Index: clang-tools-extra/clangd/refactor/tweaks/CMakeLists.txt
===================================================================
--- clang-tools-extra/clangd/refactor/tweaks/CMakeLists.txt
+++ clang-tools-extra/clangd/refactor/tweaks/CMakeLists.txt
@@ -8,4 +8,5 @@
 # $<TARGET_OBJECTS:obj.clangDaemonTweaks> to a list of sources, see
 # clangd/tool/CMakeLists.txt for an example.
 add_clang_library(clangDaemonTweaks OBJECT
+  QualifyName.cpp
   )
Index: clang-tools-extra/clangd/refactor/tweaks/QualifyName.cpp
===================================================================
--- /dev/null
+++ clang-tools-extra/clangd/refactor/tweaks/QualifyName.cpp
@@ -0,0 +1,180 @@
+//===--- QualifyName.cpp -----------------------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+#include "AST.h"
+#include "ClangdUnit.h"
+#include "SourceCode.h"
+#include "refactor/Tweak.h"
+#include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/AST/TypeLoc.h"
+#include "clang/Basic/SourceLocation.h"
+#include "clang/Tooling/Tooling.h"
+#include "llvm/Support/Error.h"
+
+namespace clang {
+namespace clangd {
+namespace {
+/// Fully qualifies a name under a cursor.
+/// Before:
+///   using namespace std;
+///   ^vector<int> foo;
+/// After:
+///   std::vector<int> foo;
+class QualifyName : public Tweak {
+public:
+  TweakID id() const override final;
+
+  bool prepare(const Selection &Inputs) override;
+  Expected<tooling::Replacements> apply(const Selection &Inputs) override;
+  std::string title() const override;
+
+private:
+  SourceLocation InsertLoc;
+  std::string Qualifier;
+};
+
+REGISTER_TWEAK(QualifyName);
+
+struct Reference {
+  SourceLocation Begin;
+  NamedDecl *Target = nullptr;
+
+  operator bool() const { return Target != nullptr; }
+};
+
+NamedDecl *toReferencedDecl(NestedNameSpecifier *NNS) {
+  switch (NNS->getKind()) {
+  case clang::NestedNameSpecifier::Namespace:
+    return NNS->getAsNamespace();
+  case clang::NestedNameSpecifier::NamespaceAlias:
+    return NNS->getAsNamespaceAlias();
+  case clang::NestedNameSpecifier::TypeSpec:
+  case clang::NestedNameSpecifier::TypeSpecWithTemplate:
+    return nullptr; // FIXME: handle this situation, retrieve the thing
+                    // referenced inside a type.
+  case clang::NestedNameSpecifier::Identifier:
+  case clang::NestedNameSpecifier::Super:
+  case clang::NestedNameSpecifier::Global: // FIXME: could return
+                                           // TranslationUnitDecl.
+    return nullptr;
+    return nullptr;
+  }
+  llvm_unreachable("unhandled NestedNameSpecifier kind.");
+}
+
+class LocateInsertLoc : public RecursiveASTVisitor<LocateInsertLoc> {
+public:
+  LocateInsertLoc(ASTContext &Ctx, SourceLocation CursorLoc,
+                  Reference &UnqualRef)
+      : Ctx(Ctx), CursorLoc(CursorLoc), UnqualRef(UnqualRef) {}
+
+  bool shouldWalkTypesOfTypeLocs() const { return false; }
+
+  // FIXME: RAT does not have VisitNestedNameSpecifierLoc. Should we add that?
+  bool TraverseNestedNameSpecifierLoc(NestedNameSpecifierLoc NNS) {
+    if (!RecursiveASTVisitor::TraverseNestedNameSpecifierLoc(NNS))
+      return false;
+    return VisitNestedNameSpecifierLoc(NNS);
+  }
+
+  bool VisitNestedNameSpecifierLoc(NestedNameSpecifierLoc NNS) {
+    if (NNS.getPrefix())
+      return true; // we want only unqualified names.
