Index: include/clang-c/Index.h
===================================================================
--- include/clang-c/Index.h
+++ include/clang-c/Index.h
@@ -32,7 +32,7 @@
  * compatible, thus CINDEX_VERSION_MAJOR is expected to remain stable.
  */
 #define CINDEX_VERSION_MAJOR 0
-#define CINDEX_VERSION_MINOR 39
+#define CINDEX_VERSION_MINOR 40
 
 #define CINDEX_VERSION_ENCODE(major, minor) ( \
       ((major) * 10000)                       \
@@ -1419,6 +1419,15 @@
                                              unsigned options);
 
 /**
+ * \brief Suspend a translation unit in order to free memory associated with it.
+ *
+ * A suspended translation unit uses significantly less memory but on the other
+ * side does not support any other calls than \c clang_reparseTranslationUnit
+ * to resume it or \c clang_disposeTranslationUnit to dispose it completely.
+ */
+CINDEX_LINKAGE unsigned clang_suspendTranslationUnit(CXTranslationUnit);
+
+/**
  * \brief Destroy the specified CXTranslationUnit object.
  */
 CINDEX_LINKAGE void clang_disposeTranslationUnit(CXTranslationUnit);
Index: include/clang/Frontend/ASTUnit.h
===================================================================
--- include/clang/Frontend/ASTUnit.h
+++ include/clang/Frontend/ASTUnit.h
@@ -852,6 +852,11 @@
   bool Reparse(std::shared_ptr<PCHContainerOperations> PCHContainerOps,
                ArrayRef<RemappedFile> RemappedFiles = None);
 
+  /// \brief Free data that will be re-generated on the next parse.
+  ///
+  /// Preamble-related data is not affected.
+  void ResetForParse();
+
   /// \brief Perform code completion at the given file, line, and
   /// column within this translation unit.
   ///
Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -1020,8 +1020,6 @@
 /// contain any translation-unit information, false otherwise.
 bool ASTUnit::Parse(std::shared_ptr<PCHContainerOperations> PCHContainerOps,
                     std::unique_ptr<llvm::MemoryBuffer> OverrideMainBuffer) {
-  SavedMainFileBuffer.reset();
-
   if (!Invocation)
     return true;
 
@@ -1068,17 +1066,11 @@
     Clang->createFileManager();
     FileMgr = &Clang->getFileManager();
   }
-  SourceMgr = new SourceManager(getDiagnostics(), *FileMgr,
-                                UserFilesAreVolatile);
-  TheSema.reset();
-  Ctx = nullptr;
-  PP = nullptr;
-  Reader = nullptr;
 
-  // Clear out old caches and data.
-  TopLevelDecls.clear();
-  clearFileLevelDecls();
+  ResetForParse();
 
+  SourceMgr = new SourceManager(getDiagnostics(), *FileMgr,
+                                UserFilesAreVolatile);
   if (!OverrideMainBuffer) {
     checkAndRemoveNonDriverDiags(StoredDiagnostics);
     TopLevelDeclsInPreamble.clear();
@@ -2071,6 +2063,19 @@
   return Result;
 }
 
+void ASTUnit::ResetForParse() {
+  SavedMainFileBuffer.reset();
+
+  SourceMgr.reset();
+  TheSema.reset();
+  Ctx.reset();
+  PP.reset();
+  Reader.reset();
+
+  TopLevelDecls.clear();
+  clearFileLevelDecls();
+}
+
 //----------------------------------------------------------------------------//
 // Code completion
 //----------------------------------------------------------------------------//
Index: tools/c-index-test/c-index-test.c
===================================================================
--- tools/c-index-test/c-index-test.c
+++ tools/c-index-test/c-index-test.c
@@ -1742,6 +1742,8 @@
       return -1;
 
     if (Repeats > 1) {
+      clang_suspendTranslationUnit(TU);
+
       Err = clang_reparseTranslationUnit(TU, num_unsaved_files, unsaved_files,
                                          clang_defaultReparseOptions(TU));
       if (Err != CXError_Success) {
Index: tools/libclang/CIndex.cpp
===================================================================
--- tools/libclang/CIndex.cpp
+++ tools/libclang/CIndex.cpp
@@ -3918,6 +3918,20 @@
   }
 }
 
+unsigned clang_suspendTranslationUnit(CXTranslationUnit CTUnit) {
+  if (CTUnit) {
+    ASTUnit *Unit = cxtu::getASTUnit(CTUnit);
+
+    if (Unit && Unit->isUnsafeToFree())
+      return false;
+
+    Unit->ResetForParse();
+    return true;
+  }
+
+  return false;
+}
+
 unsigned clang_defaultReparseOptions(CXTranslationUnit TU) {
   return CXReparse_None;
 }
Index: tools/libclang/libclang.exports
===================================================================
--- tools/libclang/libclang.exports
+++ tools/libclang/libclang.exports
@@ -305,6 +305,7 @@
 clang_remap_getNumFiles
 clang_reparseTranslationUnit
 clang_saveTranslationUnit
+clang_suspendTranslationUnit
 clang_sortCodeCompletionResults
 clang_toggleCrashRecovery
 clang_tokenize