clang-tools-extra/clangd/CodeComplete.cpp

//===--- CodeComplete.cpp ---------------------------------------*- C++-*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===---------------------------------------------------------------------===//
//
// AST-based completions are provided using the completion hooks in Sema.
//
// Signature help works in a similar way as code completion, but it is simpler
// as there are typically fewer candidates.
//
//===---------------------------------------------------------------------===//

#include "CodeComplete.h"
#include "CodeCompletionStrings.h"
#include "Compiler.h"
#include "FuzzyMatch.h"
#include "Headers.h"
#include "Logger.h"
#include "Quality.h"
#include "SourceCode.h"
#include "Trace.h"
#include "URI.h"
#include "index/Index.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
#include "clang/Basic/LangOptions.h"
#include "clang/Format/Format.h"
#include "clang/Frontend/CompilerInstance.h"
#include "clang/Frontend/FrontendActions.h"
#include "clang/Index/USRGeneration.h"
#include "clang/Sema/CodeCompleteConsumer.h"
#include "clang/Sema/Sema.h"
#include "clang/Tooling/Core/Replacement.h"
#include "llvm/Support/Format.h"
#include <queue>

// We log detailed candidate here if you run with -debug-only=codecomplete.
#define DEBUG_TYPE "codecomplete"

namespace clang {
namespace clangd {
namespace {

CompletionItemKind toCompletionItemKind(CXCursorKind CursorKind) {
  switch (CursorKind) {
  case CXCursor_MacroInstantiation:
  case CXCursor_MacroDefinition:
    return CompletionItemKind::Text;
  case CXCursor_CXXMethod:
  case CXCursor_Destructor:
    return CompletionItemKind::Method;
  case CXCursor_FunctionDecl:
  case CXCursor_FunctionTemplate:
    return CompletionItemKind::Function;
  case CXCursor_Constructor:
    return CompletionItemKind::Constructor;
  case CXCursor_FieldDecl:
    return CompletionItemKind::Field;
  case CXCursor_VarDecl:
  case CXCursor_ParmDecl:
    return CompletionItemKind::Variable;
  // FIXME(ioeric): use LSP struct instead of class when it is suppoted in the
  // protocol.
  case CXCursor_StructDecl:
  case CXCursor_ClassDecl:
  case CXCursor_UnionDecl:
  case CXCursor_ClassTemplate:
  case CXCursor_ClassTemplatePartialSpecialization:
    return CompletionItemKind::Class;
  case CXCursor_Namespace:
  case CXCursor_NamespaceAlias:
  case CXCursor_NamespaceRef:
    return CompletionItemKind::Module;
  case CXCursor_EnumConstantDecl:
    return CompletionItemKind::Value;
  case CXCursor_EnumDecl:
    return CompletionItemKind::Enum;
  // FIXME(ioeric): figure out whether reference is the right type for aliases.
  case CXCursor_TypeAliasDecl:
  case CXCursor_TypeAliasTemplateDecl:
  case CXCursor_TypedefDecl:
  case CXCursor_MemberRef:
  case CXCursor_TypeRef:
    return CompletionItemKind::Reference;
  default:
    return CompletionItemKind::Missing;
  }
}

CompletionItemKind
toCompletionItemKind(CodeCompletionResult::ResultKind ResKind,
                     CXCursorKind CursorKind) {
  switch (ResKind) {
  case CodeCompletionResult::RK_Declaration:
    return toCompletionItemKind(CursorKind);
  case CodeCompletionResult::RK_Keyword:
    return CompletionItemKind::Keyword;
  case CodeCompletionResult::RK_Macro:
    return CompletionItemKind::Text; // unfortunately, there's no 'Macro'
                                     // completion items in LSP.
  case CodeCompletionResult::RK_Pattern:
    return CompletionItemKind::Snippet;
  }
  llvm_unreachable("Unhandled CodeCompletionResult::ResultKind.");
}

CompletionItemKind toCompletionItemKind(index::SymbolKind Kind) {
  using SK = index::SymbolKind;
  switch (Kind) {
  case SK::Unknown:
    return CompletionItemKind::Missing;
  case SK::Module:
  case SK::Namespace:
  case SK::NamespaceAlias:
    return CompletionItemKind::Module;
  case SK::Macro:
    return CompletionItemKind::Text;
  case SK::Enum:
    return CompletionItemKind::Enum;
  // FIXME(ioeric): use LSP struct instead of class when it is suppoted in the
  // protocol.
  case SK::Struct:
  case SK::Class:
  case SK::Protocol:
  case SK::Extension:
  case SK::Union:
    return CompletionItemKind::Class;
  // FIXME(ioeric): figure out whether reference is the right type for aliases.
  case SK::TypeAlias:
  case SK::Using:
    return CompletionItemKind::Reference;
  case SK::Function:
  // FIXME(ioeric): this should probably be an operator. This should be fixed
  // when `Operator` is support type in the protocol.
  case SK::ConversionFunction:
    return CompletionItemKind::Function;
  case SK::Variable:
  case SK::Parameter:
    return CompletionItemKind::Variable;
  case SK::Field:
    return CompletionItemKind::Field;
  // FIXME(ioeric): use LSP enum constant when it is supported in the protocol.
  case SK::EnumConstant:
    return CompletionItemKind::Value;
  case SK::InstanceMethod:
  case SK::ClassMethod:
  case SK::StaticMethod:
  case SK::Destructor:
    return CompletionItemKind::Method;
  case SK::InstanceProperty:
  case SK::ClassProperty:
  case SK::StaticProperty:
    return CompletionItemKind::Property;
  case SK::Constructor:
    return CompletionItemKind::Constructor;
  }
  llvm_unreachable("Unhandled clang::index::SymbolKind.");
}

