaml::ScalarNode *Param) {
   llvm::SmallString<10> Storage;
   return URI::fromUri(Param->getValue(Storage));
 }
 
 std::string URI::unparse(const URI &U) {
-  return U.uri;
+  return "\"" + U.uri + "\"";
 }
 
 llvm::Optional<TextDocumentIdentifier>
 TextDocumentIdentifier::parse(llvm::yaml::MappingNode *Params) {
   TextDocumentIdentifier Result;
   for (auto &NextKeyValue : *Params) {
     auto *KeyString = dyn_cast<llvm::yaml::ScalarNode>(NextKeyValue.getKey());
     if (!KeyString)

 std::string Range::unparse(const Range &P) {
   std::string Result;
   llvm::raw_string_ostream(Result) << llvm::format(
       R"({"start": %s, "end": %s})", Position::unparse(P.start).c_str(),
       Position::unparse(P.end).c_str());
   return Result;
 }
 
+std::string Location::unparse(const Location &P) {
+  std::string Result;
+  llvm::raw_string_ostream(Result) << llvm::format(
+      R"({"uri": %s, "range": %s})", URI::unparse(P.uri).c_str(),
+      Range::unparse(P.range).c_str());
+  return Result;
+}
+
 llvm::Optional<TextDocumentItem>
 TextDocumentItem::parse(llvm::yaml::MappingNode *Params) {
   TextDocumentItem Result;
   for (auto &NextKeyValue : *Params) {
     auto *KeyString = dyn_cast<llvm::yaml::ScalarNode>(NextKeyValue.getKey());
     if (!KeyString)
       return llvm::None;
 

clangd/ProtocolHandlers.h

public:
   virtual void onDocumentOnTypeFormatting(DocumentOnTypeFormattingParams Params,
                                           StringRef ID, JSONOutput &Out) = 0;
   virtual void onDocumentRangeFormatting(DocumentRangeFormattingParams Params,
                                          StringRef ID, JSONOutput &Out) = 0;
   virtual void onCodeAction(CodeActionParams Params, StringRef ID,
                             JSONOutput &Out) = 0;
   virtual void onCompletion(TextDocumentPositionParams Params, StringRef ID,
                             JSONOutput &Out) = 0;
+  virtual void onGoToDefinition(TextDocumentPositionParams Params, StringRef ID,
+                            JSONOutput &Out) = 0;
 };
 
 void regiterCallbackHandlers(JSONRPCDispatcher &Dispatcher, JSONOutput &Out,
                              ProtocolCallbacks &Callbacks);
 
 } // namespace clangd
 } // namespace clang
 
 #endif

clangd/ProtocolHandlers.cpp

void handleMethod(llvm::yaml::MappingNode *Params, StringRef ID) override {
 
     Callbacks.onCompletion(*TDPP, ID, Output);
   }
 
 private:
   ProtocolCallbacks &Callbacks;
 };
 
+struct GotoDefinitionHandler : Handler {
+  GotoDefinitionHandler(JSONOutput &Output, ProtocolCallbacks &Callbacks)
+      : Handler(Output), Callbacks(Callbacks) {}
+
+  void handleMethod(llvm::yaml::MappingNode *Params, StringRef ID) override {
+    auto TDPP = TextDocumentPositionParams::parse(Params);
+    if (!TDPP) {
+      Output.log("Failed to decode TextDocumentPositionParams!\n");
+      return;
+    }
+
+    Callbacks.onGoToDefinition(*TDPP, ID, Output);
+  }
+
+private:
+  ProtocolCallbacks &Callbacks;
+};
+
 } // namespace
 
 void clangd::regiterCallbackHandlers(JSONRPCDispatcher &Dispatcher,
                                      JSONOutput &Out,
                                      ProtocolCallbacks &Callbacks) {
   Dispatcher.registerHandler(
       "initialize", llvm::make_unique<InitializeHandler>(Out, Callbacks));
   Dispatcher.registerHandler(
Dispatcher.registerHandler(
       "textDocument/formatting",
       llvm::make_unique<TextDocumentFormattingHandler>(Out, Callbacks));
   Dispatcher.registerHandler(
       "textDocument/codeAction",
       llvm::make_unique<CodeActionHandler>(Out, Callbacks));
   Dispatcher.registerHandler(
       "textDocument/completion",
       llvm::make_unique<CompletionHandler>(Out, Callbacks));
+  Dispatcher.registerHandler("textDocument/definition",
+      llvm::make_unique<GotoDefinitionHandler>(Out, Callbacks));
 }

test/clangd/definitions.test

+# RUN: clangd -run-synchronously < %s | FileCheck %s
+# It is absolutely vital that this file has CRLF line endings.
+#
+Content-Length: 125
+
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
+
+Content-Length: 172
+
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"file:///main.cpp","languageId":"cpp","version":1,"text":"int main() {\nint a;\na;\n}\n"}}}
+
+Content-Length: 148
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":2,"character":0}}}
+# Go to local variable
+# CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 1, "character": 0}, "end": {"line": 1, "character": 5}}}]}
+
+#Content-Length: 148
+# FAILS for now
+#{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":2,"character":1}}}
+# no-CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 1, "character": 0}, "end": {"line": 1, "character": 5}}}]}
+
+Content-Length: 214
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":2},"contentChanges":[{"text":"struct Foo {\nint x;\n};\nint main() {\n  Foo bar = { x : 1 };\n}\n"}]}}
+
+Content-Length: 149
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":4,"character":14}}}
+# Go to field, GNU old-style field designator 
+# CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 1, "character": 0}, "end": {"line": 1, "character": 5}}}]}
+
+Content-Length: 215
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":3},"contentChanges":[{"text":"struct Foo {\nint x;\n};\nint main() {\n  Foo baz = { .x = 2 };\n}\n"}]}}
+
+Content-Length: 149
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":4,"character":15}}}
+# Go to field, field designator 
+# no-CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 1, "character": 0}, "end": {"line": 1, "character": 5}}}]}
+
+Content-Length: 187
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":4},"contentChanges":[{"text":"int main() {\n   main();\n   return 0;\n}"}]}}
+
+Content-Length: 148
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":1,"character":3}}}
+# Go to function declaration, function call 
+# CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 0, "character": 0}, "end": {"line": 3, "character": 1}}}]}
+
+Content-Length: 208
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":5},"contentChanges":[{"text":"struct Foo {\n};\nint main() {\n   Foo bar;\n   return 0;\n}\n"}]}}
+
+Content-Length: 148
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":3,"character":3}}}
+# Go to struct declaration, new struct instance 
+# no-CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 0, "character": 0}, "end": {"line": 1, "character": 1}}}]}
+
+Content-Length: 231
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":5},"contentChanges":[{"text":"namespace n1 {\nstruct Foo {\n};\n}\nint main() {\n   n1::Foo bar;\n   return 0;\n}\n"}]}}
+
+Content-Length: 148
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":5,"character":4}}}
+# Go to struct declaration, new struct instance, qualified name 
+# no-CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 0, "character": 0}, "end": {"line": 3, "character": 100}}}]}
+
+Content-Length: 215
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":6},"contentChanges":[{"text":"struct Foo {\n  int x;\n};\nint main() {\n   Foo bar;\n   bar.x;\n}\n"}]}}
+
+Content-Length: 148
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":5,"character":7}}}
+# Go to field declaration, field reference 
+# CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 1, "character": 2}, "end": {"line": 1, "character": 7}}}]}
+
+Content-Length: 220
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":7},"contentChanges":[{"text":"struct Foo {\n  void x();\n};\nint main() {\n   Foo bar;\n   bar.x();\n}\n"}]}}
+
+Content-Length: 148
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":5,"character":7}}}
+# Go to method declaration, method call 
+# CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 1, "character": 2}, "end": {"line": 1, "character": 10}}}]}
+
+Content-Length: 240
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":7},"contentChanges":[{"text":"struct Foo {\n};\ntypedef Foo TypedefFoo;\nint main() {\n   TypedefFoo bar;\n   return 0;\n}\n"}]}}
+
+Content-Length: 149
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":4,"character":10}}}
+# Go to typedef 
+# no-CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 2, "character": 0}, "end": {"line": 2, "character": 22}}}]}
+
+Content-Length: 254
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":7},"contentChanges":[{"text":"template <typename MyTemplateParam>\nvoid foo() {\n MyTemplateParam a;\n}\nint main() {\n   return 0;\n}\n"}]}}
+
+Content-Length: 149
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":2,"character":13}}}
+# Go to template type parameter. Fails until clangIndex is modified to handle those.
+# no-CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 0, "character": 10}, "end": {"line": 0, "character": 34}}}]}
+
+Content-Length: 256
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":7},"contentChanges":[{"text":"namespace ns {\nstruct Foo {\nstatic void bar() {}\n};\n}\nint main() {\n   ns::Foo::bar();\n   return 0;\n}\n"}]}}
+
+Content-Length: 148
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":6,"character":4}}}
+# Go to namespace, static method call 
+# CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 0, "character": 0}, "end": {"line": 4, "character": 1}}}]}
+
+Content-Length: 265
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":7},"contentChanges":[{"text":"namespace ns {\nstruct Foo {\n  int field;\n  Foo(int param) : field(param) {}\n};\n}\nint main() {\n   return 0;\n}\n"}]}}
+
+Content-Length: 149
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":3,"character":21}}}
+# Go to field, member initializer 
+# CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 2, "character": 2}, "end": {"line": 2, "character": 11}}}]}
+
+Content-Length: 204
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///main.cpp","version":7},"contentChanges":[{"text":"#define MY_MACRO 0\nint main() {\n  return MY_MACRO;\n}\n"}]}}
+
+Content-Length: 148
+
+{"jsonrpc":"2.0","id":1,"method":"textDocument/definition","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":2,"character":9}}}
+# Go to macro
+# CHECK: {"jsonrpc":"2.0","id":1,"result":[{"uri": "file:///main.cpp", "range": {"start": {"line": 0, "character": 8}, "end": {"line": 0, "character": 18}}}]}
+
+Content-Length: 44
+
+{"jsonrpc":"2.0","id":3,"method":"shutdown"}

test/clangd/formatting.test

 # RUN: clangd < %s | FileCheck %s
 # It is absolutely vital that this file has CRLF line endings.
 #
 Content-Length: 125
 
 {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
-# CHECK: Content-Length: 424
+# CHECK: Content-Length: 462
 # CHECK: {"jsonrpc":"2.0","id":0,"result":{"capabilities":{
 # CHECK:   "textDocumentSync": 1,
 # CHECK:   "documentFormattingProvider": true,
 # CHECK:   "documentRangeFormattingProvider": true,
 # CHECK:   "documentOnTypeFormattingProvider": {"firstTriggerCharacter":"}","moreTriggerCharacter":[]},
 # CHECK:   "codeActionProvider": true,
-# CHECK:   "completionProvider": {"resolveProvider": false, "triggerCharacters": [".",">"]}
+# CHECK:   "completionProvider": {"resolveProvider": false, "triggerCharacters": [".",">"]},
+# CHECK:   "definitionProvider": true
 # CHECK: }}}
 #
 Content-Length: 193
 
 {"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"file:///foo.c","languageId":"c","version":1,"text":"int foo ( int x ) {\n    x = x+1;\n    return x;\n    }"}}}
 #
 #
 Content-Length: 233