Index: clang-tools-extra/trunk/clangd/CodeComplete.h
===================================================================
--- clang-tools-extra/trunk/clangd/CodeComplete.h
+++ clang-tools-extra/trunk/clangd/CodeComplete.h
@@ -68,6 +68,11 @@
   /// If more results are available, we set CompletionList.isIncomplete.
   size_t Limit = 0;
 
+  enum IncludeInsertion {
+    IWYU,
+    NeverInsert,
+  } InsertIncludes = IncludeInsertion::IWYU;
+
   /// A visual indicator to prepend to the completion label to indicate whether
   /// completion result would trigger an #include insertion or not.
   struct IncludeInsertionIndicator {
Index: clang-tools-extra/trunk/clangd/CodeComplete.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/CodeComplete.cpp
+++ clang-tools-extra/trunk/clangd/CodeComplete.cpp
@@ -191,7 +191,9 @@
 
   // Returns a token identifying the overload set this is part of.
   // 0 indicates it's not part of any overload set.
-  size_t overloadSet() const {
+  size_t overloadSet(const CodeCompleteOptions &Opts) const {
+    if (!Opts.BundleOverloads)
+      return 0;
     llvm::SmallString<256> Scratch;
     if (IndexResult) {
       switch (IndexResult->SymInfo.Kind) {
@@ -208,7 +210,7 @@
         // This could break #include insertion.
         return llvm::hash_combine(
             (IndexResult->Scope + IndexResult->Name).toStringRef(Scratch),
-            headerToInsertIfAllowed().getValueOr(""));
+            headerToInsertIfAllowed(Opts).getValueOr(""));
       default:
         return 0;
       }
@@ -223,12 +225,14 @@
       D->printQualifiedName(OS);
     }
     return llvm::hash_combine(Scratch,
-                              headerToInsertIfAllowed().getValueOr(""));
+                              headerToInsertIfAllowed(Opts).getValueOr(""));
   }
 
   // The best header to include if include insertion is allowed.
-  llvm::Optional<llvm::StringRef> headerToInsertIfAllowed() const {
-    if (RankedIncludeHeaders.empty())
+  llvm::Optional<llvm::StringRef>
+  headerToInsertIfAllowed(const CodeCompleteOptions &Opts) const {
+    if (Opts.InsertIncludes == CodeCompleteOptions::NeverInsert ||
+        RankedIncludeHeaders.empty())
       return None;
     if (SemaResult && SemaResult->Declaration) {
       // Avoid inserting new #include if the declaration is found in the current
@@ -338,7 +342,7 @@
           Includes.calculateIncludePath(*ResolvedDeclaring, *ResolvedInserted),
           Includes.shouldInsertInclude(*ResolvedDeclaring, *ResolvedInserted));
     };
-    bool ShouldInsert = C.headerToInsertIfAllowed().hasValue();
+    bool ShouldInsert = C.headerToInsertIfAllowed(Opts).hasValue();
     // Calculate include paths and edits for all possible headers.
     for (const auto &Inc : C.RankedIncludeHeaders) {
       if (auto ToInclude = Inserted(Inc)) {
@@ -1373,7 +1377,7 @@
       if (C.IndexResult)
         C.RankedIncludeHeaders = getRankedIncludes(*C.IndexResult);
       C.Name = IndexResult ? IndexResult->Name : Recorder->getName(*SemaResult);
-      if (auto OverloadSet = Opts.BundleOverloads ? C.overloadSet() : 0) {
+      if (auto OverloadSet = C.overloadSet(Opts)) {
         auto Ret = BundleLookup.try_emplace(OverloadSet, Bundles.size());
         if (Ret.second)
           Bundles.emplace_back();
Index: clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp
+++ clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp
@@ -6,8 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "Features.inc"
 #include "ClangdLSPServer.h"
+#include "CodeComplete.h"
+#include "Features.inc"
 #include "Path.h"
 #include "Protocol.h"
 #include "Trace.h"
@@ -154,6 +155,20 @@
     "debug-origin", llvm::cl::desc("Show origins of completion items"),
     llvm::cl::init(CodeCompleteOptions().ShowOrigins), llvm::cl::Hidden);
 
+static llvm::cl::opt<CodeCompleteOptions::IncludeInsertion> HeaderInsertion(
+    "header-insertion",
+    llvm::cl::desc("Add #include directives when accepting code completions"),
+    llvm::cl::init(CodeCompleteOptions().InsertIncludes),
+    llvm::cl::values(
+        clEnumValN(CodeCompleteOptions::IWYU, "iwyu",
+                   "Include what you use. "
+                   "Insert the owning header for top-level symbols, unless the "
+                   "header is already directly included or the symbol is "
+                   "forward-declared."),
+        clEnumValN(
+            CodeCompleteOptions::NeverInsert, "never",
+            "Never insert #include directives as part of code completion")));
+
 static llvm::cl::opt<bool> HeaderInsertionDecorators(
     "header-insertion-decorators",
     llvm::cl::desc("Prepend a circular dot or space before the completion "
@@ -438,6 +453,7 @@
   CCOpts.Limit = LimitResults;
   CCOpts.BundleOverloads = CompletionStyle != Detailed;
   CCOpts.ShowOrigins = ShowOrigins;
+  CCOpts.InsertIncludes = HeaderInsertion;
   if (!HeaderInsertionDecorators) {
     CCOpts.IncludeIndicator.Insert.clear();
     CCOpts.IncludeIndicator.NoInsert.clear();
Index: clang-tools-extra/trunk/unittests/clangd/CodeCompleteTests.cpp
===================================================================
--- clang-tools-extra/trunk/unittests/clangd/CodeCompleteTests.cpp
+++ clang-tools-extra/trunk/unittests/clangd/CodeCompleteTests.cpp
@@ -555,6 +555,16 @@
                              {Sym});
   EXPECT_THAT(Results.Completions,
               ElementsAre(AllOf(Named("X"), InsertInclude("\"bar.h\""))));
+  // Can be disabled via option.
+  CodeCompleteOptions NoInsertion;
+  NoInsertion.InsertIncludes = CodeCompleteOptions::NeverInsert;
+  Results = completions(Server,
+                             R"cpp(
+          int main() { ns::^ }
+      )cpp",
+                             {Sym}, NoInsertion);
+  EXPECT_THAT(Results.Completions,
+              ElementsAre(AllOf(Named("X"), Not(InsertInclude()))));
   // Duplicate based on inclusions in preamble.
   Results = completions(Server,
                         R"cpp(