[clangd] Code hover for Clangd

[Nebiroth]

Summary

Implemention of Code Hover as described in LSP definition.

[malaperle]

clangd/ClangdUnit.cpp

1117 ↗

(On Diff #124116)

I think we need to simplify this part a bit. I'll try to find a way. Feel free to wait until more comments before updating.

[malaperle]

clangd/ClangdUnit.cpp

1173 ↗

(On Diff #124116)

I think it has to do with macro expansions, the plot thickens..

[malaperle]

clangd/ClangdUnit.cpp

1173 ↗

(On Diff #124116)

ObjCMethodDecl is forward-declared in astfwd.h as part of a macro expansion. We have to either use the expansion loc or spelling loc to get the appropriate file entry. Probably expansion loc makes more sense but I'll test a bit.

[malaperle]

clangd/Protocol.h

454

The doc should use /// like the others

[malaperle]

clangd/ClangdUnit.cpp

1135 ↗

(On Diff #124116)

Not sure why but I don't get the comments when I hover on "push_back" but I do get it on many other methods. This can be investigated separately I think. It could be a bug in getRawCommentForDeclNoCache

[malaperle]

clangd/ClangdUnit.h

320 ↗

(On Diff #124116)

I think this can be only in the source file, in a an anonymous namespace.

[malaperle]

clangd/ClangdUnit.cpp

1029 ↗

(On Diff #124116)

const FileEntry *F =
        SourceMgr.getFileEntryForID(SourceMgr.getFileID(LocStart));
if (!F)
  return llvm::errc::no_such_file_or_directory;

1039 ↗

(On Diff #124116)

FE can be null. How about returning a llvm::ErrorOr ?

1242 ↗

(On Diff #124116)

FE can be null. How about returning a llvm::ErrorOr ?

[Nebiroth]

Invalid FileEntries now return llvm:ErrorOr

[malaperle]

clangd/ClangdUnit.cpp

1117 ↗

(On Diff #124116)

Here's the version in which I tried to simplify this *a bit*. With the new ErrorOr checks as well.

Hover clangd::getHover(ParsedAST &AST, Position Pos, clangd::Logger &Logger) {
  const SourceManager &SourceMgr = AST.getASTContext().getSourceManager();
  const LangOptions &LangOpts = AST.getASTContext().getLangOpts();
  const FileEntry *FE = SourceMgr.getFileEntryForID(SourceMgr.getMainFileID());
  if (!FE)
    return Hover();

  SourceLocation SourceLocationBeg = getBeginningOfIdentifier(AST, Pos, FE);

  auto DeclMacrosFinder = std::make_shared<DeclarationAndMacrosFinder>(
      llvm::errs(), SourceLocationBeg, AST.getASTContext(),
      AST.getPreprocessor());
  index::IndexingOptions IndexOpts;
  IndexOpts.SystemSymbolFilter =
      index::IndexingOptions::SystemSymbolFilterKind::All;
  IndexOpts.IndexFunctionLocals = true;
  indexTopLevelDecls(AST.getASTContext(), AST.getTopLevelDecls(),
                     DeclMacrosFinder, IndexOpts);

  auto Decls = DeclMacrosFinder->takeDecls();
  if (!Decls.empty() && Decls[0]) {
    const Decl *LocationDecl = Decls[0];
    std::vector<MarkedString> HoverContents;

    // Compute scope as the first "section" of the hover.
    if (const NamedDecl *NamedD = dyn_cast<NamedDecl>(LocationDecl)) {
      std::string Scope = getScope(NamedD, AST.getASTContext().getLangOpts());
      if (!Scope.empty()) {
        MarkedString Info = {"", "C++", "In " + Scope};
        HoverContents.push_back(Info);
      }
    }

    // Adjust beginning of hover text depending on the presence of templates and comments.
    TemplateDecl * TDec = LocationDecl->getDescribedTemplate();
    const Decl * BeginDecl = TDec ? TDec : LocationDecl;
    SourceLocation BeginLocation = BeginDecl->getSourceRange().getBegin();
    RawComment *RC = AST.getASTContext().getRawCommentForDeclNoCache(
        BeginDecl);
    if (RC)
      BeginLocation = RC->getLocStart();

    // Adjust end of hover text for things that have braces/bodies. We don't
    // want to show the whole definition of a function, class, etc.
    SourceLocation EndLocation = LocationDecl->getSourceRange().getEnd();
    if (auto FuncDecl = dyn_cast<FunctionDecl>(LocationDecl)) {
      if (FuncDecl->isThisDeclarationADefinition() && FuncDecl->getBody())
        EndLocation = FuncDecl->getBody()->getLocStart();
    } else if (auto TagDeclaration = dyn_cast<TagDecl>(LocationDecl)) {
      if (TagDeclaration->isThisDeclarationADefinition())
        EndLocation = TagDeclaration->getBraceRange().getBegin();
    } else if (auto NameDecl = dyn_cast<NamespaceDecl>(LocationDecl)) {
      SourceLocation BeforeLBraceLoc = Lexer::getLocForEndOfToken(
          LocationDecl->getLocation(), 0, SourceMgr, LangOpts);
      if (BeforeLBraceLoc.isValid())
        EndLocation = BeforeLBraceLoc;
    }

    SourceRange SR(BeginLocation, EndLocation);
    if (SR.isValid()) {
      auto L = getDeclarationLocation(AST, SR);
      if (L) {
        auto Ref = getBufferDataFromSourceRange(AST, *L);
        if (Ref) {
          Hover H;
          if (SourceMgr.isInMainFile(BeginLocation))
            H.range = L->range;
          MarkedString MS = {"", "C++", *Ref};
          HoverContents.push_back(MS);
          H.contents = std::move(HoverContents);
          return H;
        }
      }
    }
  }

  auto MacroInfos = DeclMacrosFinder->takeMacroInfos();
  if (!MacroInfos.empty() && MacroInfos[0]) {
    const MacroInfo *MacroInf = MacroInfos[0];
    SourceRange SR(MacroInf->getDefinitionLoc(),
                                 MacroInf->getDefinitionEndLoc());
    if (SR.isValid()) {
      auto L = getDeclarationLocation(AST, SR);
      if (L) {
        auto Ref = getBufferDataFromSourceRange(AST, *L);
        if (Ref) {
          Hover H;
          if (SourceMgr.isInMainFile(SR.getBegin()))
            H.range = L->range;
          MarkedString MS = {"", "C++", "#define " + Ref->str()};
          H.contents.push_back(MS);
          return H;
        }
      }
    }
  }

  return Hover();
}

1172 ↗

(On Diff #124116)

The range could be in another file but we can only highlight things in the file that the user current has open. For example, if I'm foo.cpp and I hover on something declared in foo.h, it will change the background color in foo.cpp but using the line numbers of foo.h! The protocol should be more clear about this but since there are no Uri in the Hover struct, we can assume this range should only apply to the open file, i.e. the main file. So I suggest this check:

if (SourceMgr.isInMainFile(BeginLocation))
  H.range = L->range;

[malaperle]

clangd/Protocol.cpp

367

remove, we have to return the contents of the H that was passed as parameter, not a new one. I hit this bug while testing with no range (hover text in another file)

[malaperle]

I copy/pasted the comments again so that you don't miss them.

clangd/ClangdUnit.cpp

1117 ↗

(On Diff #124116)

Here's the version in which I tried to simplify this *a bit*. With the new ErrorOr checks as well.

Hover clangd::getHover(ParsedAST &AST, Position Pos, clangd::Logger &Logger) {
  const SourceManager &SourceMgr = AST.getASTContext().getSourceManager();
  const LangOptions &LangOpts = AST.getASTContext().getLangOpts();
  const FileEntry *FE = SourceMgr.getFileEntryForID(SourceMgr.getMainFileID());
  if (!FE)
    return Hover();

  SourceLocation SourceLocationBeg = getBeginningOfIdentifier(AST, Pos, FE);

  auto DeclMacrosFinder = std::make_shared<DeclarationAndMacrosFinder>(
      llvm::errs(), SourceLocationBeg, AST.getASTContext(),
      AST.getPreprocessor());
  index::IndexingOptions IndexOpts;
  IndexOpts.SystemSymbolFilter =
      index::IndexingOptions::SystemSymbolFilterKind::All;
  IndexOpts.IndexFunctionLocals = true;
  indexTopLevelDecls(AST.getASTContext(), AST.getTopLevelDecls(),
                     DeclMacrosFinder, IndexOpts);

  auto Decls = DeclMacrosFinder->takeDecls();
  if (!Decls.empty() && Decls[0]) {
    const Decl *LocationDecl = Decls[0];
    std::vector<MarkedString> HoverContents;

    // Compute scope as the first "section" of the hover.
    if (const NamedDecl *NamedD = dyn_cast<NamedDecl>(LocationDecl)) {
      std::string Scope = getScope(NamedD, AST.getASTContext().getLangOpts());
      if (!Scope.empty()) {
        MarkedString Info = {"", "C++", "In " + Scope};
        HoverContents.push_back(Info);
      }
    }

    // Adjust beginning of hover text depending on the presence of templates and comments.
    TemplateDecl * TDec = LocationDecl->getDescribedTemplate();
    const Decl * BeginDecl = TDec ? TDec : LocationDecl;
    SourceLocation BeginLocation = BeginDecl->getSourceRange().getBegin();
    RawComment *RC = AST.getASTContext().getRawCommentForDeclNoCache(
        BeginDecl);
    if (RC)
      BeginLocation = RC->getLocStart();

    // Adjust end of hover text for things that have braces/bodies. We don't
    // want to show the whole definition of a function, class, etc.
    SourceLocation EndLocation = LocationDecl->getSourceRange().getEnd();
    if (auto FuncDecl = dyn_cast<FunctionDecl>(LocationDecl)) {
      if (FuncDecl->isThisDeclarationADefinition() && FuncDecl->getBody())
        EndLocation = FuncDecl->getBody()->getLocStart();
    } else if (auto TagDeclaration = dyn_cast<TagDecl>(LocationDecl)) {
      if (TagDeclaration->isThisDeclarationADefinition())
        EndLocation = TagDeclaration->getBraceRange().getBegin();
    } else if (auto NameDecl = dyn_cast<NamespaceDecl>(LocationDecl)) {
      SourceLocation BeforeLBraceLoc = Lexer::getLocForEndOfToken(
          LocationDecl->getLocation(), 0, SourceMgr, LangOpts);
      if (BeforeLBraceLoc.isValid())
        EndLocation = BeforeLBraceLoc;
    }

