Index: include/clang/Rewrite/Core/Rewriter.h
===================================================================
--- include/clang/Rewrite/Core/Rewriter.h
+++ include/clang/Rewrite/Core/Rewriter.h
@@ -184,7 +184,18 @@
   /// Returns true if any files were not saved successfully.
   /// Outputs diagnostics via the source manager's diagnostic engine
   /// in case of an error.
-  bool overwriteChangedFiles();
+  ///
+  /// prewrite(FID) is called after all changes to FID have been written to a
+  /// temporary file, but before that temporary file is moved into FID's
+  /// location.  Windows's ReplaceFileW(to, from) internally creates (another)
+  /// temporary file and swaps the data streams of that file with the one of
+  /// |to| so that it's possible to replace an opened file.  However, if |to|
+  /// _was_ opened, the temp file now has that open data stream, and
+  /// ReplaceFileW() fails to delete it, causing a tempo file leak.  To fix,
+  /// clients of this function must close all file mappings of a file before
+  /// it's written to (but after it's been read by Rewriter), and this prewrite
+  /// callback is a convenient place for doing this.
+  bool overwriteChangedFiles(llvm::function_ref<void(FileID)> prewrite);
 
 private:
   unsigned getLocationOffsetAndFileID(SourceLocation Loc, FileID &FID) const;
Index: lib/Frontend/Rewrite/FixItRewriter.cpp
===================================================================
--- lib/Frontend/Rewrite/FixItRewriter.cpp
+++ lib/Frontend/Rewrite/FixItRewriter.cpp
@@ -82,9 +82,9 @@
   Editor.applyRewrites(Rec);
 
   if (FixItOpts->InPlace) {
-    // Overwriting open files on Windows is tricky, but the rewriter can do it
-    // for us.
-    Rewrite.overwriteChangedFiles();
+    // FIXME: This likely leaks temp files on Windows, see comment on
+    // Rewriter::overwriteChangedFiles().
+    Rewrite.overwriteChangedFiles([](FileID){});
     return false;
   }
 
Index: lib/Rewrite/Rewriter.cpp
===================================================================
--- lib/Rewrite/Rewriter.cpp
+++ lib/Rewrite/Rewriter.cpp
@@ -443,7 +443,7 @@
 };
 } // end anonymous namespace
 
-bool Rewriter::overwriteChangedFiles() {
+bool Rewriter::overwriteChangedFiles(llvm::function_ref<void(FileID)> prewrite) {
   bool AllWritten = true;
   for (buffer_iterator I = buffer_begin(), E = buffer_end(); I != E; ++I) {
     const FileEntry *Entry =
@@ -452,6 +452,7 @@
                              AllWritten);
     if (File.ok()) {
       I->second.write(File.getStream());
+      prewrite(I->first);
     }
   }
   return !AllWritten;
Index: lib/Tooling/Refactoring.cpp
===================================================================
--- lib/Tooling/Refactoring.cpp
+++ lib/Tooling/Refactoring.cpp
@@ -64,7 +64,9 @@
 }
 
 int RefactoringTool::saveRewrittenFiles(Rewriter &Rewrite) {
-  return Rewrite.overwriteChangedFiles() ? 1 : 0;
+  // FIXME: This likely leaks temp files on Windows, see comment on
+  // Rewriter::overwriteChangedFiles().
+  return Rewrite.overwriteChangedFiles([](FileID){}) ? 1 : 0;
 }
 
 bool formatAndApplyAllReplacements(
Index: tools/clang-format/ClangFormat.cpp
===================================================================
--- tools/clang-format/ClangFormat.cpp
+++ tools/clang-format/ClangFormat.cpp
@@ -299,7 +299,10 @@
     if (Inplace) {
       if (FileName == "-")
         errs() << "error: cannot use -i when reading from stdin.\n";
-      else if (Rewrite.overwriteChangedFiles())
+      else if (Rewrite.overwriteChangedFiles([&Code, ID](FileID FID) {
+                 assert(FID == ID);
+                 Code.reset();
+               }))
         return true;
     } else {
       if (Cursor.getNumOccurrences() != 0)
Index: unittests/Tooling/RefactoringTest.cpp
===================================================================
--- unittests/Tooling/RefactoringTest.cpp
+++ unittests/Tooling/RefactoringTest.cpp
@@ -639,7 +639,9 @@
   Replacements Replaces = toReplacements({Replacement(
       Context.Sources, Context.getLocation(ID, 2, 1), 5, "replaced")});
   EXPECT_TRUE(applyAllReplacements(Replaces, Context.Rewrite));
-  EXPECT_FALSE(Context.Rewrite.overwriteChangedFiles());
+  // This test doesn't create file mappings, so overwriteChangedFiles() needs
+  // no parameter.
+  EXPECT_FALSE(Context.Rewrite.overwriteChangedFiles([](FileID){}));
   EXPECT_EQ("line1\nreplaced\nline3\nline4",
             getFileContentFromDisk("input.cpp"));
 }
Index: unittests/Tooling/RewriterTest.cpp
===================================================================
--- unittests/Tooling/RewriterTest.cpp
+++ unittests/Tooling/RewriterTest.cpp
@@ -19,7 +19,9 @@
   RewriterTestContext Context;
   FileID ID = Context.createOnDiskFile("t.cpp", "line1\nline2\nline3\nline4");
   Context.Rewrite.ReplaceText(Context.getLocation(ID, 2, 1), 5, "replaced");
-  EXPECT_FALSE(Context.Rewrite.overwriteChangedFiles());
+  // This test doesn't create file mappings, so overwriteChangedFiles() needs
+  // no parameter.
+  EXPECT_FALSE(Context.Rewrite.overwriteChangedFiles([](FileID){}));
   EXPECT_EQ("line1\nreplaced\nline3\nline4",
             Context.getFileContentFromDisk("t.cpp")); 
 }
@@ -32,7 +34,9 @@
     "working.cpp", "line1\nline2\nline3\nline4");
   Context.Rewrite.ReplaceText(Context.getLocation(WorkingID, 2, 1), 5,
                               "replaced");
-  EXPECT_TRUE(Context.Rewrite.overwriteChangedFiles());
+  // This test doesn't create file mappings, so overwriteChangedFiles() needs
+  // no parameter.
+  EXPECT_TRUE(Context.Rewrite.overwriteChangedFiles([](FileID){}));
   EXPECT_EQ("line1\nreplaced\nline3\nline4",
             Context.getFileContentFromDisk("working.cpp")); 
 }