/// Get the optional chunk as a string. This function is possibly recursive.
///
/// The parameter info for each parameter is appended to the Parameters.
std::string
getOptionalParameters(const CodeCompletionString &CCS,
                      std::vector<ParameterInformation> &Parameters) {
  std::string Result;
  for (const auto &Chunk : CCS) {
    switch (Chunk.Kind) {
    case CodeCompletionString::CK_Optional:
      assert(Chunk.Optional &&
             "Expected the optional code completion string to be non-null.");
      Result += getOptionalParameters(*Chunk.Optional, Parameters);
      break;
    case CodeCompletionString::CK_VerticalSpace:
      break;
    case CodeCompletionString::CK_Placeholder:
      // A string that acts as a placeholder for, e.g., a function call
      // argument.
      // Intentional fallthrough here.
    case CodeCompletionString::CK_CurrentParameter: {
      // A piece of text that describes the parameter that corresponds to
      // the code-completion location within a function call, message send,
      // macro invocation, etc.
      Result += Chunk.Text;
      ParameterInformation Info;
      Info.label = Chunk.Text;
      Parameters.push_back(std::move(Info));
      break;
    }
    default:
      Result += Chunk.Text;
      break;
    }
  }
  return Result;
}

/// Creates a `HeaderFile` from \p Header which can be either a URI or a literal
/// include.
static llvm::Expected<HeaderFile> toHeaderFile(StringRef Header,
                                               llvm::StringRef HintPath) {
  if (isLiteralInclude(Header))
    return HeaderFile{Header.str(), /*Verbatim=*/true};
  auto U = URI::parse(Header);
  if (!U)
    return U.takeError();

  auto IncludePath = URI::includeSpelling(*U);
  if (!IncludePath)
    return IncludePath.takeError();
  if (!IncludePath->empty())
    return HeaderFile{std::move(*IncludePath), /*Verbatim=*/true};

  auto Resolved = URI::resolve(*U, HintPath);
  if (!Resolved)
    return Resolved.takeError();
  return HeaderFile{std::move(*Resolved), /*Verbatim=*/false};
}

/// A code completion result, in clang-native form.
/// It may be promoted to a CompletionItem if it's among the top-ranked results.
struct CompletionCandidate {
  llvm::StringRef Name; // Used for filtering and sorting.
  // We may have a result from Sema, from the index, or both.
  const CodeCompletionResult *SemaResult = nullptr;
  const Symbol *IndexResult = nullptr;

  // Builds an LSP completion item.
  CompletionItem build(StringRef FileName, const CompletionItemScores &Scores,
                       const CodeCompleteOptions &Opts,
                       CodeCompletionString *SemaCCS,
                       const IncludeInserter *Includes,
                       llvm::StringRef SemaDocComment) const {
    assert(bool(SemaResult) == bool(SemaCCS));
    CompletionItem I;
    bool ShouldInsertInclude = true;
    if (SemaResult) {
      I.kind = toCompletionItemKind(SemaResult->Kind, SemaResult->CursorKind);
      getLabelAndInsertText(*SemaCCS, &I.label, &I.insertText,
                            Opts.EnableSnippets);
      I.filterText = getFilterText(*SemaCCS);
      I.documentation = formatDocumentation(*SemaCCS, SemaDocComment);
      I.detail = getDetail(*SemaCCS);
      // Avoid inserting new #include if the declaration is found in the current
      // file e.g. the symbol is forward declared.
      if (SemaResult->Kind == CodeCompletionResult::RK_Declaration) {
        if (const auto *D = SemaResult->getDeclaration()) {
          const auto &SM = D->getASTContext().getSourceManager();
          ShouldInsertInclude =
              ShouldInsertInclude &&
              std::none_of(D->redecls_begin(), D->redecls_end(),
                           [&SM](const Decl *RD) {
                             return SM.isInMainFile(
                                 SM.getExpansionLoc(RD->getLocStart()));
                           });
        }
      }
    }
    if (IndexResult) {
      if (I.kind == CompletionItemKind::Missing)
        I.kind = toCompletionItemKind(IndexResult->SymInfo.Kind);
      // FIXME: reintroduce a way to show the index source for debugging.
      if (I.label.empty())
        I.label = IndexResult->CompletionLabel;
      if (I.filterText.empty())
        I.filterText = IndexResult->Name;

      // FIXME(ioeric): support inserting/replacing scope qualifiers.
      if (I.insertText.empty())
        I.insertText = Opts.EnableSnippets
                           ? IndexResult->CompletionSnippetInsertText
                           : IndexResult->CompletionPlainInsertText;

      if (auto *D = IndexResult->Detail) {
        if (I.documentation.empty())
          I.documentation = D->Documentation;
        if (I.detail.empty())
          I.detail = D->CompletionDetail;
        if (ShouldInsertInclude && Includes && !D->IncludeHeader.empty()) {
          auto Edit = [&]() -> Expected<Optional<TextEdit>> {
            auto ResolvedDeclaring = toHeaderFile(
                IndexResult->CanonicalDeclaration.FileURI, FileName);
            if (!ResolvedDeclaring)
              return ResolvedDeclaring.takeError();
            auto ResolvedInserted = toHeaderFile(D->IncludeHeader, FileName);
            if (!ResolvedInserted)
              return ResolvedInserted.takeError();
            return Includes->insert(*ResolvedDeclaring, *ResolvedInserted);
          }();
          if (!Edit) {
            std::string ErrMsg =
                ("Failed to generate include insertion edits for adding header "
                 "(FileURI=\"" +
                 IndexResult->CanonicalDeclaration.FileURI +
                 "\", IncludeHeader=\"" + D->IncludeHeader + "\") into " +
                 FileName)
                    .str();
            log(ErrMsg + ":" + llvm::toString(Edit.takeError()));
          } else if (*Edit) {
            I.additionalTextEdits = {std::move(**Edit)};
          }
        }
      }
    }
    I.scoreInfo = Scores;
    I.sortText = sortText(Scores.finalScore, Name);
    I.insertTextFormat = Opts.EnableSnippets ? InsertTextFormat::Snippet
                                             : InsertTextFormat::PlainText;
    return I;
  }
};
using ScoredCandidate = std::pair<CompletionCandidate, CompletionItemScores>;