    SourceRange SR(BeginLocation, EndLocation);
    if (SR.isValid()) {
      auto L = getDeclarationLocation(AST, SR);
      if (L) {
        auto Ref = getBufferDataFromSourceRange(AST, *L);
        if (Ref) {
          Hover H;
          if (SourceMgr.isInMainFile(BeginLocation))
            H.range = L->range;
          MarkedString MS = {"", "C++", *Ref};
          HoverContents.push_back(MS);
          H.contents = std::move(HoverContents);
          return H;
        }
      }
    }
  }

  auto MacroInfos = DeclMacrosFinder->takeMacroInfos();
  if (!MacroInfos.empty() && MacroInfos[0]) {
    const MacroInfo *MacroInf = MacroInfos[0];
    SourceRange SR(MacroInf->getDefinitionLoc(),
                                 MacroInf->getDefinitionEndLoc());
    if (SR.isValid()) {
      auto L = getDeclarationLocation(AST, SR);
      if (L) {
        auto Ref = getBufferDataFromSourceRange(AST, *L);
        if (Ref) {
          Hover H;
          if (SourceMgr.isInMainFile(SR.getBegin()))
            H.range = L->range;
          MarkedString MS = {"", "C++", "#define " + Ref->str()};
          H.contents.push_back(MS);
          return H;
        }
      }
    }
  }

  return Hover();
}

1172 ↗

(On Diff #124116)

The range could be in another file but we can only highlight things in the file that the user current has open. For example, if I'm foo.cpp and I hover on something declared in foo.h, it will change the background color in foo.cpp but using the line numbers of foo.h! The protocol should be more clear about this but since there are no Uri in the Hover struct, we can assume this range should only apply to the open file, i.e. the main file. So I suggest this check:

if (SourceMgr.isInMainFile(BeginLocation))
  H.range = L->range;

[malaperle]

clangd/ClangdUnit.cpp

1071 ↗

(On Diff #124833)

llvm::ErrorOr<Location>

1075 ↗

(On Diff #124833)

To fix the macro expansion crash:

SourceLocation LocStart = SourceMgr.getExpansionLoc(ValSourceRange.getBegin());
const FileEntry *F =
        SourceMgr.getFileEntryForID(SourceMgr.getFileID(LocStart));

and simple early return

if (!F)
  return llvm::errc::no_such_file_or_directory;

1079 ↗

(On Diff #124833)

getExpansionLineNumber

1080 ↗

(On Diff #124833)

getExpansionColumnNumber

1082 ↗

(On Diff #124833)

getExpansionLineNumber

1083 ↗

(On Diff #124833)

getExpansionColumnNumber

1087 ↗

(On Diff #124833)

You can do this earlier and simpler, see comment above.

[malaperle]

clangd/ClangdUnit.cpp

1117 ↗

(On Diff #124116)

if (SourceMgr.isInMainFile(SR.getBegin()))
  H.range = L->range;

This is not quite right in case you have a macro definition in the preamble but still in the same "document" (they are different FileIDs for the SourceManager!). For example:

#include "foo.h"
#define MY_MACRO 1

void bar() {
   MY_MACRO;
}

The #define is *also* part of the preamble so isMainFile will return false. So in that case, I'm not sure what's the best way to make sure the macro expansion and the definition are in the same file but comparing FileEntry should work although a bit verbose:

FileID FID = SourceMgr.getFileID(SourceMgr.getExpansionLoc(SR.getBegin()));
bool IsInMainFile = FID.isValid() && SourceMgr.getMainFileID() == FID;
if (!IsInMainFile) {
  // Special case when a macro is defined in a preamble, it could still be in the "main file", below the inclusions.
  // The SourceManager considers the preamble and the main file as two different FileIDs but the FileEntries should match.
  bool IsInPreamble = FID == SourceMgr.getPreambleFileID();
  if (IsInPreamble)
    IsInMainFile = SourceMgr.getFileEntryForID(SourceMgr.getMainFileID()) == SourceMgr.getFileEntryForID(FID);
}
if (IsInMainFile)
  H.range = L->range;

[Nebiroth]

Simplified getHover() function
Proper usage of ErrorOr to return errors
Given range for Hover struct now only applies to the open file
Fixed crash on MacroExpansion

[malaperle]

clangd/ClangdLSPServer.cpp

284

remove extra line

clangd/ClangdUnit.cpp

1054 ↗

(On Diff #124945)

else is not needed because it returns in the if

1080 ↗

(On Diff #124945)

else is not needed since you return in the if

1209 ↗

(On Diff #124945)

else
           H.range = Range();

Remove this, once the bug in Protocol.cpp is fixed, the Range will be truly optional.

1247 ↗

(On Diff #124945)

else
           H.range = Range();

Remove this, once the bug in Protocol.cpp is fixed, the Range will be truly optional.

1505 ↗

(On Diff #124945)

revert this change

clangd/Protocol.cpp

367

remove, we have to return the contents of the H that was passed as parameter, not a new one. I hit this bug while testing with no range (hover text in another file)

So this should be

if (H.range.hasValue()) {
    return json::obj{
        {"contents", json::ary(H.contents)},
        {"range", H.range.getValue()},
    };
  }

  return json::obj{
    {"contents", json::ary(H.contents)},
};

clangd/Protocol.h

30

revert this change?

482

Documentation should use /// like the others

487

Documentation should use /// like the others

[Nebiroth]

clangd/Protocol.h

30

#include <string> is not needed.

[malaperle]

clangd/Protocol.h

30

I meant removing YAMLParser.h

[Nebiroth]

clangd/Protocol.h

30

That's what I figured. I'll remove that.

[Nebiroth]

Minor code cleanup
Merge with master

[malaperle]

This looks good to me. @ilya-biryukov Any objections?
I think we can do further iterations later to handle macros and other specific cases (multiple Decls at the position, etc) . It's already very useful functionality.

[malaperle]

Needs to be rebased.

[Nebiroth]

Rebase on master

[ilya-biryukov]

clangd/ClangdLSPServer.cpp

328

NIT: remove empty line

clangd/ClangdServer.cpp

352–353

We don't need to capture this, remove it from the capture list.

440–441

Return an error instead of asserting to follow how other functions do that:

if (!Resources)
  return llvm::make_error<llvm::StringError>(
      "findHover called on non-added file",
      llvm::errc::invalid_argument);

clangd/ClangdServer.h

232

Please put this right after findDocumentHighlights. It is really weird to see findHover in the middle of formatting functions.

235

These functions are duplicates of existing ones, they should not be here.
Bad merge?

clangd/ClangdUnit.cpp

222 ↗

(On Diff #127131)

NIT: remove the empty line

288 ↗

(On Diff #127131)

NIT: remove the empty line

303 ↗

(On Diff #127131)

Bad merge? This comment should not be changed.

308 ↗

(On Diff #127131)

std::move(Decls) is better. Bad merge?

313 ↗

(On Diff #127131)

This comment is not upstream now and we're removed it in the other patch. Remove it here too.

438 ↗

(On Diff #127131)

We should avoid doing the string-matching when we have the AST.
Use something like this instead (haven't tested, but you should get the idea):

/// Returns a NamedDecl that could be used to print the qualifier.
Decl* getScopeOfDecl(Decl* D) {
  // Code from NamedDecl::printQualifiedName, we should probably move it into 
  const DeclContext *Ctx = D->getDeclContext();

  // For ObjC methods, look through categories and use the interface as context.
  if (auto *MD = dyn_cast<ObjCMethodDecl>(D))
    if (auto *ID = MD->getClassInterface())
      Ctx = ID;

  if (Ctx->isFunctionOrMethod()) {
    /// This is a function-local entity.
    return nullptr;
  }

  return Ctx;
}


std::string printScopeOfDecl(Decl* Scope) {
  if (isa<TranslationUnitDecl>(Scope))
    return "global namespace";

  if (isa<NamedDecl>(Scope))
    return cast<NamedDecl>(Scope)->printQualifiedName();

  return "";
}


// Later use like this:
auto ScopeText = printScopeOfDecl(getScopeOfDecl(D));

462 ↗

(On Diff #127131)

NIT: llvm::Expected instead of llvm::ErrorOr

485 ↗

(On Diff #127131)

NIT: llvm::Expected instead of llvm::ErrorOr

498 ↗

(On Diff #127131)

Why are we changing the semantics here? Why should we use expansion locations instead of spelling locations here?

591 ↗

(On Diff #127131)

This code seems pretty involved. Don't RawComments handle this case?

658 ↗

(On Diff #127131)

Body of this if statement is a great candidate to be extracted into a separate function. Same for macros. The code is really hard to read as it is right now.

std::vector<MarkedString> getHoverContents(Decl *D);
std::vector<MarkedString> getHoverContents(MacroDef *M);
`

670 ↗

(On Diff #127131)

the indent is off here. Use clang-format.

968 ↗

(On Diff #127131)

This should clearly go before the comment!
Also, we're currently setting this flag when building the preamble, but not when parsing the AST. We should definitely do that in both cases.

clangd/Protocol.h

454

NIT: remove empty line

480

NIT: remove empty line

483

NIT: remove empty comment line

489

NIT: remove empty comment line

test/clangd/initialize-params-invalid.test

2

indent is off by one

test/clangd/initialize-params.test

2

indent is off by one

[Nebiroth]

clangd/ClangdServer.h

235

Yes, this is a bad merge.

clangd/ClangdUnit.cpp

438 ↗

(On Diff #127131)

I'm not sure I understand. The intention was to have a separate MarkedString display the scope for whatever we are hovering on. The way I understand the above code, this would display a scope that isn't as precise as I would like which defeats the purpose of having this feature in the first place.

498 ↗

(On Diff #127131)

Bad merge.

591 ↗

(On Diff #127131)

Bad merge.

968 ↗

(On Diff #127131)

Do you mean also adding this line somewhere in ASTUnit.cpp?

