Index: clangd/ClangdUnit.h
===================================================================
--- clangd/ClangdUnit.h
+++ clangd/ClangdUnit.h
@@ -9,7 +9,7 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CLANGDUNIT_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CLANGDUNIT_H
 
-#include "../clang-tidy/ClangTidyOptions.h"
+#include "Compiler.h"
 #include "Diagnostics.h"
 #include "FS.h"
 #include "Function.h"
@@ -60,14 +60,6 @@
   std::unique_ptr<PreambleFileStatusCache> StatCache;
 };
 
-/// Information required to run clang, e.g. to parse AST or do code completion.
-struct ParseInputs {
-  tooling::CompileCommand CompileCommand;
-  IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS;
-  std::string Contents;
-  tidy::ClangTidyOptions ClangTidyOpts;
-};
-
 /// Stores and provides access to parsed AST.
 class ParsedAST {
 public:
@@ -137,10 +129,6 @@
 using PreambleParsedCallback =
     std::function<void(ASTContext &, std::shared_ptr<clang::Preprocessor>)>;
 
-/// Builds compiler invocation that could be used to build AST or preamble.
-std::unique_ptr<CompilerInvocation>
-buildCompilerInvocation(const ParseInputs &Inputs);
-
 /// Rebuild the preamble for the new inputs unless the old one can be reused.
 /// If \p OldPreamble can be reused, it is returned unchanged.
 /// If \p OldPreamble is null, always builds the preamble.
Index: clangd/ClangdUnit.cpp
===================================================================
--- clangd/ClangdUnit.cpp
+++ clangd/ClangdUnit.cpp
@@ -418,34 +418,6 @@
   assert(this->Action);
 }
 
-std::unique_ptr<CompilerInvocation>
-buildCompilerInvocation(const ParseInputs &Inputs) {
-  std::vector<const char *> ArgStrs;
-  for (const auto &S : Inputs.CompileCommand.CommandLine)
-    ArgStrs.push_back(S.c_str());
-
-  if (Inputs.FS->setCurrentWorkingDirectory(Inputs.CompileCommand.Directory)) {
-    log("Couldn't set working directory when creating compiler invocation.");
-    // We proceed anyway, our lit-tests rely on results for non-existing working
-    // dirs.
-  }
-
-  // FIXME(ibiryukov): store diagnostics from CommandLine when we start
-  // reporting them.
-  IgnoreDiagnostics IgnoreDiagnostics;
-  llvm::IntrusiveRefCntPtr<DiagnosticsEngine> CommandLineDiagsEngine =
-      CompilerInstance::createDiagnostics(new DiagnosticOptions,
-                                          &IgnoreDiagnostics, false);
-  std::unique_ptr<CompilerInvocation> CI = createInvocationFromCommandLine(
-      ArgStrs, CommandLineDiagsEngine, Inputs.FS);
-  if (!CI)
-    return nullptr;
-  // createInvocationFromCommandLine sets DisableFree.
-  CI->getFrontendOpts().DisableFree = false;
-  CI->getLangOpts()->CommentOpts.ParseAllComments = true;
-  return CI;
-}
-
 std::shared_ptr<const PreambleData>
 buildPreamble(PathRef FileName, CompilerInvocation &CI,
               std::shared_ptr<const PreambleData> OldPreamble,
Index: clangd/CodeComplete.cpp
===================================================================
--- clangd/CodeComplete.cpp
+++ clangd/CodeComplete.cpp
@@ -1016,33 +1016,17 @@
                       const SemaCompleteInput &Input,
                       IncludeStructure *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.
-  }
-
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS = Input.VFS;
   if (Input.Preamble && Input.Preamble->StatCache)
     VFS = Input.Preamble->StatCache->getConsumingFS(std::move(VFS));
-  IgnoreDiagnostics DummyDiagsConsumer;
-  auto CI = createInvocationFromCommandLine(
-      ArgStrs,
-      CompilerInstance::createDiagnostics(new DiagnosticOptions,
-                                          &DummyDiagsConsumer, false),
-      VFS);
+  auto CI =
+      buildCompilerInvocation(ParseInputs{Input.Command, VFS, Input.Contents});
   if (!CI) {
     elog("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.
@@ -1072,6 +1056,7 @@
       *Offset;
   // NOTE: we must call BeginSourceFile after prepareCompilerInstance. Otherwise
   // the remapped buffers do not get freed.
+  IgnoreDiagnostics DummyDiagsConsumer;
   auto Clang = prepareCompilerInstance(
       std::move(CI),
       (Input.Preamble && !CompletingInPreamble) ? &Input.Preamble->Preamble
Index: clangd/Compiler.h
===================================================================
--- clangd/Compiler.h
+++ clangd/Compiler.h
@@ -15,9 +15,11 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_COMPILER_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_COMPILER_H
 
+#include "../clang-tidy/ClangTidyOptions.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Frontend/PrecompiledPreamble.h"
+#include "clang/Tooling/CompilationDatabase.h"
 
 namespace clang {
 namespace clangd {
@@ -31,6 +33,18 @@
                         const clang::Diagnostic &Info) override;
 };
 
+/// Information required to run clang, e.g. to parse AST or do code completion.
+struct ParseInputs {
+  tooling::CompileCommand CompileCommand;
+  IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS;
+  std::string Contents;
+  tidy::ClangTidyOptions ClangTidyOpts;
+};
+
+/// Builds compiler invocation that could be used to build AST or preamble.
+std::unique_ptr<CompilerInvocation>
+buildCompilerInvocation(const ParseInputs &Inputs);
+
 /// Creates a compiler instance, configured so that:
 ///   - Contents of the parsed file are remapped to \p MainFile.
 ///   - Preamble is overriden to use PCH passed to this function. It means the
Index: clangd/Compiler.cpp
===================================================================
--- clangd/Compiler.cpp
+++ clangd/Compiler.cpp
@@ -39,6 +39,34 @@
   IgnoreDiagnostics::log(DiagLevel, Info);
 }
 
+std::unique_ptr<CompilerInvocation>
+buildCompilerInvocation(const ParseInputs &Inputs) {
+  std::vector<const char *> ArgStrs;
+  for (const auto &S : Inputs.CompileCommand.CommandLine)
+    ArgStrs.push_back(S.c_str());
+
+  if (Inputs.FS->setCurrentWorkingDirectory(Inputs.CompileCommand.Directory)) {
+    log("Couldn't set working directory when creating compiler invocation.");
+    // We proceed anyway, our lit-tests rely on results for non-existing working
+    // dirs.
+  }
+
+  // FIXME(ibiryukov): store diagnostics from CommandLine when we start
+  // reporting them.
+  IgnoreDiagnostics IgnoreDiagnostics;
+  llvm::IntrusiveRefCntPtr<DiagnosticsEngine> CommandLineDiagsEngine =
+      CompilerInstance::createDiagnostics(new DiagnosticOptions,
+                                          &IgnoreDiagnostics, false);
+  std::unique_ptr<CompilerInvocation> CI = createInvocationFromCommandLine(
+      ArgStrs, CommandLineDiagsEngine, Inputs.FS);
+  if (!CI)
+    return nullptr;
+  // createInvocationFromCommandLine sets DisableFree.
+  CI->getFrontendOpts().DisableFree = false;
+  CI->getLangOpts()->CommentOpts.ParseAllComments = true;
+  return CI;
+}
+
 std::unique_ptr<CompilerInstance>
 prepareCompilerInstance(std::unique_ptr<clang::CompilerInvocation> CI,
                         const PrecompiledPreamble *Preamble,