// Determine the symbol ID for a Sema code completion result, if possible.
llvm::Optional<SymbolID> getSymbolID(const CodeCompletionResult &R) {
  switch (R.Kind) {
  case CodeCompletionResult::RK_Declaration:
  case CodeCompletionResult::RK_Pattern: {
    llvm::SmallString<128> USR;
    if (/*Ignore=*/clang::index::generateUSRForDecl(R.Declaration, USR))
      return None;
    return SymbolID(USR);
  }
  case CodeCompletionResult::RK_Macro:
    // FIXME: Macros do have USRs, but the CCR doesn't contain enough info.
  case CodeCompletionResult::RK_Keyword:
    return None;
  }
  llvm_unreachable("unknown CodeCompletionResult kind");
}

// Scopes of the paritial identifier we're trying to complete.
// It is used when we query the index for more completion results.
struct SpecifiedScope {
  // The scopes we should look in, determined by Sema.
  //
  // If the qualifier was fully resolved, we look for completions in these
  // scopes; if there is an unresolved part of the qualifier, it should be
  // resolved within these scopes.
  //
  // Examples of qualified completion:
  //
  //   "::vec"                                      => {""}
  //   "using namespace std; ::vec^"                => {"", "std::"}
  //   "namespace ns {using namespace std;} ns::^"  => {"ns::", "std::"}
  //   "std::vec^"                                  => {""}  // "std" unresolved
  //
  // Examples of unqualified completion:
  //
  //   "vec^"                                       => {""}
  //   "using namespace std; vec^"                  => {"", "std::"}
  //   "using namespace std; namespace ns { vec^ }" => {"ns::", "std::", ""}
  //
  // "" for global namespace, "ns::" for normal namespace.
  std::vector<std::string> AccessibleScopes;
  // The full scope qualifier as typed by the user (without the leading "::").
  // Set if the qualifier is not fully resolved by Sema.
  llvm::Optional<std::string> UnresolvedQualifier;

  // Construct scopes being queried in indexes.
  // This method format the scopes to match the index request representation.
  std::vector<std::string> scopesForIndexQuery() {
    std::vector<std::string> Results;
    for (llvm::StringRef AS : AccessibleScopes) {
      Results.push_back(AS);
      if (UnresolvedQualifier)
        Results.back() += *UnresolvedQualifier;
    }
    return Results;
  }
};

// Get all scopes that will be queried in indexes.
std::vector<std::string> getQueryScopes(CodeCompletionContext &CCContext,
                                        const SourceManager &SM) {
  auto GetAllAccessibleScopes = [](CodeCompletionContext &CCContext) {
    SpecifiedScope Info;
    for (auto *Context : CCContext.getVisitedContexts()) {
      if (isa<TranslationUnitDecl>(Context))
        Info.AccessibleScopes.push_back(""); // global namespace
      else if (const auto *NS = dyn_cast<NamespaceDecl>(Context))
        Info.AccessibleScopes.push_back(NS->getQualifiedNameAsString() + "::");
    }
    return Info;
  };

  auto SS = CCContext.getCXXScopeSpecifier();

  // Unqualified completion (e.g. "vec^").
  if (!SS) {
    // FIXME: Once we can insert namespace qualifiers and use the in-scope
    //        namespaces for scoring, search in all namespaces.
    // FIXME: Capture scopes and use for scoring, for example,
    //        "using namespace std; namespace foo {v^}" =>
    //        foo::value > std::vector > boost::variant
    return GetAllAccessibleScopes(CCContext).scopesForIndexQuery();
  }

  // Qualified completion ("std::vec^"), we have two cases depending on whether
  // the qualifier can be resolved by Sema.
  if ((*SS)->isValid()) { // Resolved qualifier.
    return GetAllAccessibleScopes(CCContext).scopesForIndexQuery();
  }

  // Unresolved qualifier.
  // FIXME: When Sema can resolve part of a scope chain (e.g.
  // "known::unknown::id"), we should expand the known part ("known::") rather
  // than treating the whole thing as unknown.
  SpecifiedScope Info;
  Info.AccessibleScopes.push_back(""); // global namespace

  Info.UnresolvedQualifier =
      Lexer::getSourceText(CharSourceRange::getCharRange((*SS)->getRange()), SM,
                           clang::LangOptions())
          .ltrim("::");
  // Sema excludes the trailing "::".
  if (!Info.UnresolvedQualifier->empty())
    *Info.UnresolvedQualifier += "::";

  return Info.scopesForIndexQuery();
}

// Should we perform index-based completion in a context of the specified kind?
// FIXME: consider allowing completion, but restricting the result types.
bool contextAllowsIndex(enum CodeCompletionContext::Kind K) {
  switch (K) {
  case CodeCompletionContext::CCC_TopLevel:
  case CodeCompletionContext::CCC_ObjCInterface:
  case CodeCompletionContext::CCC_ObjCImplementation:
  case CodeCompletionContext::CCC_ObjCIvarList:
  case CodeCompletionContext::CCC_ClassStructUnion:
  case CodeCompletionContext::CCC_Statement:
  case CodeCompletionContext::CCC_Expression:
  case CodeCompletionContext::CCC_ObjCMessageReceiver:
  case CodeCompletionContext::CCC_EnumTag:
  case CodeCompletionContext::CCC_UnionTag:
  case CodeCompletionContext::CCC_ClassOrStructTag:
  case CodeCompletionContext::CCC_ObjCProtocolName:
  case CodeCompletionContext::CCC_Namespace:
  case CodeCompletionContext::CCC_Type:
  case CodeCompletionContext::CCC_Name: // FIXME: why does ns::^ give this?
  case CodeCompletionContext::CCC_PotentiallyQualifiedName:
  case CodeCompletionContext::CCC_ParenthesizedExpression:
  case CodeCompletionContext::CCC_ObjCInterfaceName:
  case CodeCompletionContext::CCC_ObjCCategoryName:
    return true;
  case CodeCompletionContext::CCC_Other: // Be conservative.
  case CodeCompletionContext::CCC_OtherWithMacros:
  case CodeCompletionContext::CCC_DotMemberAccess:
  case CodeCompletionContext::CCC_ArrowMemberAccess:
  case CodeCompletionContext::CCC_ObjCPropertyAccess:
  case CodeCompletionContext::CCC_MacroName:
  case CodeCompletionContext::CCC_MacroNameUse:
  case CodeCompletionContext::CCC_PreprocessorExpression:
  case CodeCompletionContext::CCC_PreprocessorDirective:
  case CodeCompletionContext::CCC_NaturalLanguage:
  case CodeCompletionContext::CCC_SelectorName:
  case CodeCompletionContext::CCC_TypeQualifiers:
  case CodeCompletionContext::CCC_ObjCInstanceMessage:
  case CodeCompletionContext::CCC_ObjCClassMessage:
  case CodeCompletionContext::CCC_Recovery:
    return false;
  }
  llvm_unreachable("unknown code completion context");
}