[Nebiroth]

findHover properly returns an error
Removed many cases of bad merging
Separated getHover in two functions
Minor indenting and NIT fixes

[sammccall]

clangd/ClangdUnit.cpp

941 ↗

(On Diff #127413)

Doesn't this slow down parsing by a lot? I thought @ilya-biryukov had seen that it was responsible for big performance losses in code complete.

ISTM we might prefer to look up the comment from the index by USR, and just live without it if we can't find it.

(Apologies if this has been discussed)

clangd/ClangdUnit.h

1 ↗

(On Diff #127413)

Please don't cram more stuff into clangdunit that's not directly related to building ASTs and managing their lifetimes. Instead, can we pull as much as possible out into Documentation.h or similar?

Note not Hover.h as there are other use cases, like we should use this logic for the code completion item documentation too.

test/clangd/hover.test

2

Could I ask you to do primary/exhaustive testing of this with unit tests, and just a basic smoke test with lit?
Then it's possible to read the tests, understand whether they're correct, and understand the failure messages.

I'm trying to pull more of the features in this direction - see CodeCompleteTests.cpp for an example of how this can work.

We've had quite a few cases of bugs that had tests, but nobody noticed that the tests were wrong because it's just not possible to read them.

[simark]

New version

Here's a new version of the patch, quite reworked. I scaled down the scope a
little bit to make it simpler (see commit log). We'll add more features later
on.

[ilya-biryukov]

Thanks for picking this up!
I have just one major comment related to how we generate the hover text.

Current implementation tries to get the actual source code for every declaration and then patch it up to look nice on in the output.
It is quite a large piece of code (spanning most of the patch) and tries to mix and match source code and output from the AST when there's not enough information in the AST (e.g. tryFixupIndentation and stripOpeningBracket). This approach is really complex as we'll inevitably try to parse "parts" of C++ and figure out how to deal with macros, etc.
Worse than that, I would expect this code to grow uncontrollably with time and be very hard to maintain.
Moreover, in presence of macros it arguably gives non-useful results. For example, consider this:

#define DEFINITIONS int foo(); int bar(); int baz();

// somewhere else
DEFINITIONS

// somewhere else
int test() {
  foo(); // <-- hover currently doesn't work here. And even if it did, showing a line with just DEFINITIONS is not that useful.
}

I suggest we move to a different approach of pretty-printing the relevant parts of the AST. It is already implemented in clang, handles all the cases in the AST (and will be updated along when AST is changed), shows useful information in presence of macros and is much easier to implement.
The version of getHoverContents using this is just a few lines of code:

static std::string getHoverContents(const Decl *D) {
  if (TemplateDecl *TD = D->getDescribedTemplate())
    D = TD; // we want to see the template bits.

  std::string Result;
  llvm::raw_string_ostream OS(Result);

  PrintingPolicy Policy(D->getASTContext().getLangOpts());
  Policy.TerseOutput = true;
  D->print(OS, Policy);

  OS.flush();
  return Result;
}

It doesn't add the "Defined in ..." piece, but illustrates the APIs we can use. We should use the same APIs for the scope as well, avoiding fallbacks to manual printing if we can.
If there's something missing/wrong in the upstream pretty printers, it's fine and we can fix them (e.g., the thing that I've immediately noticed is that namespaces are always printed with curly braces).

clangd/ClangdLSPServer.cpp

328

NIT: remove the empty line at the start of function

clangd/Protocol.cpp

360

NIT: use nullptr instead of NULL
(irrelevant if we use StringRef, see other comment below)

367

• StringRef is a nicer abstraction, maybe use it instead of const char*?
• Maybe move declaration of KindStr closer to its usage?
• Maybe extract a helper function to mark the code path for invalid kinds as unreachable? I.e.

static StringRef toTextKind(MarkupKind Kind) {
  switch (Kind) {
    case MarkupKind::PlainText:
      return "plaintext";
    case MarkupKind::Markdown:
      return "markdown";
  }
  llvm_unreachable("Invalid MarkupKind");
}

clangd/XRefs.cpp

346

Same suggestion as before. Could we extract a helper function to mark invalid enum values unreachable?

572

This assert seems rather unlikely and just makes the code more complex. Let's assume it's true and remove it (along with the subsequent if statement)

580

This assert seems rather unlikely and just makes the code more complex. Let's assume it's true and remove it (along with the subsequent if statement)

test/clangd/hover.test

3

We have a more readable format for the lit-tests now. Could we update this test to use it?
(see other tests in test/clangd for example)

unittests/clangd/XRefsTests.cpp

238

NIT: remove empty line at the start of the function

240

NIT: Use StringRef instead of const char*

[simark]

In D35894#1001875, @ilya-biryukov wrote:

Thanks for picking this up!
I have just one major comment related to how we generate the hover text.

Current implementation tries to get the actual source code for every declaration and then patch it up to look nice on in the output.
It is quite a large piece of code (spanning most of the patch) and tries to mix and match source code and output from the AST when there's not enough information in the AST (e.g. tryFixupIndentation and stripOpeningBracket). This approach is really complex as we'll inevitably try to parse "parts" of C++ and figure out how to deal with macros, etc.
Worse than that, I would expect this code to grow uncontrollably with time and be very hard to maintain.
Moreover, in presence of macros it arguably gives non-useful results. For example, consider this:

#define DEFINITIONS int foo(); int bar(); int baz();

// somewhere else
DEFINITIONS

// somewhere else
int test() {
  foo(); // <-- hover currently doesn't work here. And even if it did, showing a line with just DEFINITIONS is not that useful.
}

I suggest we move to a different approach of pretty-printing the relevant parts of the AST. It is already implemented in clang, handles all the cases in the AST (and will be updated along when AST is changed), shows useful information in presence of macros and is much easier to implement.
The version of getHoverContents using this is just a few lines of code:

static std::string getHoverContents(const Decl *D) {
  if (TemplateDecl *TD = D->getDescribedTemplate())
    D = TD; // we want to see the template bits.

  std::string Result;
  llvm::raw_string_ostream OS(Result);

  PrintingPolicy Policy(D->getASTContext().getLangOpts());
  Policy.TerseOutput = true;
  D->print(OS, Policy);

  OS.flush();
  return Result;
}

It doesn't add the "Defined in ..." piece, but illustrates the APIs we can use. We should use the same APIs for the scope as well, avoiding fallbacks to manual printing if we can.
If there's something missing/wrong in the upstream pretty printers, it's fine and we can fix them (e.g., the thing that I've immediately noticed is that namespaces are always printed with curly braces).

I thought it would be nice to show in the hover the declaration formatted as it is written in the source file, because that's how the developer is used to read it. But on the other hand, I completely agree that trying to do string formatting ourselves is opening a can of worms. I'll try the approach you suggest.

[simark]

clangd/ClangdLSPServer.cpp

328

Done.

clangd/Protocol.cpp

360

Ack.

367

Done.

clangd/XRefs.cpp

346

Done.

572

Done...

580

... and done.

test/clangd/hover.test

3

Indeed the new format looks much more readable. I'll modify this one to look like completion.test.

unittests/clangd/XRefsTests.cpp

238

Done.

240

Done.

[simark]

Another example where pretty-printing the AST gives better results:

int x, y = 5, z = 6;

Hover the z will now show int z = 6, before it would have shown int x, y = 5, z = 6. I think new version is better because it only shows the variable we care about.

Is there something similar we can do when printing macros, or for them the approach of getting the text from the source file is still the best thing we have?

[simark]

The only problem I see is that when hovering a function/struct name, it now prints the whole function/struct definition. When talking with @malaperle, he told me that you had discussed it before and we should not have the definition in the hover, just the prototype for a function and the name (e.g. "struct foo") for a struct. Glancing quickly at the PrintingPolicy, I don't see a way to do that. Do you have any idea?

[simark]

In D35894#1003342, @simark wrote:

The only problem I see is that when hovering a function/struct name, it now prints the whole function/struct definition. When talking with @malaperle, he told me that you had discussed it before and we should not have the definition in the hover, just the prototype for a function and the name (e.g. "struct foo") for a struct. Glancing quickly at the PrintingPolicy, I don't see a way to do that. Do you have any idea?

Arrg, Sorry, I just re-read your comment and saw that you used TerseOutput which does that. I did not catch that when transcribing the code (I did not want to copy paste to understand the code better, but got bitten).

[ilya-biryukov]

In D35894#1003356, @simark wrote:

In D35894#1003342, @simark wrote:

The only problem I see is that when hovering a function/struct name, it now prints the whole function/struct definition. When talking with @malaperle, he told me that you had discussed it before and we should not have the definition in the hover, just the prototype for a function and the name (e.g. "struct foo") for a struct. Glancing quickly at the PrintingPolicy, I don't see a way to do that. Do you have any idea?

Arrg, Sorry, I just re-read your comment and saw that you used TerseOutput which does that. I did not catch that when transcribing the code (I did not want to copy paste to understand the code better, but got bitten).

Yeah, the TerseOutput bit was important :-)

In D35894#1003342, @simark wrote:

The only problem I see is that when hovering a function/struct name, it now prints the whole function/struct definition. When talking with @malaperle, he told me that you had discussed it before and we should not have the definition in the hover, just the prototype for a function and the name (e.g. "struct foo") for a struct. Glancing quickly at the PrintingPolicy, I don't see a way to do that. Do you have any idea?

I'm not aware of the code that pretty-prints macros. There's a code that dumps debug output, but it's not gonna help in that case.
Alternatively, we could simply print the macro name and not print the definition. That's super-simple and is in line with hovers for the AST decls. Let's do that in the initial version.

For macros using the source code is also less of a problem, so we may try to do it. C++ doesn't allow to generate a macro inside another macro, so we don't run into the same problems we may hit with the AST. And the overall syntax is much simpler than for AST, so we can hope to get string manipulation right with reasonable effort. (Simply reindenting should do the trick).

[simark]

I'm not aware of the code that pretty-prints macros. There's a code that dumps debug output, but it's not gonna help in that case.
Alternatively, we could simply print the macro name and not print the definition. That's super-simple and is in line with hovers for the AST decls. Let's do that in the initial version.

Ok, I'm considering printing "#define MACRO". Just printing the name would not be very useful, at least printing "#define" gives the information that the hovered entity is defined as a macro (and not a function or variable).

