diff --git a/include/swift/Basic/LangOptions.h b/include/swift/Basic/LangOptions.h
index 1401f7324fd0f..13cc2bd42b0a4 100644
--- a/include/swift/Basic/LangOptions.h
+++ b/include/swift/Basic/LangOptions.h
@@ -496,6 +496,12 @@ namespace swift {
     /// Enable verification when every SubstitutionMap is constructed.
     bool VerifyAllSubstitutionMaps = false;
 
+    /// If set to true, the source manager will avoid memory mapping source files
+    /// with the expectation they may change on disk. This is most useful when
+    /// opening files under sourcekitd on Windows, as memory mapping on Windows
+    /// prevents files from being written.
+    bool OpenSourcesAsVolatile = false;
+
     /// Load swiftmodule files in memory as volatile and avoid mmap.
     bool EnableVolatileModules = false;
 
diff --git a/include/swift/Basic/SourceManager.h b/include/swift/Basic/SourceManager.h
index 350ea0744d948..ced1aa5b88f7c 100644
--- a/include/swift/Basic/SourceManager.h
+++ b/include/swift/Basic/SourceManager.h
@@ -94,6 +94,7 @@ class SourceManager {
 private:
   llvm::SourceMgr LLVMSourceMgr;
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FileSystem;
+  bool OpenSourcesAsVolatile = false;
   unsigned IDEInspectionTargetBufferID = 0U;
   unsigned IDEInspectionTargetOffset;
 
@@ -166,6 +167,12 @@ class SourceManager {
     return FileSystem;
   }
 
+  // Setting this prevents SourceManager from memory mapping sources (via opening files
+  // with isVolatile=true);
+  void setOpenSourcesAsVolatile() {
+    OpenSourcesAsVolatile = true;
+  }
+
   void setIDEInspectionTarget(unsigned BufferID, unsigned Offset) {
     assert(BufferID != 0U && "Buffer should be valid");
 
diff --git a/lib/Basic/SourceLoc.cpp b/lib/Basic/SourceLoc.cpp
index 07e2b5bc743cb..fc201c1f8eeac 100644
--- a/lib/Basic/SourceLoc.cpp
+++ b/lib/Basic/SourceLoc.cpp
@@ -695,7 +695,11 @@ unsigned SourceManager::getExternalSourceBufferID(StringRef Path) {
     return It->getSecond();
   }
   unsigned Id = 0u;
-  auto InputFileOrErr = swift::vfs::getFileOrSTDIN(*getFileSystem(), Path);
+  auto InputFileOrErr =
+      swift::vfs::getFileOrSTDIN(*getFileSystem(), Path,
+                                 /* FileSize */ -1,
+                                 /* RequiresNullTerminator */ true,
+                                 /* isVolatile */ this->OpenSourcesAsVolatile);
   if (InputFileOrErr) {
     // This assertion ensures we can look up from the map in the future when
     // using the same Path.
diff --git a/lib/Frontend/Frontend.cpp b/lib/Frontend/Frontend.cpp
index 19fef52dca664..2cd914838e57d 100644
--- a/lib/Frontend/Frontend.cpp
+++ b/lib/Frontend/Frontend.cpp
@@ -553,6 +553,10 @@ bool CompilerInstance::setup(const CompilerInvocation &Invoke,
     Invocation.getSearchPathOptions().dump(LangOpts.Target.isOSDarwin());
   }
 
+  if (LangOpts.OpenSourcesAsVolatile) {
+    this->getSourceMgr().setOpenSourcesAsVolatile();
+  }
+
   // If we expect an implicit stdlib import, load in the standard library. If we
   // either fail to find it or encounter an error while loading it, bail early. Continuing will at best
   // trigger a bunch of other errors due to the stdlib being missing, or at
diff --git a/lib/IDETool/CompilerInvocation.cpp b/lib/IDETool/CompilerInvocation.cpp
index 47556d28f91a2..a3a7a5cd0e89d 100644
--- a/lib/IDETool/CompilerInvocation.cpp
+++ b/lib/IDETool/CompilerInvocation.cpp
@@ -225,6 +225,11 @@ bool ide::initCompilerInvocation(
   LangOpts.AttachCommentsToDecls = true;
   LangOpts.DiagnosticsEditorMode = true;
   LangOpts.CollectParsedToken = true;
+  #if defined(_WIN32)
+  // Source files that might be open in an editor should not be memory mapped on Windows,
+  // as they will become read-only.
+  LangOpts.OpenSourcesAsVolatile = true;
+  #endif
   if (LangOpts.PlaygroundTransform) {
     // The playground instrumenter changes the AST in ways that disrupt the
     // SourceKit functionality. Since we don't need the instrumenter, and all we