diff --git a/clang-tools-extra/clangd/unittests/PreambleTests.cpp b/clang-tools-extra/clangd/unittests/PreambleTests.cpp
--- a/clang-tools-extra/clangd/unittests/PreambleTests.cpp
+++ b/clang-tools-extra/clangd/unittests/PreambleTests.cpp
@@ -36,35 +36,19 @@
   return arg.first() == File && arg.second == D;
 }
 
-std::shared_ptr<const PreambleData>
-createPreamble(llvm::StringRef Contents = "") {
-  auto TU = TestTU::withCode(Contents);
-  // ms-compatibility changes meaning of #import, make sure it is turned off.
-  TU.ExtraArgs = {"-fno-ms-compatibility"};
-  TU.Filename = "preamble.cpp";
-  auto PI = TU.inputs();
-  IgnoreDiagnostics Diags;
-  auto CI = buildCompilerInvocation(PI, Diags);
-  if (!CI) {
-    ADD_FAILURE() << "failed to build compiler invocation";
-    return nullptr;
-  }
-  if (auto Preamble = buildPreamble(TU.Filename, *CI, PI, true, nullptr))
-    return Preamble;
-  ADD_FAILURE() << "failed to build preamble";
-  return nullptr;
-}
-
 // Builds a preamble for BaselineContents, patches it for ModifiedContents and
 // returns the includes in the patch.
 IncludeStructure
 collectPatchedIncludes(llvm::StringRef ModifiedContents,
                        llvm::StringRef BaselineContents,
                        llvm::StringRef MainFileName = "main.cpp") {
-  auto BaselinePreamble = createPreamble(BaselineContents);
-  // Create the patch.
-  auto TU = TestTU::withCode(ModifiedContents);
+  auto TU = TestTU::withCode(BaselineContents);
   TU.Filename = MainFileName.str();
+  // ms-compatibility changes meaning of #import, make sure it is turned off.
+  TU.ExtraArgs = {"-fno-ms-compatibility"};
+  auto BaselinePreamble = TU.buildPreamble();
+  // Create the patch.
+  TU = TestTU::withCode(ModifiedContents);
   auto PI = TU.inputs();
   auto PP = PreamblePatch::create(testPath(TU.Filename), PI, *BaselinePreamble);
   // Collect patch contents.
diff --git a/clang-tools-extra/clangd/unittests/TestTU.h b/clang-tools-extra/clangd/unittests/TestTU.h
--- a/clang-tools-extra/clangd/unittests/TestTU.h
+++ b/clang-tools-extra/clangd/unittests/TestTU.h
@@ -68,6 +68,7 @@
   // By default, build() will report Error diagnostics as GTest errors.
   // Suppress this behavior by adding an 'error-ok' comment to the code.
   ParsedAST build() const;
+  std::shared_ptr<const PreambleData> buildPreamble() const;
   ParseInputs inputs() const;
   SymbolSlab headerSymbols() const;
   std::unique_ptr<SymbolIndex> index() const;
diff --git a/clang-tools-extra/clangd/unittests/TestTU.cpp b/clang-tools-extra/clangd/unittests/TestTU.cpp
--- a/clang-tools-extra/clangd/unittests/TestTU.cpp
+++ b/clang-tools-extra/clangd/unittests/TestTU.cpp
@@ -65,14 +65,24 @@
   return Inputs;
 }
 
+std::shared_ptr<const PreambleData> TestTU::buildPreamble() const {
+  auto Inputs = inputs();
+  IgnoreDiagnostics Diags;
+  auto CI = buildCompilerInvocation(Inputs, Diags);
+  assert(CI && "Failed to build compilation invocation.");
+  return clang::clangd::buildPreamble(testPath(Filename), *CI, Inputs,
+                                      /*StoreInMemory=*/true,
+                                      /*PreambleCallback=*/nullptr);
+}
+
 ParsedAST TestTU::build() const {
   auto Inputs = inputs();
   StoreDiags Diags;
   auto CI = buildCompilerInvocation(Inputs, Diags);
   assert(CI && "Failed to build compilation invocation.");
-  auto Preamble =
-      buildPreamble(testPath(Filename), *CI, Inputs,
-                    /*StoreInMemory=*/true, /*PreambleCallback=*/nullptr);
+  auto Preamble = clang::clangd::buildPreamble(testPath(Filename), *CI, Inputs,
+                                               /*StoreInMemory=*/true,
+                                               /*PreambleCallback=*/nullptr);
   auto AST = ParsedAST::build(testPath(Filename), Inputs, std::move(CI),
                               Diags.take(), Preamble);
   if (!AST.hasValue()) {