Is there a way to get the macro name from the MacroInfo object? I couldn't find any, so the solution I'm considering is to make DeclarationAndMacrosFinder::takeMacroInfos return an std::vector<std::pair<StringRef, const MacroInfo *>>, where the first member of the pair is the macro name. It would come from IdentifierInfo->getName() in DeclarationAndMacrosFinder::finish. Does that make sense, or is there a simpler way?

[ilya-biryukov]

In D35894#1006124, @simark wrote:

Is there a way to get the macro name from the MacroInfo object? I couldn't find any, so the solution I'm considering is to make DeclarationAndMacrosFinder::takeMacroInfos return an std::vector<std::pair<StringRef, const MacroInfo *>>, where the first member of the pair is the macro name. It would come from IdentifierInfo->getName() in DeclarationAndMacrosFinder::finish. Does that make sense, or is there a simpler way?

I don't think there's a way to get macro name from MacroInfo. pair<Name, MacroInfo*> sounds good to me, I'd probably even use a named struct here: struct MacroDecl { StringRef Name; const MacroInfo &Info; }

[simark]

Generate Hover by pretty-printing the AST

This new version of the patch generates the hover by pretty-printing the AST.
The unit tests are updated accordingly. There are some inconsistencies in how
things are printed. For example, I find it a bit annoying that we print empty
curly braces after class names (e.g. "class Foo {}"). Also, namespace and
enums are printed with a newline between the two braces. These are things that
could get fixed by doing changes to the clang AST printer.

The hover.test test now uses a more readable format.

[simark]

unittests/clangd/XRefsTests.cpp

563

Note that I used .str() here to make the output of failing tests readable and useful. By default, gtest tries to print StringRef as if it was a container. I tried to make add a PrintTo function to specify how it should be printed, but couldn't get it to work (to have it called by the compiler), so I settled for this.

[ilya-biryukov]

Just a few last remarks and this is good to go.
Should I land it for you after the last comments are fixed?

clangd/XRefs.cpp

354

We should call flush() before returning Name here. The raw_string_ostream is buffered.

373

NIT: use llvm::None here instead of {}

394

Accidental leftover from previous code?

unittests/clangd/XRefsTests.cpp

264

NIT: LLVM uses UpperCamelCase for field names.

563

Thanks for spotting that.
We have a fix for that in LLVM's gtest extensions (D43330).
str() can now be removed.

[simark]

clangd/XRefs.cpp

354

I replaced it with Stream.str().

373

Done.

394

Oops, thanks.

unittests/clangd/XRefsTests.cpp

264

Ok, I fixed all the structs this patch adds.

563

Removed. Even if this patch is merged before D43330 it's not a big deal, since it only affects debug output of failing test cases.

[simark]

New version

• Rebase on master, change findHover to have a callback-based interface
• Fix nits from previous review comments

[simark]

In D35894#1008593, @ilya-biryukov wrote:

Just a few last remarks and this is good to go.
Should I land it for you after the last comments are fixed?

I just received my commit access, so if you are ok with it, I would try to push it once I get your final OK.

[ilya-biryukov]

Only the naming of fields left.

Also, please make sure to run git-clang-format on the code before submitting to make sure it's formatted properly.

clangd/ClangdLSPServer.cpp

331

NIT: use return replyError to be consistent with other methods.

clangd/XRefs.cpp

354

Also works, albeit less efficient (will probably do a copy where move is enough).

unittests/clangd/XRefsTests.cpp

264

Argh, sorry that I didn't mention it before, but we have an exception for structs in Procotol.h, they follow the spelling in the LSP spec (i.e. should be lowerCamelCase).

[simark]

In D35894#1008861, @ilya-biryukov wrote:

Only the naming of fields left.

Also, please make sure to run git-clang-format on the code before submitting to make sure it's formatted properly.

Ahh, I was running clang-format by hand, didn't know about git-clang-format (and obviously forgot some files). Thanks.

clangd/ClangdLSPServer.cpp

331

Ok.

clangd/XRefs.cpp

354

Ah, didn't think about that. I'll change it to flush + return Name.

unittests/clangd/XRefsTests.cpp

264

Haha, no problem, changing them back is one keystroke away.

[simark]

Fix some nits

• Revert case change of struct fields in Protocol.h
• Change return formatting in onHover
• Use flush() + return Name in NamedDeclQualifiedName and TypeDeclToString

[ilya-biryukov]

Thanks for fixing all the NITs!

[simark]

Fix field names in XRefsTests.cpp

I realized I forgot to add some fixups in the previous version, the field names
in XRefsTests.cpp were not updated and therefore it did not build.

[ilya-biryukov]

LGTM

[simark]

Rebase

[simark]

Rebase

[malaperle]

Rebase.

[malaperle]

Add a missing std::move, fix some formatting.

[malaperle]

Works well and code looks good. There were only minor tweaks since the last approval. I'll land this since there are some issues with Simon's svn account.

clangd/ClangdLSPServer.h

private:
   void onCompletion(TextDocumentPositionParams &Params) override;
   void onSignatureHelp(TextDocumentPositionParams &Params) override;
   void onGoToDefinition(TextDocumentPositionParams &Params) override;
   void onSwitchSourceHeader(TextDocumentIdentifier &Params) override;
   void onDocumentHighlight(TextDocumentPositionParams &Params) override;
   void onFileEvent(DidChangeWatchedFilesParams &Params) override;
   void onCommand(ExecuteCommandParams &Params) override;
   void onRename(RenameParams &Parames) override;
+  void onHover(TextDocumentPositionParams &Params) override;
 
   std::vector<TextEdit> getFixIts(StringRef File, const clangd::Diagnostic &D);
 
   JSONOutput &Out;
   /// Used to indicate that the 'shutdown' request was received from the
   /// Language Server client.
   bool ShutdownRequestReceived = false;
 

clangd/ClangdLSPServer.cpp

 //===--- ClangdLSPServer.cpp - LSP server ------------------------*- C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===---------------------------------------------------------------------===//
 
 #include "ClangdLSPServer.h"
 #include "JSONRPCDispatcher.h"
 #include "SourceCode.h"
 #include "URI.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Path.h"
 
 using namespace clang::clangd;
 using namespace clang;
 
reply(json::obj{
                  {"triggerCharacters", {".", ">", ":"}},
              }},
             {"signatureHelpProvider",
              json::obj{
                  {"triggerCharacters", {"(", ","}},
              }},
             {"definitionProvider", true},
             {"documentHighlightProvider", true},
+            {"hoverProvider", true},
             {"renameProvider", true},
             {"executeCommandProvider",
              json::obj{
                  {"commands", {ExecuteCommandParams::CLANGD_APPLY_FIX_COMMAND}},
              }},
         }}}});
 }
 

 
 void ClangdLSPServer::onCodeAction(CodeActionParams &Params) {
   // We provide a code action for each diagnostic at the requested location
   // which has FixIts available.
   auto Code = Server.getDocument(Params.textDocument.uri.file);
   if (!Code)
     return replyError(ErrorCode::InvalidParams,
                       "onCodeAction called for non-added file");
 
   json::ary Commands;
   for (Diagnostic &D : Params.context.diagnostics) {
     auto Edits = getFixIts(Params.textDocument.uri.file, D);
     if (!Edits.empty()) {
       WorkspaceEdit WE;
       WE.changes = {{Params.textDocument.uri.uri(), std::move(Edits)}};
       Commands.push_back(json::obj{
           {"title", llvm::formatv("Apply FixIt {0}", D.message)},
           {"command", ExecuteCommandParams::CLANGD_APPLY_FIX_COMMAND},
           {"arguments", {WE}},
       });
     }
   }
   reply(std::move(Commands));
 }
 
 void ClangdLSPServer::onCompletion(TextDocumentPositionParams &Params) {
   Server.codeComplete(Params.textDocument.uri.file, Params.position, CCOpts,
                       [](Tagged<CompletionList> List) { reply(List.Value); });
 }
 
 void ClangdLSPServer::onSignatureHelp(TextDocumentPositionParams &Params) {
   Server.signatureHelp(Params.textDocument.uri.file, Params.position,
                        [](llvm::Expected<Tagged<SignatureHelp>> SignatureHelp) {
                          if (!SignatureHelp)
                            return replyError(
                                ErrorCode::InvalidParams,
                                llvm::toString(SignatureHelp.takeError()));
                          reply(SignatureHelp->Value);
                        });
 }
 
 void ClangdLSPServer::onGoToDefinition(TextDocumentPositionParams &Params) {
Server.findDocumentHighlights(
       [](llvm::Expected<Tagged<std::vector<DocumentHighlight>>> Highlights) {
         if (!Highlights)
           return replyError(ErrorCode::InternalError,
                             llvm::toString(Highlights.takeError()));
         reply(json::ary(Highlights->Value));
       });
 }
 
+void ClangdLSPServer::onHover(TextDocumentPositionParams &Params) {
+  Server.findHover(Params.textDocument.uri.file, Params.position,
+                   [](llvm::Expected<Tagged<Hover>> H) {
+                     if (!H) {
+                       replyError(ErrorCode::InternalError,
+                                  llvm::toString(H.takeError()));
+                       return;
+                     }
+
+                     reply(H->Value);
+                   });
+}
+
 ClangdLSPServer::ClangdLSPServer(JSONOutput &Out, unsigned AsyncThreadsCount,
                                  bool StorePreamblesInMemory,
                                  const clangd::CodeCompleteOptions &CCOpts,
                                  llvm::Optional<StringRef> ResourceDir,
                                  llvm::Optional<Path> CompileCommandsDir,
                                  bool BuildDynamicSymbolIndex,
                                  SymbolIndex *StaticIdx)
     : Out(Out), CDB(std::move(CompileCommandsDir)), CCOpts(CCOpts),
       Server(CDB, /*DiagConsumer=*/*this, FSProvider, AsyncThreadsCount,

clangd/ClangdServer.h

public:
 
   template <class U>
   Tagged(const Tagged<U> &Other) : Value(Other.Value), Tag(Other.Tag) {}
 
   template <class U>
   Tagged(Tagged<U> &&Other)
       : Value(std::move(Other.Value)), Tag(std::move(Other.Tag)) {}
 
