diff --git a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
--- a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
@@ -20,6 +20,7 @@
 #include "index/MemIndex.h"
 #include "support/Context.h"
 #include "support/Path.h"
+#include "clang/AST/Attr.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticSema.h"
 #include "llvm/Support/ScopedPrinter.h"
@@ -745,6 +746,19 @@
   EXPECT_THAT(TU.build().getLocalTopLevelDecls(), SizeIs(1));
 }
 
+TEST(DiagnosticsTest, PreambleWithPragmaAssumeNonnull) {
+  auto TU = TestTU::withCode(R"cpp(
+#pragma clang assume_nonnull begin
+void foo(int *x);
+#pragma clang assume_nonnull end
+)cpp");
+  auto AST = TU.build();
+  EXPECT_THAT(*AST.getDiagnostics(), IsEmpty());
+  const auto *X = cast<FunctionDecl>(findDecl(AST, "foo")).getParamDecl(0);
+  ASSERT_TRUE(X->getOriginalType()->getNullability(X->getASTContext()) ==
+      NullabilityKind::NonNull);
+}
+
 TEST(DiagnosticsTest, InsideMacros) {
   Annotations Test(R"cpp(
     #define TEN 10
diff --git a/clang/include/clang/Lex/PreprocessorOptions.h b/clang/include/clang/Lex/PreprocessorOptions.h
--- a/clang/include/clang/Lex/PreprocessorOptions.h
+++ b/clang/include/clang/Lex/PreprocessorOptions.h
@@ -128,7 +128,8 @@
   ///
   /// When the lexer is done, one of the things that need to be preserved is the
   /// conditional #if stack, so the ASTWriter/ASTReader can save/restore it when
-  /// processing the rest of the file.
+  /// processing the rest of the file. Similarly, we track an unterminated
+  /// #pragma assume_nonnull.
   bool GeneratePreamble = false;
 
   /// Whether to write comment locations into the PCH when building it.
diff --git a/clang/include/clang/Serialization/ASTBitCodes.h b/clang/include/clang/Serialization/ASTBitCodes.h
--- a/clang/include/clang/Serialization/ASTBitCodes.h
+++ b/clang/include/clang/Serialization/ASTBitCodes.h
@@ -698,6 +698,9 @@
 
   /// Record code for included files.
   PP_INCLUDED_FILES = 66,
+
+  /// Record code for an unterminated \#pragma clang assume_nonnull begin.
+  PP_ASSUME_NONNULL_LOC = 67,
 };
 
 /// Record types used within a source manager block.
diff --git a/clang/lib/Lex/PPLexerChange.cpp b/clang/lib/Lex/PPLexerChange.cpp
--- a/clang/lib/Lex/PPLexerChange.cpp
+++ b/clang/lib/Lex/PPLexerChange.cpp
@@ -427,10 +427,16 @@
     PragmaARCCFCodeAuditedInfo = {nullptr, SourceLocation()};
   }
 
-  // Complain about reaching a true EOF within assume_nonnull.
+  // Complain about reaching a true EOF.
+  //
   // We don't want to complain about reaching the end of a macro
   // instantiation or a _Pragma.
-  if (PragmaAssumeNonNullLoc.isValid() &&
+  //
+  // In addition, we don't want to complain about reaching the end of the
+  // preamble that we're generating, nor the end of the preamble that the main
+  // file is using.
+  if (PragmaAssumeNonNullLoc.isValid() && !this->PPOpts->GeneratePreamble &&
+      !(CurLexer && CurLexer->getFileID() == PredefinesFileID) &&
       !isEndOfMacro && !(CurLexer && CurLexer->Is_PragmaLexer)) {
     Diag(PragmaAssumeNonNullLoc, diag::err_pp_eof_in_assume_nonnull);
 
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3109,6 +3109,7 @@
       case IDENTIFIER_OFFSET:
       case INTERESTING_IDENTIFIERS:
       case STATISTICS:
+      case PP_ASSUME_NONNULL_LOC:
       case PP_CONDITIONAL_STACK:
       case PP_COUNTER_VALUE:
       case SOURCE_LOCATION_OFFSETS:
@@ -3371,6 +3372,13 @@
       }
       break;
 
+    case PP_ASSUME_NONNULL_LOC: {
+      unsigned Idx = 0;
+      if (!Record.empty())
+        PP.setPragmaAssumeNonNullLoc(ReadSourceLocation(F, Record, Idx));
+      break;
+    }
+
     case PP_CONDITIONAL_STACK:
       if (!Record.empty()) {
         unsigned Idx = 0, End = Record.size() - 1;
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -872,6 +872,7 @@
   RECORD(PP_CONDITIONAL_STACK);
   RECORD(DECLS_TO_CHECK_FOR_DEFERRED_DIAGS);
   RECORD(PP_INCLUDED_FILES);
+  RECORD(PP_ASSUME_NONNULL_LOC);
 
   // SourceManager Block.
   BLOCK(SOURCE_MANAGER_BLOCK);
@@ -2299,6 +2300,16 @@
     Stream.EmitRecord(PP_COUNTER_VALUE, Record);
   }
 
+  // If we have an unterminated #pragma assume_nonnull, remember it since we
+  // should be at the end of a preamble.
+  SourceLocation AssumeNonNullLoc = PP.getPragmaAssumeNonNullLoc();
+  if (AssumeNonNullLoc.isValid()) {
+    assert(PP.isRecordingPreamble());
+    AddSourceLocation(AssumeNonNullLoc, Record);
+    Stream.EmitRecord(PP_ASSUME_NONNULL_LOC, Record);
+    Record.clear();
+  }
+
   if (PP.isRecordingPreamble() && PP.hasRecordedPreamble()) {
     assert(!IsModule);
     auto SkipInfo = PP.getPreambleSkipInfo();
diff --git a/clang/test/Index/preamble-assume-nonnull.c b/clang/test/Index/preamble-assume-nonnull.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Index/preamble-assume-nonnull.c
@@ -0,0 +1,6 @@
+// RUN: env CINDEXTEST_EDITING=1 c-index-test -test-load-source local %s 2>&1 \
+// RUN: | FileCheck %s --implicit-check-not "error:"
+
+#pragma clang assume_nonnull begin
+void foo(int *x);
+#pragma clang assume_nonnull end