Index: clangd/ClangdUnit.h
===================================================================
--- clangd/ClangdUnit.h
+++ clangd/ClangdUnit.h
@@ -48,6 +48,8 @@
   llvm::SmallVector<tooling::Replacement, 1> FixIts;
 };
 
+struct PreambleData;
+
 /// Stores and provides access to parsed AST.
 class ParsedAST {
 public:
@@ -55,8 +57,7 @@
   /// it is reused during parsing.
   static llvm::Optional<ParsedAST>
   Build(std::unique_ptr<clang::CompilerInvocation> CI,
-        const PrecompiledPreamble *Preamble,
-        ArrayRef<serialization::DeclID> PreambleDeclIDs,
+        std::shared_ptr<const PreambleData> Preamble,
         std::unique_ptr<llvm::MemoryBuffer> Buffer,
         std::shared_ptr<PCHContainerOperations> PCHs,
         IntrusiveRefCntPtr<vfs::FileSystem> VFS, clangd::Logger &Logger);
@@ -80,15 +81,18 @@
   const std::vector<DiagWithFixIts> &getDiagnostics() const;
 
 private:
-  ParsedAST(std::unique_ptr<CompilerInstance> Clang,
+  ParsedAST(std::shared_ptr<const PreambleData> Preamble,
+            std::unique_ptr<CompilerInstance> Clang,
             std::unique_ptr<FrontendAction> Action,
             std::vector<const Decl *> TopLevelDecls,
-            std::vector<serialization::DeclID> PendingTopLevelDecls,
             std::vector<DiagWithFixIts> Diags);
 
 private:
   void ensurePreambleDeclsDeserialized();
 
+  // In-memory preambles must outlive the AST, it is important that this member
+  // goes before Clang and Action.
+  std::shared_ptr<const PreambleData> Preamble;
   // We store an "incomplete" FrontendAction (i.e. no EndSourceFile was called
   // on it) and CompilerInstance used to run it. That way we don't have to do
   // complex memory management of all Clang structures on our own. (They are
@@ -100,7 +104,7 @@
   // Data, stored after parsing.
   std::vector<DiagWithFixIts> Diags;
   std::vector<const Decl *> TopLevelDecls;
-  std::vector<serialization::DeclID> PendingTopLevelDecls;
+  bool PreambleDeclsDeserialized;
 };
 
 // Provides thread-safe access to ParsedAST.
Index: clangd/ClangdUnit.cpp
===================================================================
--- clangd/ClangdUnit.cpp
+++ clangd/ClangdUnit.cpp
@@ -878,8 +878,7 @@
 
 llvm::Optional<ParsedAST>
 ParsedAST::Build(std::unique_ptr<clang::CompilerInvocation> CI,
-                 const PrecompiledPreamble *Preamble,
-                 ArrayRef<serialization::DeclID> PreambleDeclIDs,
+                 std::shared_ptr<const PreambleData> Preamble,
                  std::unique_ptr<llvm::MemoryBuffer> Buffer,
                  std::shared_ptr<PCHContainerOperations> PCHs,
                  IntrusiveRefCntPtr<vfs::FileSystem> VFS,
@@ -888,8 +887,10 @@
   std::vector<DiagWithFixIts> ASTDiags;
   StoreDiagsConsumer UnitDiagsConsumer(/*ref*/ ASTDiags);
 
+  const PrecompiledPreamble *PreamblePCH =
+      Preamble ? &Preamble->Preamble : nullptr;
   auto Clang = prepareCompilerInstance(
-      std::move(CI), Preamble, std::move(Buffer), std::move(PCHs),
+      std::move(CI), PreamblePCH, std::move(Buffer), std::move(PCHs),
       std::move(VFS), /*ref*/ UnitDiagsConsumer);
 
   // Recover resources if we crash before exiting this method.
@@ -911,15 +912,8 @@
   Clang->getDiagnostics().setClient(new EmptyDiagsConsumer);
 
   std::vector<const Decl *> ParsedDecls = Action->takeTopLevelDecls();
-  std::vector<serialization::DeclID> PendingDecls;
-  if (Preamble) {
-    PendingDecls.reserve(PreambleDeclIDs.size());
-    PendingDecls.insert(PendingDecls.begin(), PreambleDeclIDs.begin(),
-                        PreambleDeclIDs.end());
-  }
-
-  return ParsedAST(std::move(Clang), std::move(Action), std::move(ParsedDecls),
-                   std::move(PendingDecls), std::move(ASTDiags));
+  return ParsedAST(std::move(Preamble), std::move(Clang), std::move(Action),
+                   std::move(ParsedDecls), std::move(ASTDiags));
 }
 
 namespace {
@@ -1060,24 +1054,25 @@
 }
 
 void ParsedAST::ensurePreambleDeclsDeserialized() {
-  if (PendingTopLevelDecls.empty())
+  if (PreambleDeclsDeserialized || !Preamble)
     return;
 
   std::vector<const Decl *> Resolved;
-  Resolved.reserve(PendingTopLevelDecls.size());
+  Resolved.reserve(Preamble->TopLevelDeclIDs.size());
 
   ExternalASTSource &Source = *getASTContext().getExternalSource();
-  for (serialization::DeclID TopLevelDecl : PendingTopLevelDecls) {
+  for (serialization::DeclID TopLevelDecl : Preamble->TopLevelDeclIDs) {
     // Resolve the declaration ID to an actual declaration, possibly
     // deserializing the declaration in the process.
     if (Decl *D = Source.GetExternalDecl(TopLevelDecl))
       Resolved.push_back(D);
   }
 
-  TopLevelDecls.reserve(TopLevelDecls.size() + PendingTopLevelDecls.size());
+  TopLevelDecls.reserve(TopLevelDecls.size() +
+                        Preamble->TopLevelDeclIDs.size());
   TopLevelDecls.insert(TopLevelDecls.begin(), Resolved.begin(), Resolved.end());
 
-  PendingTopLevelDecls.clear();
+  PreambleDeclsDeserialized = true;
 }
 
 ParsedAST::ParsedAST(ParsedAST &&Other) = default;
@@ -1111,14 +1106,15 @@
   return Diags;
 }
 
-ParsedAST::ParsedAST(std::unique_ptr<CompilerInstance> Clang,
+ParsedAST::ParsedAST(std::shared_ptr<const PreambleData> Preamble,
+                     std::unique_ptr<CompilerInstance> Clang,
                      std::unique_ptr<FrontendAction> Action,
                      std::vector<const Decl *> TopLevelDecls,
-                     std::vector<serialization::DeclID> PendingTopLevelDecls,
                      std::vector<DiagWithFixIts> Diags)
-    : Clang(std::move(Clang)), Action(std::move(Action)),
-      Diags(std::move(Diags)), TopLevelDecls(std::move(TopLevelDecls)),
-      PendingTopLevelDecls(std::move(PendingTopLevelDecls)) {
+    : Preamble(std::move(Preamble)), Clang(std::move(Clang)),
+      Action(std::move(Action)), Diags(std::move(Diags)),
+      TopLevelDecls(std::move(TopLevelDecls)),
+      PreambleDeclsDeserialized(false) {
   assert(this->Clang);
   assert(this->Action);
 }
@@ -1328,12 +1324,8 @@
     } // unlock Mutex
 
     // Prepare the Preamble and supplementary data for rebuilding AST.
-    const PrecompiledPreamble *PreambleForAST = nullptr;
-    ArrayRef<serialization::DeclID> SerializedPreambleDecls = llvm::None;
     std::vector<DiagWithFixIts> Diagnostics;
     if (NewPreamble) {
-      PreambleForAST = &NewPreamble->Preamble;
-      SerializedPreambleDecls = NewPreamble->TopLevelDeclIDs;
       Diagnostics.insert(Diagnostics.begin(), NewPreamble->Diags.begin(),
                          NewPreamble->Diags.end());
     }
@@ -1342,9 +1334,9 @@
     llvm::Optional<ParsedAST> NewAST;
     {
       trace::Span Tracer(llvm::Twine("Build: ") + That->FileName);
-      NewAST = ParsedAST::Build(
-          std::move(CI), PreambleForAST, SerializedPreambleDecls,
-          std::move(ContentsBuffer), PCHs, VFS, That->Logger);
+      NewAST =
+          ParsedAST::Build(std::move(CI), std::move(NewPreamble),
+                           std::move(ContentsBuffer), PCHs, VFS, That->Logger);
     }
 
     if (NewAST) {