   T Value = T();
   VFSTag Tag = VFSTag();
 };
 
 template <class T>
 Tagged<typename std::decay<T>::type> make_tagged(T &&Value, VFSTag Tag) {
   return Tagged<typename std::decay<T>::type>(std::forward<T>(Value), Tag);
 }
 
public:
   /// vfs::FileSystem used for signature help. This method should only be called
   /// for currently tracked files.
   void signatureHelp(
       PathRef File, Position Pos,
       UniqueFunction<void(llvm::Expected<Tagged<SignatureHelp>>)> Callback,
       llvm::Optional<StringRef> OverridenContents = llvm::None,
       IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS = nullptr);
 
   /// Get definition of symbol at a specified \p Line and \p Column in \p File.
   void findDefinitions(
       PathRef File, Position Pos,
       UniqueFunction<void(llvm::Expected<Tagged<std::vector<Location>>>)>
           Callback);
 
   /// Helper function that returns a path to the corresponding source file when
   /// given a header file and vice versa.
   llvm::Optional<Path> switchSourceHeader(PathRef Path);
 
   /// Get document highlights for a given position.
   void findDocumentHighlights(
       PathRef File, Position Pos,
       UniqueFunction<
           void(llvm::Expected<Tagged<std::vector<DocumentHighlight>>>)>
           Callback);
 
+  /// Get code hover for a given position.
+  void findHover(PathRef File, Position Pos,
+                 UniqueFunction<void(llvm::Expected<Tagged<Hover>>)> Callback);
+
   /// Run formatting for \p Rng inside \p File with content \p Code.
   llvm::Expected<tooling::Replacements> formatRange(StringRef Code,
                                                     PathRef File, Range Rng);
 
   /// Run formatting for the whole \p File with content \p Code.
   llvm::Expected<tooling::Replacements> formatFile(StringRef Code,
                                                    PathRef File);
 
   /// Run formatting after a character was typed at \p Pos in \p File with
   /// content \p Code.
   llvm::Expected<tooling::Replacements>
   formatOnType(StringRef Code, PathRef File, Position Pos);
 
   /// Rename all occurrences of the symbol at the \p Pos in \p File to
   /// \p NewName.
   void rename(PathRef File, Position Pos, llvm::StringRef NewName,
               UniqueFunction<void(Expected<std::vector<tooling::Replacement>>)>
                   Callback);
 
   /// Gets current document contents for \p File. Returns None if \p File is not
   /// currently tracked.
   /// FIXME(ibiryukov): This function is here to allow offset-to-Position
   /// conversions in outside code, maybe there's a way to get rid of it.
   llvm::Optional<std::string> getDocument(PathRef File);
 
   /// Only for testing purposes.
   /// Waits until all requests to worker thread are finished and dumps AST for
   /// \p File. \p File must be in the list of added documents.
   void dumpAST(PathRef File, UniqueFunction<void(std::string)> Callback);

clangd/ClangdServer.cpp

 //===--- ClangdServer.cpp - Main clangd server code --------------*- C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===-------------------------------------------------------------------===//
 
 #include "ClangdServer.h"
 #include "CodeComplete.h"
 #include "SourceCode.h"
 #include "XRefs.h"
 #include "index/Merge.h"
 #include "clang/Format/Format.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Tooling/CompilationDatabase.h"
 #include "clang/Tooling/Refactoring/RefactoringResultConsumer.h"
 #include "clang/Tooling/Refactoring/Rename/RenamingAction.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 #include <future>
 
 using namespace clang;
 using namespace clang::clangd;
 
 namespace {
 
 void ignoreError(llvm::Error Err) {
   handleAllErrors(std::move(Err), [](const llvm::ErrorInfoBase &) {});
 }
 
 std::string getStandardResourceDir() {
   static int Dummy; // Just an address in this process.
   return CompilerInvocation::GetResourcesPath("clangd", (void *)&Dummy);
auto Action = [](decltype(Callback) Callback,
 
     Callback(Result);
   };
 
   WorkScheduler.runWithAST(File, BindWithForward(Action, std::move(Callback)));
 }
 
 void ClangdServer::findDefinitions(
     PathRef File, Position Pos,
     UniqueFunction<void(llvm::Expected<Tagged<std::vector<Location>>>)>
         Callback) {
   auto TaggedFS = FSProvider.getTaggedFileSystem(File);
   auto Action = [Pos, TaggedFS](decltype(Callback) Callback,
                                 llvm::Expected<InputsAndAST> InpAST) {
     if (!InpAST)
       return Callback(InpAST.takeError());
     auto Result = clangd::findDefinitions(InpAST->AST, Pos);
     Callback(make_tagged(std::move(Result), TaggedFS.Tag));
   };
 
   WorkScheduler.runWithAST(File, BindWithForward(Action, std::move(Callback)));
 }
 
 llvm::Optional<Path> ClangdServer::switchSourceHeader(PathRef Path) {
 
   StringRef SourceExtensions[] = {".cpp", ".c", ".cc", ".cxx",
                                   ".c++", ".m", ".mm"};
   StringRef HeaderExtensions[] = {".h", ".hh", ".hpp", ".hxx", ".inc"};
 
   StringRef PathExt = llvm::sys::path::extension(Path);
 
for (StringRef NewExt : NewExts) {
     if (FS->exists(NewPath))
       return NewPath.str().str();
   }
 
   return llvm::None;
 }
 
 llvm::Expected<tooling::Replacements>
 ClangdServer::formatCode(llvm::StringRef Code, PathRef File,
                          ArrayRef<tooling::Range> Ranges) {
   // Call clang-format.
   auto TaggedFS = FSProvider.getTaggedFileSystem(File);
   auto StyleOrError =
       format::getStyle("file", File, "LLVM", Code, TaggedFS.Value.get());
   if (!StyleOrError) {
     return StyleOrError.takeError();
   } else {
     return format::reformat(StyleOrError.get(), Code, Ranges, File);
   }
 }
 
 void ClangdServer::findDocumentHighlights(
     PathRef File, Position Pos,
     UniqueFunction<void(llvm::Expected<Tagged<std::vector<DocumentHighlight>>>)>
         Callback) {
   auto FileContents = DraftMgr.getDraft(File);
   if (!FileContents.Draft)
     return Callback(llvm::make_error<llvm::StringError>(
         "findDocumentHighlights called on non-added file",
         llvm::errc::invalid_argument));
 
   auto TaggedFS = FSProvider.getTaggedFileSystem(File);
 
   auto Action = [TaggedFS, Pos](decltype(Callback) Callback,
                                 llvm::Expected<InputsAndAST> InpAST) {
     if (!InpAST)
       return Callback(InpAST.takeError());
     auto Result = clangd::findDocumentHighlights(InpAST->AST, Pos);
     Callback(make_tagged(std::move(Result), TaggedFS.Tag));
   };
 
   WorkScheduler.runWithAST(File, BindWithForward(Action, std::move(Callback)));
 }
 
+void ClangdServer::findHover(
+    PathRef File, Position Pos,
+    UniqueFunction<void(llvm::Expected<Tagged<Hover>>)> Callback) {
+  Hover FinalHover;
+  auto FileContents = DraftMgr.getDraft(File);
+  if (!FileContents.Draft)
+    return Callback(llvm::make_error<llvm::StringError>(
+        "findHover called on non-added file",
+        llvm::errc::invalid_argument));
+
+  auto TaggedFS = FSProvider.getTaggedFileSystem(File);
+
+  auto Action = [Pos, TaggedFS](decltype(Callback) Callback,
+                                llvm::Expected<InputsAndAST> InpAST) {
+    if (!InpAST)
+      return Callback(InpAST.takeError());
+
+    Hover Result = clangd::getHover(InpAST->AST, Pos);
+    Callback(make_tagged(std::move(Result), TaggedFS.Tag));
+  };
+
+  WorkScheduler.runWithAST(File, BindWithForward(Action, std::move(Callback)));
+}
+
 void ClangdServer::scheduleReparseAndDiags(
     PathRef File, VersionedDraft Contents,
     Tagged<IntrusiveRefCntPtr<vfs::FileSystem>> TaggedFS) {
   tooling::CompileCommand Command = CompileArgs.getCompileCommand(File);
 
   using OptDiags = llvm::Optional<std::vector<DiagWithFixIts>>;
 
   DocVersion Version = Contents.Version;

 std::vector<std::pair<Path, std::size_t>>
 ClangdServer::getUsedBytesPerFile() const {
   return WorkScheduler.getUsedBytesPerFile();
 }
 
 LLVM_NODISCARD bool
 ClangdServer::blockUntilIdleForTest(llvm::Optional<double> TimeoutSeconds) {
   return WorkScheduler.blockUntilIdle(timeoutSeconds(TimeoutSeconds));
 }

clangd/Protocol.h

 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_PROTOCOL_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_PROTOCOL_H
 
 #include "JSONExpr.h"
 #include "URI.h"
 #include "llvm/ADT/Optional.h"
 #include <string>
 #include <vector>
 
 namespace clang {
 namespace clangd {
 
 enum class ErrorCode {
   // Defined by JSON RPC.
   ParseError = -32700,
   InvalidRequest = -32600,

 bool fromJSON(const json::Expr &, Range &);
 json::Expr toJSON(const Range &);
 llvm::raw_ostream &operator<<(llvm::raw_ostream &, const Range &);
 
 struct Location {
   /// The text document's URI.
   URIForFile uri;
   Range range;
 
   friend bool operator==(const Location &LHS, const Location &RHS) {
     return LHS.uri == RHS.uri && LHS.range == RHS.range;
   }
 
   friend bool operator!=(const Location &LHS, const Location &RHS) {
     return !(LHS == RHS);
   }
 
struct TextDocumentPositionParams {
   /// The text document.
   TextDocumentIdentifier textDocument;
 
   /// The position inside the text document.
   Position position;
 };
 bool fromJSON(const json::Expr &, TextDocumentPositionParams &);
 