// Some member calls are blacklisted because they're so rarely useful.
static bool isBlacklistedMember(const NamedDecl &D) {
  // Destructor completion is rarely useful, and works inconsistently.
  // (s.^ completes ~string, but s.~st^ is an error).
  if (D.getKind() == Decl::CXXDestructor)
    return true;
  // Injected name may be useful for A::foo(), but who writes A::A::foo()?
  if (auto *R = dyn_cast_or_null<RecordDecl>(&D))
    if (R->isInjectedClassName())
      return true;
  // Explicit calls to operators are also rare.
  auto NameKind = D.getDeclName().getNameKind();
  if (NameKind == DeclarationName::CXXOperatorName ||
      NameKind == DeclarationName::CXXLiteralOperatorName ||
      NameKind == DeclarationName::CXXConversionFunctionName)
    return true;
  return false;
}

// The CompletionRecorder captures Sema code-complete output, including context.
// It filters out ignored results (but doesn't apply fuzzy-filtering yet).
// It doesn't do scoring or conversion to CompletionItem yet, as we want to
// merge with index results first.
// Generally the fields and methods of this object should only be used from
// within the callback.
struct CompletionRecorder : public CodeCompleteConsumer {
  CompletionRecorder(const CodeCompleteOptions &Opts,
                     UniqueFunction<void()> ResultsCallback)
      : CodeCompleteConsumer(Opts.getClangCompleteOpts(),
                             /*OutputIsBinary=*/false),
        CCContext(CodeCompletionContext::CCC_Other), Opts(Opts),
        CCAllocator(std::make_shared<GlobalCodeCompletionAllocator>()),
        CCTUInfo(CCAllocator), ResultsCallback(std::move(ResultsCallback)) {
    assert(this->ResultsCallback);
  }

  std::vector<CodeCompletionResult> Results;
  CodeCompletionContext CCContext;
  Sema *CCSema = nullptr; // Sema that created the results.
  // FIXME: Sema is scary. Can we store ASTContext and Preprocessor, instead?

  void ProcessCodeCompleteResults(class Sema &S, CodeCompletionContext Context,
                                  CodeCompletionResult *InResults,
                                  unsigned NumResults) override final {
    // If a callback is called without any sema result and the context does not
    // support index-based completion, we simply skip it to give way to
    // potential future callbacks with results.
    if (NumResults == 0 && !contextAllowsIndex(Context.getKind()))
      return;
    if (CCSema) {
      log(llvm::formatv(
          "Multiple code complete callbacks (parser backtracked?). "
          "Dropping results from context {0}, keeping results from {1}.",
          getCompletionKindString(Context.getKind()),
          getCompletionKindString(this->CCContext.getKind())));
      return;
    }
    // Record the completion context.
    CCSema = &S;
    CCContext = Context;

    // Retain the results we might want.
    for (unsigned I = 0; I < NumResults; ++I) {
      auto &Result = InResults[I];
      // Drop hidden items which cannot be found by lookup after completion.
      // Exception: some items can be named by using a qualifier.
      if (Result.Hidden && (!Result.Qualifier || Result.QualifierIsInformative))
        continue;
      if (!Opts.IncludeIneligibleResults &&
          (Result.Availability == CXAvailability_NotAvailable ||
           Result.Availability == CXAvailability_NotAccessible))
        continue;
      if (Result.Declaration &&
          !Context.getBaseType().isNull() // is this a member-access context?
          && isBlacklistedMember(*Result.Declaration))
        continue;
      // We choose to never append '::' to completion results in clangd.
      Result.StartsNestedNameSpecifier = false;
      Results.push_back(Result);
    }
    ResultsCallback();
  }

  CodeCompletionAllocator &getAllocator() override { return *CCAllocator; }
  CodeCompletionTUInfo &getCodeCompletionTUInfo() override { return CCTUInfo; }

  // Returns the filtering/sorting name for Result, which must be from Results.
  // Returned string is owned by this recorder (or the AST).
  llvm::StringRef getName(const CodeCompletionResult &Result) {
    switch (Result.Kind) {
    case CodeCompletionResult::RK_Declaration:
      if (auto *ID = Result.Declaration->getIdentifier())
        return ID->getName();
      break;
    case CodeCompletionResult::RK_Keyword:
      return Result.Keyword;
    case CodeCompletionResult::RK_Macro:
      return Result.Macro->getName();
    case CodeCompletionResult::RK_Pattern:
      return Result.Pattern->getTypedText();
    }
    auto *CCS = codeCompletionString(Result);
    return CCS->getTypedText();
  }

