[Sema] Add cancellation flag to typechecker options

Once this flag is set, the type checker will bail out with a “expression to complex” error.

Author: ahoppen

include/swift/Basic/LangOptions.h

@@ -595,6 +595,12 @@ namespace swift {
     /// than this many seconds.
     unsigned ExpressionTimeoutThreshold = 600;
 
+    /// If the shared pointer is not a \c nullptr and the pointee is \c true,
+    /// typechecking should be aborted at the next possible opportunity.
+    /// This is used by SourceKit to cancel requests for which the result is no
+    /// longer of interest.
+    std::shared_ptr<bool> CancellationFlag = nullptr;
+
     /// If non-zero, abort the switch statement exhaustiveness checker if
     /// the Space::minus function is called more than this many times.
     ///

include/swift/Sema/ConstraintSystem.h

@@ -5155,6 +5155,10 @@ class ConstraintSystem {
     if (isExpressionAlreadyTooComplex)
       return true;
 
+    auto CancellationFlag = getASTContext().TypeCheckerOpts.CancellationFlag;
+    if (CancellationFlag && *CancellationFlag)
+      return true;
+
     auto used = getASTContext().getSolverMemory() + solutionMemory;
     MaxMemory = std::max(used, MaxMemory);
     auto threshold = getASTContext().TypeCheckerOpts.SolverMemoryThreshold;