+enum class MarkupKind {
+  PlainText,
+  Markdown,
+};
+
+struct MarkupContent {
+  MarkupKind Kind = MarkupKind::PlainText;
+  std::string Value;
+};
+json::Expr toJSON(const MarkupContent &MC);
+
+struct Hover {
+  /// The hover's content
+  MarkupContent Contents;
+
+  /// An optional range is a range inside a text document
+  /// that is used to visualize a hover, e.g. by changing the background color.
+  llvm::Optional<Range> Range;
+};
+json::Expr toJSON(const Hover &H);
+
 /// The kind of a completion entry.
 enum class CompletionItemKind {
   Missing = 0,
   Text = 1,
   Method = 2,
   Function = 3,
   Constructor = 4,
   Field = 5,
   Variable = 6,
   Class = 7,
   Interface = 8,
   Module = 9,
   Property = 10,
   Unit = 11,
   Value = 12,
   Enum = 13,
   Keyword = 14,
   Snippet = 15,
   Color = 16,
   File = 17,
   Reference = 18,
 };
 
 /// Defines whether the insert text in a completion item should be interpreted
 /// as plain text or a snippet.
 enum class InsertTextFormat {
   Missing = 0,
   /// The primary text to be inserted is treated as a plain string.
   PlainText = 1,
   /// The primary text to be inserted is treated as a snippet.
   ///
   /// A snippet can define tab stops and placeholders with `$1`, `$2`
   /// and `${3:foo}`. `$0` defines the final tab stop, it defaults to the end
   /// of the snippet. Placeholders with equal identifiers are linked, that is
   /// typing in one will update others too.
   ///
   /// See also:
   /// https//github.com/Microsoft/vscode/blob/master/src/vs/editor/contrib/snippet/common/snippet.md
   Snippet = 2,
 };
 
 /// Provides details for how a completion item was scored.
 /// This can be used for client-side filtering of completion items as the
 /// user keeps typing.
 /// This is a clangd extension.

clangd/Protocol.cpp

 }
 
 bool fromJSON(const json::Expr &Params, TextDocumentPositionParams &R) {
   json::ObjectMapper O(Params);
   return O && O.map("textDocument", R.textDocument) &&
          O.map("position", R.position);
 }
 
+static StringRef toTextKind(MarkupKind Kind)
+{
+  switch (Kind) {
+  case MarkupKind::PlainText:
+    return "plaintext";
+  case MarkupKind::Markdown:
+    return "markdown";
+  }
+  llvm_unreachable("Invalid MarkupKind");
+}
+
+json::Expr toJSON(const MarkupContent &MC) {
+  if (MC.Value.empty())
+    return nullptr;
+
+  return json::obj{
+      {"kind", toTextKind(MC.Kind)},
+      {"value", MC.Value},
+  };
+}
+
+json::Expr toJSON(const Hover &H) {
+  json::obj Result{{"contents", toJSON(H.Contents)}};
+
+  if (H.Range.hasValue())
+    Result["range"] = toJSON(*H.Range);
+
+  return Result;
+}
+
 json::Expr toJSON(const CompletionItem &CI) {
   assert(!CI.label.empty() && "completion item label is required");
   json::obj Result{{"label", CI.label}};
   if (CI.kind != CompletionItemKind::Missing)
     Result["kind"] = static_cast<int>(CI.kind);
   if (!CI.detail.empty())
     Result["detail"] = CI.detail;
   if (!CI.documentation.empty())

clangd/ProtocolHandlers.h

public:
   virtual void onCompletion(TextDocumentPositionParams &Params) = 0;
   virtual void onSignatureHelp(TextDocumentPositionParams &Params) = 0;
   virtual void onGoToDefinition(TextDocumentPositionParams &Params) = 0;
   virtual void onSwitchSourceHeader(TextDocumentIdentifier &Params) = 0;
   virtual void onFileEvent(DidChangeWatchedFilesParams &Params) = 0;
   virtual void onCommand(ExecuteCommandParams &Params) = 0;
   virtual void onRename(RenameParams &Parames) = 0;
   virtual void onDocumentHighlight(TextDocumentPositionParams &Params) = 0;
+  virtual void onHover(TextDocumentPositionParams &Params) = 0;
 };
 
 void registerCallbackHandlers(JSONRPCDispatcher &Dispatcher, JSONOutput &Out,
                               ProtocolCallbacks &Callbacks);
 
 } // namespace clangd
 } // namespace clang
 
 #endif

clangd/ProtocolHandlers.cpp

struct HandlerRegisterer {
 
   JSONRPCDispatcher &Dispatcher;
   JSONOutput *Out;
   ProtocolCallbacks *Callbacks;
 };
 
 } // namespace
 
 void clangd::registerCallbackHandlers(JSONRPCDispatcher &Dispatcher,
                                       JSONOutput &Out,
                                       ProtocolCallbacks &Callbacks) {
   HandlerRegisterer Register{Dispatcher, &Out, &Callbacks};
 
   Register("initialize", &ProtocolCallbacks::onInitialize);
   Register("shutdown", &ProtocolCallbacks::onShutdown);
   Register("exit", &ProtocolCallbacks::onExit);
   Register("textDocument/didOpen", &ProtocolCallbacks::onDocumentDidOpen);
   Register("textDocument/didClose", &ProtocolCallbacks::onDocumentDidClose);
   Register("textDocument/didChange", &ProtocolCallbacks::onDocumentDidChange);
   Register("textDocument/rangeFormatting",
            &ProtocolCallbacks::onDocumentRangeFormatting);
   Register("textDocument/onTypeFormatting",
            &ProtocolCallbacks::onDocumentOnTypeFormatting);
   Register("textDocument/formatting", &ProtocolCallbacks::onDocumentFormatting);
   Register("textDocument/codeAction", &ProtocolCallbacks::onCodeAction);
   Register("textDocument/completion", &ProtocolCallbacks::onCompletion);
   Register("textDocument/signatureHelp", &ProtocolCallbacks::onSignatureHelp);
   Register("textDocument/definition", &ProtocolCallbacks::onGoToDefinition);
   Register("textDocument/switchSourceHeader",
            &ProtocolCallbacks::onSwitchSourceHeader);
   Register("textDocument/rename", &ProtocolCallbacks::onRename);
+  Register("textDocument/hover", &ProtocolCallbacks::onHover);
   Register("workspace/didChangeWatchedFiles", &ProtocolCallbacks::onFileEvent);
   Register("workspace/executeCommand", &ProtocolCallbacks::onCommand);
   Register("textDocument/documentHighlight",
            &ProtocolCallbacks::onDocumentHighlight);
 }

clangd/XRefs.h

 
 /// Get definition of symbol at a specified \p Pos.
 std::vector<Location> findDefinitions(ParsedAST &AST, Position Pos);
 
 /// Returns highlights for all usages of a symbol at \p Pos.
 std::vector<DocumentHighlight> findDocumentHighlights(ParsedAST &AST,
                                                       Position Pos);
 
+/// Get the hover information when hovering at \p Pos.
+Hover getHover(ParsedAST &AST, Position Pos);
+
 } // namespace clangd
 } // namespace clang
 #endif

clangd/XRefs.cpp

 //===--- XRefs.cpp ----------------------------------------------*- C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===---------------------------------------------------------------------===//
 #include "XRefs.h"
 #include "Logger.h"
 #include "URI.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/Index/IndexDataConsumer.h"
 #include "clang/Index/IndexingAction.h"
 #include "llvm/Support/Path.h"
 namespace clang {
 namespace clangd {
 using namespace llvm;
 namespace {
 
 // Get the definition from a given declaration `D`.
 // Return nullptr if no definition is found, or the declaration type of `D` is
 // not supported.
 const Decl* GetDefinition(const Decl* D) {
   assert(D);
   if (const auto *TD = dyn_cast<TagDecl>(D))
     return TD->getDefinition();
   else if (const auto *VD = dyn_cast<VarDecl>(D))
     return VD->getDefinition();
   else if (const auto *FD = dyn_cast<FunctionDecl>(D))
     return FD->getDefinition();
   return nullptr;
 }
 
+struct MacroDecl {
+  StringRef Name;
+  const MacroInfo *Info;
+};
+
 /// Finds declarations locations that a given source location refers to.
 class DeclarationAndMacrosFinder : public index::IndexDataConsumer {
   std::vector<const Decl *> Decls;
-  std::vector<const MacroInfo *> MacroInfos;
+  std::vector<MacroDecl> MacroInfos;
   const SourceLocation &SearchedLocation;
   const ASTContext &AST;
   Preprocessor &PP;
 
 public:
   DeclarationAndMacrosFinder(raw_ostream &OS,
                              const SourceLocation &SearchedLocation,
                              ASTContext &AST, Preprocessor &PP)
       : SearchedLocation(SearchedLocation), AST(AST), PP(PP) {}
 
   std::vector<const Decl *> takeDecls() {
     // Don't keep the same declaration multiple times.
     // This can happen when nodes in the AST are visited twice.
     std::sort(Decls.begin(), Decls.end());
     auto Last = std::unique(Decls.begin(), Decls.end());
     Decls.erase(Last, Decls.end());
     return std::move(Decls);
   }
 
