diff --git a/test/SourceKit/RelatedIdents/run-parser-with-large-stack.swift b/test/SourceKit/RelatedIdents/run-parser-with-large-stack.swift
new file mode 100644
index 0000000000000..62c0eefa73dbe
--- /dev/null
+++ b/test/SourceKit/RelatedIdents/run-parser-with-large-stack.swift
@@ -0,0 +1,303 @@
+// This test used to overflow the stack because SwiftParser was run on a background thread with reduced stack size
+// RUN: %sourcekitd-test -req=related-idents -pos=%(line + 1):5 %s -- %s
+let x = 1
+
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
+if true {
diff --git a/tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp b/tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp
index 652011f5dcea8..bf3340a79a07e 100644
--- a/tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp
+++ b/tools/SourceKit/lib/SwiftLang/SwiftASTManager.cpp
@@ -174,13 +174,13 @@ namespace SourceKit {
   void ASTUnit::Implementation::consumeAsync(SwiftASTConsumerRef ConsumerRef,
                                              ASTUnitRef ASTRef) {
 #if defined(_WIN32)
-	// Windows uses more up for stack space (why?) than macOS/Linux which
-	// causes stack overflows in a dispatch thread with 64k stack. Passing
-	// useDeepStack=true means it's given a _beginthreadex thread with an 8MB
-	// stack.
-	bool useDeepStack = true;
+    // Windows uses more up for stack space (why?) than macOS/Linux which
+    // causes stack overflows in a dispatch thread with 64k stack. Passing
+    // useDeepStack=true means it's given a _beginthreadex thread with an 8MB
+    // stack.
+    bool useDeepStack = true;
 #else
-	bool useDeepStack = false;
+    bool useDeepStack = ConsumerRef->requiresDeepStack();
 #endif
     Queue.dispatch([ASTRef, ConsumerRef]{
       SwiftASTConsumer &ASTConsumer = *ConsumerRef;
diff --git a/tools/SourceKit/lib/SwiftLang/SwiftASTManager.h b/tools/SourceKit/lib/SwiftLang/SwiftASTManager.h
index bcd0c65a8ae30..7e43fe5439d45 100644
--- a/tools/SourceKit/lib/SwiftLang/SwiftASTManager.h
+++ b/tools/SourceKit/lib/SwiftLang/SwiftASTManager.h
@@ -153,6 +153,15 @@ class SwiftASTConsumer : public std::enable_shared_from_this<SwiftASTConsumer> {
 public:
   virtual ~SwiftASTConsumer() { }
 
+  /// Whether `handlePrimaryAST` should be executed with the same stack size as
+  /// the main thread.
+  ///
+  /// By default, it is assumed that `handlePrimaryAST` does not do a lot of
+  /// work and it is sufficient to run it on a background thread's stack with
+  /// reduced size. Set this to `true` if the consumer can perform additional
+  /// work that might require more stack size, such as invoking SwiftParser.
+  virtual bool requiresDeepStack() { return false; }
+
   // MARK: Cancellation
 
   /// The result of this consumer is no longer of interest to the SourceKit
diff --git a/tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp b/tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp
index cc11e3b15e124..8182a9cdc2462 100644
--- a/tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp
+++ b/tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp
@@ -2537,6 +2537,8 @@ void SwiftLangSupport::findRelatedIdentifiersInFile(
     std::function<void(const RequestResult<RelatedIdentsResult> &)> Receiver;
     SwiftInvocationRef Invok;
 
+    bool requiresDeepStack() override { return true; }
+
 #if SWIFT_BUILD_SWIFT_SYNTAX
     // FIXME: Don't silently eat errors here.
     RelatedIdentsResult getRelatedIdents(SourceFile *SrcFile,