  // Build a CodeCompletion string for R, which must be from Results.
  // The CCS will be owned by this recorder.
  CodeCompletionString *codeCompletionString(const CodeCompletionResult &R) {
    // CodeCompletionResult doesn't seem to be const-correct. We own it, anyway.
    return const_cast<CodeCompletionResult &>(R).CreateCodeCompletionString(
        *CCSema, CCContext, *CCAllocator, CCTUInfo,
        /*IncludeBriefComments=*/false);
  }

private:
  CodeCompleteOptions Opts;
  std::shared_ptr<GlobalCodeCompletionAllocator> CCAllocator;
  CodeCompletionTUInfo CCTUInfo;
  UniqueFunction<void()> ResultsCallback;
};

struct ScoredCandidateGreater {
  bool operator()(const ScoredCandidate &L, const ScoredCandidate &R) {
    if (L.second.finalScore != R.second.finalScore)
      return L.second.finalScore > R.second.finalScore;
    return L.first.Name < R.first.Name; // Earlier name is better.
  }
};

class SignatureHelpCollector final : public CodeCompleteConsumer {

public:
  SignatureHelpCollector(const clang::CodeCompleteOptions &CodeCompleteOpts,
                         SignatureHelp &SigHelp)
      : CodeCompleteConsumer(CodeCompleteOpts, /*OutputIsBinary=*/false),
        SigHelp(SigHelp),
        Allocator(std::make_shared<clang::GlobalCodeCompletionAllocator>()),
        CCTUInfo(Allocator) {}

  void ProcessOverloadCandidates(Sema &S, unsigned CurrentArg,
                                 OverloadCandidate *Candidates,
                                 unsigned NumCandidates) override {
    SigHelp.signatures.reserve(NumCandidates);
    // FIXME(rwols): How can we determine the "active overload candidate"?
    // Right now the overloaded candidates seem to be provided in a "best fit"
    // order, so I'm not too worried about this.
    SigHelp.activeSignature = 0;
    assert(CurrentArg <= (unsigned)std::numeric_limits<int>::max() &&
           "too many arguments");
    SigHelp.activeParameter = static_cast<int>(CurrentArg);
    for (unsigned I = 0; I < NumCandidates; ++I) {
      const auto &Candidate = Candidates[I];
      const auto *CCS = Candidate.CreateSignatureString(
          CurrentArg, S, *Allocator, CCTUInfo, true);
      assert(CCS && "Expected the CodeCompletionString to be non-null");
      // FIXME: for headers, we need to get a comment from the index.
      SigHelp.signatures.push_back(ProcessOverloadCandidate(
          Candidate, *CCS,
          getParameterDocComment(S.getASTContext(), Candidate, CurrentArg,
                                 /*CommentsFromHeaders=*/false)));
    }
  }

  GlobalCodeCompletionAllocator &getAllocator() override { return *Allocator; }

  CodeCompletionTUInfo &getCodeCompletionTUInfo() override { return CCTUInfo; }

private:
  // FIXME(ioeric): consider moving CodeCompletionString logic here to
  // CompletionString.h.
  SignatureInformation
  ProcessOverloadCandidate(const OverloadCandidate &Candidate,
                           const CodeCompletionString &CCS,
                           llvm::StringRef DocComment) const {
    SignatureInformation Result;
    const char *ReturnType = nullptr;

    Result.documentation = formatDocumentation(CCS, DocComment);

    for (const auto &Chunk : CCS) {
      switch (Chunk.Kind) {
      case CodeCompletionString::CK_ResultType:
        // A piece of text that describes the type of an entity or,
        // for functions and methods, the return type.
        assert(!ReturnType && "Unexpected CK_ResultType");
        ReturnType = Chunk.Text;
        break;
      case CodeCompletionString::CK_Placeholder:
        // A string that acts as a placeholder for, e.g., a function call
        // argument.
        // Intentional fallthrough here.
      case CodeCompletionString::CK_CurrentParameter: {
        // A piece of text that describes the parameter that corresponds to
        // the code-completion location within a function call, message send,
        // macro invocation, etc.
        Result.label += Chunk.Text;
        ParameterInformation Info;
        Info.label = Chunk.Text;
        Result.parameters.push_back(std::move(Info));
        break;
      }
      case CodeCompletionString::CK_Optional: {
        // The rest of the parameters are defaulted/optional.
        assert(Chunk.Optional &&
               "Expected the optional code completion string to be non-null.");
        Result.label +=
            getOptionalParameters(*Chunk.Optional, Result.parameters);
        break;
      }
      case CodeCompletionString::CK_VerticalSpace:
        break;
      default:
        Result.label += Chunk.Text;
        break;
      }
    }
    if (ReturnType) {
      Result.label += " -> ";
      Result.label += ReturnType;
    }
    return Result;
  }

  SignatureHelp &SigHelp;
  std::shared_ptr<clang::GlobalCodeCompletionAllocator> Allocator;
  CodeCompletionTUInfo CCTUInfo;

}; // SignatureHelpCollector

struct SemaCompleteInput {
  PathRef FileName;
  const tooling::CompileCommand &Command;
  PrecompiledPreamble const *Preamble;
  const std::vector<Inclusion> &PreambleInclusions;
  StringRef Contents;
  Position Pos;
  IntrusiveRefCntPtr<vfs::FileSystem> VFS;
  std::shared_ptr<PCHContainerOperations> PCHs;
};

// Invokes Sema code completion on a file.
// If \p Includes is set, it will be initialized after a compiler instance has
// been set up.
bool semaCodeComplete(std::unique_ptr<CodeCompleteConsumer> Consumer,
                      const clang::CodeCompleteOptions &Options,
                      const SemaCompleteInput &Input,
                      std::unique_ptr<IncludeInserter> *Includes = nullptr) {
  trace::Span Tracer("Sema completion");
  std::vector<const char *> ArgStrs;
  for (const auto &S : Input.Command.CommandLine)
    ArgStrs.push_back(S.c_str());

  if (Input.VFS->setCurrentWorkingDirectory(Input.Command.Directory)) {
    log("Couldn't set working directory");
    // We run parsing anyway, our lit-tests rely on results for non-existing
    // working dirs.
  }

  IgnoreDiagnostics DummyDiagsConsumer;
  auto CI = createInvocationFromCommandLine(
      ArgStrs,
      CompilerInstance::createDiagnostics(new DiagnosticOptions,
                                          &DummyDiagsConsumer, false),
      Input.VFS);
  if (!CI) {
    log("Couldn't create CompilerInvocation");
    return false;
  }
  auto &FrontendOpts = CI->getFrontendOpts();
  FrontendOpts.DisableFree = false;
  FrontendOpts.SkipFunctionBodies = true;
  CI->getLangOpts()->CommentOpts.ParseAllComments = true;
  // Disable typo correction in Sema.
  CI->getLangOpts()->SpellChecking = false;
  // Setup code completion.
  FrontendOpts.CodeCompleteOpts = Options;
  FrontendOpts.CodeCompletionAt.FileName = Input.FileName;
  auto Offset = positionToOffset(Input.Contents, Input.Pos);
  if (!Offset) {
    log("Code completion position was invalid " +
        llvm::toString(Offset.takeError()));
    return false;
  }
  std::tie(FrontendOpts.CodeCompletionAt.Line,
           FrontendOpts.CodeCompletionAt.Column) =
      offsetToClangLineColumn(Input.Contents, *Offset);

  std::unique_ptr<llvm::MemoryBuffer> ContentsBuffer =
      llvm::MemoryBuffer::getMemBufferCopy(Input.Contents, Input.FileName);
  // The diagnostic options must be set before creating a CompilerInstance.
  CI->getDiagnosticOpts().IgnoreWarnings = true;
  // We reuse the preamble whether it's valid or not. This is a
  // correctness/performance tradeoff: building without a preamble is slow, and
  // completion is latency-sensitive.
  // NOTE: we must call BeginSourceFile after prepareCompilerInstance. Otherwise
  // the remapped buffers do not get freed.
  auto Clang = prepareCompilerInstance(
      std::move(CI), Input.Preamble, std::move(ContentsBuffer),
      std::move(Input.PCHs), std::move(Input.VFS), DummyDiagsConsumer);
  Clang->setCodeCompletionConsumer(Consumer.release());

  SyntaxOnlyAction Action;
  if (!Action.BeginSourceFile(*Clang, Clang->getFrontendOpts().Inputs[0])) {
    log("BeginSourceFile() failed when running codeComplete for " +
        Input.FileName);
    return false;
  }
  if (Includes) {
    // Initialize Includes if provided.

    // FIXME(ioeric): needs more consistent style support in clangd server.
    auto Style = format::getStyle("file", Input.FileName, "LLVM",
                                  Input.Contents, Input.VFS.get());
    if (!Style) {
      log("Failed to get FormatStyle for file" + Input.FileName +
          ". Fall back to use LLVM style. Error: " +
          llvm::toString(Style.takeError()));
      Style = format::getLLVMStyle();
    }
    *Includes = llvm::make_unique<IncludeInserter>(
        Input.FileName, Input.Contents, *Style, Input.Command.Directory,
        Clang->getPreprocessor().getHeaderSearchInfo());
    for (const auto &Inc : Input.PreambleInclusions)
      Includes->get()->addExisting(Inc);
    Clang->getPreprocessor().addPPCallbacks(collectInclusionsInMainFileCallback(
        Clang->getSourceManager(), [Includes](Inclusion Inc) {
          Includes->get()->addExisting(std::move(Inc));
        }));
  }
  if (!Action.Execute()) {
    log("Execute() failed when running codeComplete for " + Input.FileName);
    return false;
  }
  Action.EndSourceFile();

  return true;
}

// Should we allow index completions in the specified context?
bool allowIndex(CodeCompletionContext &CC) {
  if (!contextAllowsIndex(CC.getKind()))
    return false;
  // We also avoid ClassName::bar (but allow namespace::bar).
  auto Scope = CC.getCXXScopeSpecifier();
  if (!Scope)
    return true;
  NestedNameSpecifier *NameSpec = (*Scope)->getScopeRep();
  if (!NameSpec)
    return true;
  // We only query the index when qualifier is a namespace.
  // If it's a class, we rely solely on sema completions.
  switch (NameSpec->getKind()) {
  case NestedNameSpecifier::Global:
  case NestedNameSpecifier::Namespace:
  case NestedNameSpecifier::NamespaceAlias:
    return true;
  case NestedNameSpecifier::Super:
  case NestedNameSpecifier::TypeSpec:
  case NestedNameSpecifier::TypeSpecWithTemplate:
  // Unresolved inside a template.
  case NestedNameSpecifier::Identifier:
    return false;
  }
  llvm_unreachable("invalid NestedNameSpecifier kind");
}

} // namespace

clang::CodeCompleteOptions CodeCompleteOptions::getClangCompleteOpts() const {
  clang::CodeCompleteOptions Result;
  Result.IncludeCodePatterns = EnableSnippets && IncludeCodePatterns;
  Result.IncludeMacros = IncludeMacros;
  Result.IncludeGlobals = true;
  // We choose to include full comments and not do doxygen parsing in
  // completion.
  // FIXME: ideally, we should support doxygen in some form, e.g. do markdown
  // formatting of the comments.
  Result.IncludeBriefComments = false;

  // When an is used, Sema is responsible for completing the main file,
  // the index can provide results from the preamble.
  // Tell Sema not to deserialize the preamble to look for results.
  Result.LoadExternal = !Index;

  return Result;
}

// Runs Sema-based (AST) and Index-based completion, returns merged results.
//
// There are a few tricky considerations:
//   - the AST provides information needed for the index query (e.g. which
//     namespaces to search in). So Sema must start first.
//   - we only want to return the top results (Opts.Limit).
//     Building CompletionItems for everything else is wasteful, so we want to
//     preserve the "native" format until we're done with scoring.
//   - the data underlying Sema completion items is owned by the AST and various
//     other arenas, which must stay alive for us to build CompletionItems.
//   - we may get duplicate results from Sema and the Index, we need to merge.
//
// So we start Sema completion first, and do all our work in its callback.
// We use the Sema context information to query the index.
// Then we merge the two result sets, producing items that are Sema/Index/Both.
// These items are scored, and the top N are synthesized into the LSP response.
// Finally, we can clean up the data structures created by Sema completion.
//
// Main collaborators are:
//   - semaCodeComplete sets up the compiler machinery to run code completion.
//   - CompletionRecorder captures Sema completion results, including context.
//   - SymbolIndex (Opts.Index) provides index completion results as Symbols
//   - CompletionCandidates are the result of merging Sema and Index results.
//     Each candidate points to an underlying CodeCompletionResult (Sema), a
//     Symbol (Index), or both. It computes the result quality score.
//     CompletionCandidate also does conversion to CompletionItem (at the end).
//   - FuzzyMatcher scores how the candidate matches the partial identifier.
//     This score is combined with the result quality score for the final score.
//   - TopN determines the results with the best score.
class CodeCompleteFlow {
  PathRef FileName;
  const CodeCompleteOptions &Opts;
  // Sema takes ownership of Recorder. Recorder is valid until Sema cleanup.
  CompletionRecorder *Recorder = nullptr;
  int NSema = 0, NIndex = 0, NBoth = 0; // Counters for logging.
  bool Incomplete = false; // Would more be available with a higher limit?
  llvm::Optional<FuzzyMatcher> Filter;       // Initialized once Sema runs.
  std::unique_ptr<IncludeInserter> Includes; // Initialized once compiler runs.

public:
  // A CodeCompleteFlow object is only useful for calling run() exactly once.
  CodeCompleteFlow(PathRef FileName, const CodeCompleteOptions &Opts)
      : FileName(FileName), Opts(Opts) {}

  CompletionList run(const SemaCompleteInput &SemaCCInput) && {
    trace::Span Tracer("CodeCompleteFlow");

    // We run Sema code completion first. It builds an AST and calculates:
    //   - completion results based on the AST.
    //   - partial identifier and context. We need these for the index query.
    CompletionList Output;
    auto RecorderOwner = llvm::make_unique<CompletionRecorder>(Opts, [&]() {
      assert(Recorder && "Recorder is not set");
      assert(Includes && "Includes is not set");
      // If preprocessor was run, inclusions from preprocessor callback should
      // already be added to Inclusions.
      Output = runWithSema();
      Includes.reset(); // Make sure this doesn't out-live Clang.
      SPAN_ATTACH(Tracer, "sema_completion_kind",
                  getCompletionKindString(Recorder->CCContext.getKind()));
    });

    Recorder = RecorderOwner.get();
    semaCodeComplete(std::move(RecorderOwner), Opts.getClangCompleteOpts(),
                     SemaCCInput, &Includes);

    SPAN_ATTACH(Tracer, "sema_results", NSema);
    SPAN_ATTACH(Tracer, "index_results", NIndex);
    SPAN_ATTACH(Tracer, "merged_results", NBoth);
    SPAN_ATTACH(Tracer, "returned_results", Output.items.size());
    SPAN_ATTACH(Tracer, "incomplete", Output.isIncomplete);
    log(llvm::formatv("Code complete: {0} results from Sema, {1} from Index, "
                      "{2} matched, {3} returned{4}.",
                      NSema, NIndex, NBoth, Output.items.size(),
                      Output.isIncomplete ? " (incomplete)" : ""));
    assert(!Opts.Limit || Output.items.size() <= Opts.Limit);
    // We don't assert that isIncomplete means we hit a limit.
    // Indexes may choose to impose their own limits even if we don't have one.
    return Output;
  }

private:
  // This is called by run() once Sema code completion is done, but before the
  // Sema data structures are torn down. It does all the real work.
  CompletionList runWithSema() {
    Filter = FuzzyMatcher(
        Recorder->CCSema->getPreprocessor().getCodeCompletionFilter());
    // Sema provides the needed context to query the index.
    // FIXME: in addition to querying for extra/overlapping symbols, we should
    //        explicitly request symbols corresponding to Sema results.
    //        We can use their signals even if the index can't suggest them.
    // We must copy index results to preserve them, but there are at most Limit.
    auto IndexResults = queryIndex();
    // Merge Sema and Index results, score them, and pick the winners.
    auto Top = mergeResults(Recorder->Results, IndexResults);
    // Convert the results to the desired LSP structs.
    CompletionList Output;
    for (auto &C : Top)
      Output.items.push_back(toCompletionItem(C.first, C.second));
    Output.isIncomplete = Incomplete;
    return Output;
  }

  SymbolSlab queryIndex() {
    if (!Opts.Index || !allowIndex(Recorder->CCContext))
      return SymbolSlab();
    trace::Span Tracer("Query index");
    SPAN_ATTACH(Tracer, "limit", Opts.Limit);

    SymbolSlab::Builder ResultsBuilder;
    // Build the query.
    FuzzyFindRequest Req;
    if (Opts.Limit)
      Req.MaxCandidateCount = Opts.Limit;
    Req.Query = Filter->pattern();
    Req.RestrictForCodeCompletion = true;
    Req.Scopes = getQueryScopes(Recorder->CCContext,
                                Recorder->CCSema->getSourceManager());
    log(llvm::formatv("Code complete: fuzzyFind(\"{0}\", scopes=[{1}])",
                      Req.Query,
                      llvm::join(Req.Scopes.begin(), Req.Scopes.end(), ",")));
    // Run the query against the index.
    if (Opts.Index->fuzzyFind(
            Req, [&](const Symbol &Sym) { ResultsBuilder.insert(Sym); }))
      Incomplete = true;
    return std::move(ResultsBuilder).build();
  }