-  std::vector<const MacroInfo *> takeMacroInfos() {
+  std::vector<MacroDecl> takeMacroInfos() {
     // Don't keep the same Macro info multiple times.
-    std::sort(MacroInfos.begin(), MacroInfos.end());
-    auto Last = std::unique(MacroInfos.begin(), MacroInfos.end());
+    std::sort(MacroInfos.begin(), MacroInfos.end(),
+              [](const MacroDecl &Left, const MacroDecl &Right) {
+                return Left.Info < Right.Info;
+              });
+
+    auto Last = std::unique(MacroInfos.begin(), MacroInfos.end(),
+                            [](const MacroDecl &Left, const MacroDecl &Right) {
+                              return Left.Info == Right.Info;
+                            });
     MacroInfos.erase(Last, MacroInfos.end());
     return std::move(MacroInfos);
   }
 
   bool
   handleDeclOccurence(const Decl *D, index::SymbolRoleSet Roles,
                       ArrayRef<index::SymbolRelation> Relations, FileID FID,
                       unsigned Offset,
if (!Lexer::getRawToken(SearchedLocation, Result, Mgr, AST.getLangOpts(),
         // referenced in a '#undef MACRO' can still be found.
         SourceLocation BeforeSearchedLocation = Mgr.getMacroArgExpandedLocation(
             Mgr.getLocForStartOfFile(DecLoc.first)
                 .getLocWithOffset(DecLoc.second - 1));
         MacroDefinition MacroDef =
             PP.getMacroDefinitionAtLoc(IdentifierInfo, BeforeSearchedLocation);
         MacroInfo *MacroInf = MacroDef.getMacroInfo();
         if (MacroInf) {
-          MacroInfos.push_back(MacroInf);
+          MacroInfos.push_back(MacroDecl{IdentifierInfo->getName(), MacroInf});
         }
       }
     }
   }
 };
 
 llvm::Optional<Location>
 getDeclarationLocation(ParsedAST &AST, const SourceRange &ValSourceRange) {
std::vector<Location> findDefinitions(ParsedAST &AST, Position Pos) {
   IndexOpts.SystemSymbolFilter =
       index::IndexingOptions::SystemSymbolFilterKind::All;
   IndexOpts.IndexFunctionLocals = true;
 
   indexTopLevelDecls(AST.getASTContext(), AST.getTopLevelDecls(),
                      DeclMacrosFinder, IndexOpts);
 
   std::vector<const Decl *> Decls = DeclMacrosFinder->takeDecls();
-  std::vector<const MacroInfo *> MacroInfos =
-      DeclMacrosFinder->takeMacroInfos();
+  std::vector<MacroDecl> MacroInfos = DeclMacrosFinder->takeMacroInfos();
   std::vector<Location> Result;
 
   for (auto Item : Decls) {
     auto L = getDeclarationLocation(AST, Item->getSourceRange());
     if (L)
       Result.push_back(*L);
   }
 
   for (auto Item : MacroInfos) {
-    SourceRange SR(Item->getDefinitionLoc(), Item->getDefinitionEndLoc());
+    SourceRange SR(Item.Info->getDefinitionLoc(),
+                   Item.Info->getDefinitionEndLoc());
     auto L = getDeclarationLocation(AST, SR);
     if (L)
       Result.push_back(*L);
   }
 
   return Result;
 }
 
auto DocHighlightsFinder = std::make_shared<DocumentHighlightsFinder>(
       llvm::errs(), AST.getASTContext(), AST.getPreprocessor(), SelectedDecls);
 
   indexTopLevelDecls(AST.getASTContext(), AST.getTopLevelDecls(),
                      DocHighlightsFinder, IndexOpts);
 
   return DocHighlightsFinder->takeHighlights();
 }
 
+static PrintingPolicy PrintingPolicyForDecls(PrintingPolicy Base) {
+  PrintingPolicy Policy(Base);
+
+  Policy.AnonymousTagLocations = false;
+  Policy.TerseOutput = true;
+  Policy.PolishForDeclaration = true;
+  Policy.ConstantsAsWritten = true;
+  Policy.SuppressTagKeyword = false;
+
+  return Policy;
+}
+
+/// Return a string representation (e.g. "class MyNamespace::MyClass") of
+/// the type declaration \p TD.
+static std::string TypeDeclToString(const TypeDecl *TD) {
+  QualType Type = TD->getASTContext().getTypeDeclType(TD);
+
+  PrintingPolicy Policy =
+      PrintingPolicyForDecls(TD->getASTContext().getPrintingPolicy());
+
+  std::string Name;
+  llvm::raw_string_ostream Stream(Name);
+  Type.print(Stream, Policy);
+
+  return Stream.str();
+}
+
+/// Return a string representation (e.g. "namespace ns1::ns2") of
+/// the named declaration \p ND.
+static std::string NamedDeclQualifiedName(const NamedDecl *ND,
+                                          StringRef Prefix) {
+  PrintingPolicy Policy =
+      PrintingPolicyForDecls(ND->getASTContext().getPrintingPolicy());
+
+  std::string Name;
+  llvm::raw_string_ostream Stream(Name);
+  Stream << Prefix << ' ';
+  ND->printQualifiedName(Stream, Policy);
+
+  return Stream.str();
+}
+
+/// Given a declaration \p D, return a human-readable string representing the
+/// scope in which it is declared.  If the declaration is in the global scope,
+/// return the string "global namespace".
+static llvm::Optional<std::string> getScopeName(const Decl *D) {
+  const DeclContext *DC = D->getDeclContext();
+
+  if (const TranslationUnitDecl *TUD = dyn_cast<TranslationUnitDecl>(DC))
+    return std::string("global namespace");
+
+  if (const TypeDecl *TD = dyn_cast<TypeDecl>(DC))
+    return TypeDeclToString(TD);
+  else if (const NamespaceDecl *ND = dyn_cast<NamespaceDecl>(DC))
+    return NamedDeclQualifiedName(ND, "namespace");
+  else if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(DC))
+    return NamedDeclQualifiedName(FD, "function");
+
+  return llvm::None;
+}
+
+/// Generate a \p Hover object given the declaration \p D.
+static Hover getHoverContents(const Decl *D) {
+  Hover H;
+  llvm::Optional<std::string> NamedScope = getScopeName(D);
+
+  // Generate the "Declared in" section.
+  if (NamedScope) {
+    assert(!NamedScope->empty());
+
+    H.Contents.Value += "Declared in ";
+    H.Contents.Value += *NamedScope;
+    H.Contents.Value += "\n\n";
+  }
+
+  // We want to include the template in the Hover.
+  if (TemplateDecl *TD = D->getDescribedTemplate())
+    D = TD;
+
+  std::string DeclText;
+  llvm::raw_string_ostream OS(DeclText);
+
+  PrintingPolicy Policy =
+      PrintingPolicyForDecls(D->getASTContext().getPrintingPolicy());
+
+  D->print(OS, Policy);
+
+  OS.flush();
+
+  H.Contents.Value += DeclText;
+  return H;
+}
+
+/// Generate a \p Hover object given the macro \p MacroInf.
+static Hover getHoverContents(StringRef MacroName) {
+  Hover H;
+
+  H.Contents.Value = "#define ";
+  H.Contents.Value += MacroName;
+
+  return H;
+}
+
+Hover getHover(ParsedAST &AST, Position Pos) {
+  const SourceManager &SourceMgr = AST.getASTContext().getSourceManager();
+  const FileEntry *FE = SourceMgr.getFileEntryForID(SourceMgr.getMainFileID());
+  if (FE == nullptr)
+    return Hover();
+
+  SourceLocation SourceLocationBeg = getBeginningOfIdentifier(AST, Pos, FE);
+  auto DeclMacrosFinder = std::make_shared<DeclarationAndMacrosFinder>(
+      llvm::errs(), SourceLocationBeg, AST.getASTContext(),
+      AST.getPreprocessor());
+
+  index::IndexingOptions IndexOpts;
+  IndexOpts.SystemSymbolFilter =
+      index::IndexingOptions::SystemSymbolFilterKind::All;
+  IndexOpts.IndexFunctionLocals = true;
+
+  indexTopLevelDecls(AST.getASTContext(), AST.getTopLevelDecls(),
+                     DeclMacrosFinder, IndexOpts);
+
+  std::vector<MacroDecl> Macros = DeclMacrosFinder->takeMacroInfos();
+  if (!Macros.empty())
+    return getHoverContents(Macros[0].Name);
+
+  std::vector<const Decl *> Decls = DeclMacrosFinder->takeDecls();
+  if (!Decls.empty())
+    return getHoverContents(Decls[0]);
+
+  return Hover();
+}
+
 } // namespace clangd
 } // namespace clang

test/clangd/hover.test

+# RUN: clangd -lit-test < %s | FileCheck %s
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
+---
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"file:///main.cpp","languageId":"cpp","version":1,"text":"void foo(); int main() { foo(); }\n"}}}
+---
+{"jsonrpc":"2.0","id":1,"method":"textDocument/hover","params":{"textDocument":{"uri":"file:///main.cpp"},"position":{"line":0,"character":27}}}
+#      CHECK:  "id": 1,
+# CHECK-NEXT:  "jsonrpc": "2.0",
+# CHECK-NEXT:  "result": {
+# CHECK-NEXT:    "contents": {
+# CHECK-NEXT:      "kind": "plaintext",
+# CHECK-NEXT:      "value": "Declared in global namespace\n\nvoid foo()"
+# CHECK-NEXT:    }
+# CHECK-NEXT:  }
+# CHECK-NEXT:}
+---
+{"jsonrpc":"2.0","id":3,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","method":"exit"}

test/clangd/initialize-params-invalid.test

 # RUN: clangd -lit-test < %s | FileCheck -strict-whitespace %s
 # Test with invalid initialize request parameters
 {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":"","rootUri":"file:///path/to/workspace","capabilities":{},"trace":"off"}}
 #      CHECK:  "id": 0,
 # CHECK-NEXT:  "jsonrpc": "2.0",
 # CHECK-NEXT:  "result": {
 # CHECK-NEXT:    "capabilities": {
 # CHECK-NEXT:      "codeActionProvider": true,
 # CHECK-NEXT:      "completionProvider": {
 # CHECK-NEXT:        "resolveProvider": false,

 # CHECK-NEXT:        "moreTriggerCharacter": []
 # CHECK-NEXT:      },
 # CHECK-NEXT:      "documentRangeFormattingProvider": true,
 # CHECK-NEXT:      "executeCommandProvider": {
 # CHECK-NEXT:        "commands": [
 # CHECK-NEXT:          "clangd.applyFix"
 # CHECK-NEXT:        ]
 # CHECK-NEXT:      },
+# CHECK-NEXT:      "hoverProvider": true,
 # CHECK-NEXT:      "renameProvider": true,
 # CHECK-NEXT:      "signatureHelpProvider": {
 # CHECK-NEXT:        "triggerCharacters": [
 # CHECK-NEXT:          "(",
 # CHECK-NEXT:          ","
 # CHECK-NEXT:        ]
 # CHECK-NEXT:      },
 # CHECK-NEXT:      "textDocumentSync": 1
 # CHECK-NEXT:    }
 # CHECK-NEXT:  }
 ---
 {"jsonrpc":"2.0","id":3,"method":"shutdown"}
 ---
 {"jsonrpc":"2.0","method":"exit"}

test/clangd/initialize-params.test

 # RUN: clangd -lit-test < %s | FileCheck -strict-whitespace %s
 # Test initialize request parameters with rootUri
 {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootUri":"file:///path/to/workspace","capabilities":{},"trace":"off"}}
 #      CHECK:  "id": 0,
 # CHECK-NEXT:  "jsonrpc": "2.0",
 # CHECK-NEXT:  "result": {
 # CHECK-NEXT:    "capabilities": {
 # CHECK-NEXT:      "codeActionProvider": true,
 # CHECK-NEXT:      "completionProvider": {
 # CHECK-NEXT:        "resolveProvider": false,