+    auto &SM = Ctx.getSourceManager();
+    auto Rng = toHalfOpenFileRange(SM, Ctx.getLangOpts(), NNS.getSourceRange());
+    if (!Rng)
+      return true;
+    if (!halfOpenRangeContains(SM, *Rng, CursorLoc))
+      return true;
+    auto *Target = toReferencedDecl(NNS.getNestedNameSpecifier());
+    if (!Target)
+      return true; // continue traversal to recurse into types, if any.
+    UnqualRef.Begin = Rng->getBegin();
+    UnqualRef.Target = Target;
+    return false; // we found the insertion point.
+  }
+
+  bool VisitDeclRefExpr(DeclRefExpr *E) {
+    if (E->hasQualifier())
+      return true; // we want only unqualified names.
+    auto &SM = Ctx.getSourceManager();
+    auto Rng = toHalfOpenFileRange(SM, Ctx.getLangOpts(), E->getSourceRange());
+    if (!Rng)
+      return true;
+    if (!halfOpenRangeContains(SM, *Rng, CursorLoc))
+      return true;
+    UnqualRef.Begin = Rng->getBegin();
+    UnqualRef.Target = E->getFoundDecl();
+    return false;
+  }
+
+  bool VisitTagTypeLoc(TagTypeLoc Loc) {
+    auto &SM = Ctx.getSourceManager();
+    auto Rng = toHalfOpenFileRange(SM, Ctx.getLangOpts(), Loc.getSourceRange());
+    if (!Rng)
+      return true;
+    if (!halfOpenRangeContains(SM, *Rng, CursorLoc))
+      return true;
+    UnqualRef.Begin = Rng->getBegin();
+    UnqualRef.Target = Loc.getDecl();
+    return false;
+  }
+
+  bool VisitTypedefTypeLoc(TypedefTypeLoc Loc) {
+    auto &SM = Ctx.getSourceManager();
+    auto Rng = toHalfOpenFileRange(SM, Ctx.getLangOpts(), Loc.getSourceRange());
+    if (!Rng)
+      return true;
+    if (!halfOpenRangeContains(SM, *Rng, CursorLoc))
+      return true;
+    UnqualRef.Begin = Rng->getBegin();
+    UnqualRef.Target = Loc.getTypedefNameDecl();
+    return false;
+  }
+
+private:
+  ASTContext &Ctx;
+  SourceLocation CursorLoc;
+  Reference &UnqualRef;
+};
+
+bool QualifyName::prepare(const Selection &Inputs) {
+  auto &Ctx = Inputs.AST.getASTContext();
+  Reference Ref;
+  LocateInsertLoc(Ctx, Inputs.Cursor, Ref).TraverseAST(Ctx);
+  if (!Ref)
+    return false;
+  auto *Parent = Ref.Target->getDeclContext();
+  if (!Parent->isNamespace())
+    return false; // avoid complicated cases, qualify only with namespaces.
+  Qualifier = printNamespaceScope(*Parent);
+  if (Qualifier.empty())
+    return false; // happens for decls from global namespaces.
+  InsertLoc = Ref.Begin;
+  return true;
+}
+
+Expected<tooling::Replacements> QualifyName::apply(const Selection &Inputs) {
+  auto &Ctx = Inputs.AST.getASTContext();
+
+  tooling::Replacements R;
+  if (auto Err = R.add(tooling::Replacement(Ctx.getSourceManager(), InsertLoc,
+                                            0, Qualifier))) {
+    llvm::consumeError(std::move(Err));
+    return llvm::createStringError(llvm::inconvertibleErrorCode(),
+                                   "building a replacement failed");
+  }
+  return R;
+}
+
+std::string QualifyName::title() const {
+  return llvm::formatv("Add '{0}' qualifier", Qualifier);
+}
+} // namespace
+} // namespace clangd
+} // namespace clang