  // Merges the Sema and Index results where possible, scores them, and
  // returns the top results from best to worst.
  std::vector<std::pair<CompletionCandidate, CompletionItemScores>>
  mergeResults(const std::vector<CodeCompletionResult> &SemaResults,
               const SymbolSlab &IndexResults) {
    trace::Span Tracer("Merge and score results");
    // We only keep the best N results at any time, in "native" format.
    TopN<ScoredCandidate, ScoredCandidateGreater> Top(
        Opts.Limit == 0 ? std::numeric_limits<size_t>::max() : Opts.Limit);
    llvm::DenseSet<const Symbol *> UsedIndexResults;
    auto CorrespondingIndexResult =
        [&](const CodeCompletionResult &SemaResult) -> const Symbol * {
      if (auto SymID = getSymbolID(SemaResult)) {
        auto I = IndexResults.find(*SymID);
        if (I != IndexResults.end()) {
          UsedIndexResults.insert(&*I);
          return &*I;
        }
      }
      return nullptr;
    };
    // Emit all Sema results, merging them with Index results if possible.
    for (auto &SemaResult : Recorder->Results)
      addCandidate(Top, &SemaResult, CorrespondingIndexResult(SemaResult));
    // Now emit any Index-only results.
    for (const auto &IndexResult : IndexResults) {
      if (UsedIndexResults.count(&IndexResult))
        continue;
      addCandidate(Top, /*SemaResult=*/nullptr, &IndexResult);
    }
    return std::move(Top).items();
  }

  Optional<float> fuzzyScore(const CompletionCandidate &C) {
    // Macros can be very spammy, so we only support prefix completion.
    // We won't end up with underfull index results, as macros are sema-only.
    if (C.SemaResult && C.SemaResult->Kind == CodeCompletionResult::RK_Macro &&
        !C.Name.startswith_lower(Filter->pattern()))
      return None;
    return Filter->match(C.Name);
  }

  // Scores a candidate and adds it to the TopN structure.
  void addCandidate(TopN<ScoredCandidate, ScoredCandidateGreater> &Candidates,
                    const CodeCompletionResult *SemaResult,
                    const Symbol *IndexResult) {
    CompletionCandidate C;
    C.SemaResult = SemaResult;
    C.IndexResult = IndexResult;
    C.Name = IndexResult ? IndexResult->Name : Recorder->getName(*SemaResult);

    SymbolQualitySignals Quality;
    SymbolRelevanceSignals Relevance;
    Relevance.Query = SymbolRelevanceSignals::CodeComplete;
    if (auto FuzzyScore = fuzzyScore(C))
      Relevance.NameMatch = *FuzzyScore;
    else
      return;
    if (IndexResult) {
      Quality.merge(*IndexResult);
      Relevance.merge(*IndexResult);
    }
    if (SemaResult) {
      Quality.merge(*SemaResult);
      Relevance.merge(*SemaResult);
    }

    float QualScore = Quality.evaluate(), RelScore = Relevance.evaluate();
    CompletionItemScores Scores;
    Scores.finalScore = evaluateSymbolAndRelevance(QualScore, RelScore);
    // The purpose of exporting component scores is to allow NameMatch to be
    // replaced on the client-side. So we export (NameMatch, final/NameMatch)
    // rather than (RelScore, QualScore).
    Scores.filterScore = Relevance.NameMatch;
    Scores.symbolScore =
        Scores.filterScore ? Scores.finalScore / Scores.filterScore : QualScore;

    LLVM_DEBUG(llvm::dbgs()
               << "CodeComplete: " << C.Name << (IndexResult ? " (index)" : "")
               << (SemaResult ? " (sema)" : "") << " = " << Scores.finalScore
               << "\n"
               << Quality << Relevance << "\n");

    NSema += bool(SemaResult);
    NIndex += bool(IndexResult);
    NBoth += SemaResult && IndexResult;
    if (Candidates.push({C, Scores}))
      Incomplete = true;
  }

  CompletionItem toCompletionItem(const CompletionCandidate &Candidate,
                                  const CompletionItemScores &Scores) {
    CodeCompletionString *SemaCCS = nullptr;
    std::string DocComment;
    if (auto *SR = Candidate.SemaResult) {
      SemaCCS = Recorder->codeCompletionString(*SR);
      if (Opts.IncludeComments) {
        assert(Recorder->CCSema);
        DocComment = getDocComment(Recorder->CCSema->getASTContext(), *SR,
                                   /*CommentsFromHeader=*/false);
      }
    }
    return Candidate.build(FileName, Scores, Opts, SemaCCS, Includes.get(),
                           DocComment);
  }
};

CompletionList codeComplete(PathRef FileName,
                            const tooling::CompileCommand &Command,
                            PrecompiledPreamble const *Preamble,
                            const std::vector<Inclusion> &PreambleInclusions,
                            StringRef Contents, Position Pos,
                            IntrusiveRefCntPtr<vfs::FileSystem> VFS,
                            std::shared_ptr<PCHContainerOperations> PCHs,
                            CodeCompleteOptions Opts) {
  return CodeCompleteFlow(FileName, Opts)
      .run({FileName, Command, Preamble, PreambleInclusions, Contents, Pos, VFS,
            PCHs});
}

SignatureHelp signatureHelp(PathRef FileName,
                            const tooling::CompileCommand &Command,
                            PrecompiledPreamble const *Preamble,
                            StringRef Contents, Position Pos,
                            IntrusiveRefCntPtr<vfs::FileSystem> VFS,
                            std::shared_ptr<PCHContainerOperations> PCHs) {
  SignatureHelp Result;
  clang::CodeCompleteOptions Options;
  Options.IncludeGlobals = false;
  Options.IncludeMacros = false;
  Options.IncludeCodePatterns = false;
  Options.IncludeBriefComments = false;
  std::vector<Inclusion> PreambleInclusions = {}; // Unused for signatureHelp
  semaCodeComplete(llvm::make_unique<SignatureHelpCollector>(Options, Result),
                   Options,
                   {FileName, Command, Preamble, PreambleInclusions, Contents,
                    Pos, std::move(VFS), std::move(PCHs)});
  return Result;
}

bool isIndexedForCodeCompletion(const NamedDecl &ND, ASTContext &ASTCtx) {
  using namespace clang::ast_matchers;
  auto InTopLevelScope = hasDeclContext(
      anyOf(namespaceDecl(), translationUnitDecl(), linkageSpecDecl()));
  return !match(decl(anyOf(InTopLevelScope,
                           hasDeclContext(
                               enumDecl(InTopLevelScope, unless(isScoped()))))),
                ND, ASTCtx)
              .empty();
}

} // namespace clangd
} // namespace clang