 # CHECK-NEXT:        "moreTriggerCharacter": []
 # CHECK-NEXT:      },
 # CHECK-NEXT:      "documentRangeFormattingProvider": true,
 # CHECK-NEXT:      "executeCommandProvider": {
 # CHECK-NEXT:        "commands": [
 # CHECK-NEXT:          "clangd.applyFix"
 # CHECK-NEXT:        ]
 # CHECK-NEXT:      },
+# CHECK-NEXT:      "hoverProvider": true,
 # CHECK-NEXT:      "renameProvider": true,
 # CHECK-NEXT:      "signatureHelpProvider": {
 # CHECK-NEXT:        "triggerCharacters": [
 # CHECK-NEXT:          "(",
 # CHECK-NEXT:          ","
 # CHECK-NEXT:        ]
 # CHECK-NEXT:      },
 # CHECK-NEXT:      "textDocumentSync": 1

unittests/clangd/XRefsTests.cpp

for (const char *Test : Tests) {
     auto AST = build(T.code());
     EXPECT_THAT(findDefinitions(AST, T.point()),
                 ElementsAre(RangeIs(T.range())))
         << Test;
   }
 }
 
 TEST(GoToDefinition, RelPathsInCompileCommand) {
   Annotations SourceAnnotations(R"cpp(
 [[int foo]];
 int baz = f^oo;
 )cpp");
 
   IgnoreDiagnostics DiagConsumer;
   MockCompilationDatabase CDB(/*UseRelPaths=*/true);
   MockFSProvider FS;
   ClangdServer Server(CDB, DiagConsumer, FS, /*AsyncThreadsCount=*/0,
                       /*StorePreambleInMemory=*/true);
 
   auto FooCpp = getVirtualTestFilePath("foo.cpp");
   FS.Files[FooCpp] = "";
 
   Server.addDocument(FooCpp, SourceAnnotations.code());
   auto Locations =
       runFindDefinitions(Server, FooCpp, SourceAnnotations.point());
   EXPECT_TRUE(bool(Locations)) << "findDefinitions returned an error";
 
   EXPECT_THAT(Locations->Value,
               ElementsAre(Location{URIForFile{FooCpp.str()},
                                    SourceAnnotations.range()}));
 }
 
+TEST(Hover, All) {
+  struct OneTest {
+    StringRef Input;
+    StringRef ExpectedHover;
+  };
+
+  OneTest Tests[] = {
+      {
+          R"cpp(// Local variable
+            int main() {
+              int bonjour;
+              ^bonjour = 2;
+              int test1 = bonjour;
+            }
+          )cpp",
+          "Declared in function main\n\nint bonjour",
+      },
+      {
+          R"cpp(// Local variable in method
+            struct s {
+              void method() {
+                int bonjour;
+                ^bonjour = 2;
+              }
+            };
+          )cpp",
+          "Declared in function s::method\n\nint bonjour",
+      },
+      {
+          R"cpp(// Struct
+            namespace ns1 {
+              struct MyClass {};
+            } // namespace ns1
+            int main() {
+              ns1::My^Class* Params;
+            }
+          )cpp",
+          "Declared in namespace ns1\n\nstruct MyClass {}",
+      },
+      {
+          R"cpp(// Class
+            namespace ns1 {
+              class MyClass {};
+            } // namespace ns1
+            int main() {
+              ns1::My^Class* Params;
+            }
+          )cpp",
+          "Declared in namespace ns1\n\nclass MyClass {}",
+      },
+      {
+          R"cpp(// Union
+            namespace ns1 {
+              union MyUnion { int x; int y; };
+            } // namespace ns1
+            int main() {
+              ns1::My^Union Params;
+            }
+          )cpp",
+          "Declared in namespace ns1\n\nunion MyUnion {}",
+      },
+      {
+          R"cpp(// Function definition via pointer
+            int foo(int) {}
+            int main() {
+              auto *X = &^foo;
+            }
+          )cpp",
+          "Declared in global namespace\n\nint foo(int)",
+      },
+      {
+          R"cpp(// Function declaration via call
+            int foo(int);
+            int main() {
+              return ^foo(42);
+            }
+          )cpp",
+          "Declared in global namespace\n\nint foo(int)",
+      },
+      {
+          R"cpp(// Field
+            struct Foo { int x; };
+            int main() {
+              Foo bar;
+              bar.^x;
+            }
+          )cpp",
+          "Declared in struct Foo\n\nint x",
+      },
+      {
+          R"cpp(// Field with initialization
+            struct Foo { int x = 5; };
+            int main() {
+              Foo bar;
+              bar.^x;
+            }
+          )cpp",
+          "Declared in struct Foo\n\nint x = 5",
+      },
+      {
+          R"cpp(// Static field
+            struct Foo { static int x; };
+            int main() {
+              Foo::^x;
+            }
+          )cpp",
+          "Declared in struct Foo\n\nstatic int x",
+      },
+      {
+          R"cpp(// Field, member initializer
+            struct Foo {
+              int x;
+              Foo() : ^x(0) {}
+            };
+          )cpp",
+          "Declared in struct Foo\n\nint x",
+      },
+      {
+          R"cpp(// Field, GNU old-style field designator
+            struct Foo { int x; };
+            int main() {
+              Foo bar = { ^x : 1 };
+            }
+          )cpp",
+          "Declared in struct Foo\n\nint x",
+      },
+      {
+          R"cpp(// Field, field designator
+            struct Foo { int x; };
+            int main() {
+              Foo bar = { .^x = 2 };
+            }
+          )cpp",
+          "Declared in struct Foo\n\nint x",
+      },
+      {
+          R"cpp(// Method call
+            struct Foo { int x(); };
+            int main() {
+              Foo bar;
+              bar.^x();
+            }
+          )cpp",
+          "Declared in struct Foo\n\nint x()",
+      },
+      {
+          R"cpp(// Static method call
+            struct Foo { static int x(); };
+            int main() {
+              Foo::^x();
+            }
+          )cpp",
+          "Declared in struct Foo\n\nstatic int x()",
+      },
+      {
+          R"cpp(// Typedef
+            typedef int Foo;
+            int main() {
+              ^Foo bar;
+            }
+          )cpp",
+          "Declared in global namespace\n\ntypedef int Foo",
+      },
+      {
+          R"cpp(// Namespace
+            namespace ns {
+            struct Foo { static void bar(); }
+            } // namespace ns
+            int main() { ^ns::Foo::bar(); }
+          )cpp",
+          "Declared in global namespace\n\nnamespace ns {\n}",
+      },
+      {
+          R"cpp(// Anonymous namespace
+            namespace ns {
+              namespace {
+                int foo;
+              } // anonymous namespace
+            } // namespace ns
+            int main() { ns::f^oo++; }
+          )cpp",
+          "Declared in namespace ns::(anonymous)\n\nint foo",
+      },
+      {
+          R"cpp(// Macro
+            #define MACRO 0
+            #define MACRO 1
+            int main() { return ^MACRO; }
+            #define MACRO 2
+            #undef macro
+          )cpp",
+          "#define MACRO",
+      },
+      {
+          R"cpp(// Forward class declaration
+            class Foo;
+            class Foo {};
+            F^oo* foo();
+          )cpp",
+          "Declared in global namespace\n\nclass Foo {}",
+      },
+      {
+          R"cpp(// Function declaration
+            void foo();
+            void g() { f^oo(); }
+            void foo() {}
+          )cpp",
+          "Declared in global namespace\n\nvoid foo()",
+      },
+      {
+          R"cpp(// Enum declaration
+            enum Hello {
+              ONE, TWO, THREE,
+            };
+            void foo() {
+              Hel^lo hello = ONE;
+            }
+          )cpp",
+          "Declared in global namespace\n\nenum Hello {\n}",
+      },
+      {
+          R"cpp(// Enumerator
+            enum Hello {
+              ONE, TWO, THREE,
+            };
+            void foo() {
+              Hello hello = O^NE;
+            }
+          )cpp",
+          "Declared in enum Hello\n\nONE",
+      },
+      {
+          R"cpp(// Enumerator in anonymous enum
+            enum {
+              ONE, TWO, THREE,
+            };
+            void foo() {
+              int hello = O^NE;
+            }
+          )cpp",
+          "Declared in enum (anonymous)\n\nONE",
+      },
+      {
+          R"cpp(// Global variable
+            static int hey = 10;
+            void foo() {
+              he^y++;
+            }
+          )cpp",
+          "Declared in global namespace\n\nstatic int hey = 10",
+      },
+      {
+          R"cpp(// Global variable in namespace
+            namespace ns1 {
+              static int hey = 10;
+            }
+            void foo() {
+              ns1::he^y++;
+            }
+          )cpp",
+          "Declared in namespace ns1\n\nstatic int hey = 10",
+      },
+      {
+          R"cpp(// Field in anonymous struct
+            static struct {
+              int hello;
+            } s;
+            void foo() {
+              s.he^llo++;
+            }
+          )cpp",
+          "Declared in struct (anonymous)\n\nint hello",
+      },
+      {
+          R"cpp(// Templated function
+            template <typename T>
+            T foo() {
+              return 17;
+            }
+            void g() { auto x = f^oo<int>(); }
+          )cpp",
+          "Declared in global namespace\n\ntemplate <typename T> T foo()",
+      },
+      {
+          R"cpp(// Anonymous union
+            struct outer {
+              union {
+                int abc, def;
+              } v;
+            };
+            void g() { struct outer o; o.v.d^ef++; }
+          )cpp",
+          "Declared in union outer::(anonymous)\n\nint def",
+      },
+  };
+
+  for (const OneTest &Test : Tests) {
+    Annotations T(Test.Input);
+    auto AST = build(T.code());
+    Hover H = getHover(AST, T.point());
+
+    EXPECT_EQ(H.Contents.Value, Test.ExpectedHover) << Test.Input;
+  }
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang