[ConstraintSystem] Save "too complex" source range if known

This would help to diagnose the initial point where constraint
system has been determine to be "too complex"

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -194,6 +194,8 @@ class ExpressionTimer {
 
   AnchorType getAnchor() const { return Anchor; }
 
+  SourceRange getAffectedRange() const;
+
   unsigned getWarnLimit() const {
     return Context.TypeCheckerOpts.WarnLongExpressionTypeChecking;
   }
@@ -5415,9 +5417,17 @@ class ConstraintSystem {
 
   /// Determine if we've already explored too many paths in an
   /// attempt to solve this expression.
-  bool isAlreadyTooComplex = false;
+  std::pair<bool, SourceRange> isAlreadyTooComplex = {false, SourceRange()};
+
+  /// If optional is not nil, result is guaranteed to point at a valid
+  /// location.
+  Optional<SourceRange> getTooComplexRange() const {
+    auto range = isAlreadyTooComplex.second;
+    return range.isValid() ? range : Optional<SourceRange>();
+  }
+
   bool isTooComplex(size_t solutionMemory) {
-    if (isAlreadyTooComplex)
+    if (isAlreadyTooComplex.first)
       return true;
 
     auto CancellationFlag = getASTContext().CancellationFlag;
@@ -5428,7 +5438,9 @@ class ConstraintSystem {
     MaxMemory = std::max(used, MaxMemory);
     auto threshold = getASTContext().TypeCheckerOpts.SolverMemoryThreshold;
     if (MaxMemory > threshold) {
-      return isAlreadyTooComplex= true;
+      // No particular location for OoM problems.
+      isAlreadyTooComplex.first = true;
+      return true;
     }
 
     if (Timer && Timer->isExpired()) {
@@ -5437,20 +5449,22 @@ class ConstraintSystem {
       // emitting an error.
       Timer->disableWarning();
 
-      return isAlreadyTooComplex = true;
+      isAlreadyTooComplex = {true, Timer->getAffectedRange()};
+      return true;
     }
 
     // Bail out once we've looked at a really large number of
     // choices.
     if (CountScopes > getASTContext().TypeCheckerOpts.SolverBindingThreshold) {
-      return isAlreadyTooComplex = true;
+      isAlreadyTooComplex.first = true;
+      return true;
     }
 
     return false;
   }
 
   bool isTooComplex(SmallVectorImpl<Solution> const &solutions) {
-    if (isAlreadyTooComplex)
+    if (isAlreadyTooComplex.first)
       return true;
 
     size_t solutionMemory = 0;

lib/Sema/ConstraintSystem.cpp

@@ -56,6 +56,22 @@ ExpressionTimer::ExpressionTimer(AnchorType Anchor, ConstraintSystem &CS,
       PrintDebugTiming(CS.getASTContext().TypeCheckerOpts.DebugTimeExpressions),
       PrintWarning(true) {}
 
+SourceRange ExpressionTimer::getAffectedRange() const {
+  ASTNode anchor;
+
+  if (auto *locator = Anchor.dyn_cast<ConstraintLocator *>()) {
+    anchor = simplifyLocatorToAnchor(locator);
+    // If locator couldn't be simplified down to a single AST
+    // element, let's use its root.
+    if (!anchor)
+      anchor = locator->getAnchor();
+  } else {
+    anchor = Anchor.get<Expr *>();
+  }
+
+  return anchor.getSourceRange();
+}
+
 ExpressionTimer::~ExpressionTimer() {
   auto elapsed = getElapsedProcessTimeInFractionalSeconds();
   unsigned elapsedMS = static_cast<unsigned>(elapsed * 1000);
@@ -81,22 +97,13 @@ ExpressionTimer::~ExpressionTimer() {
   if (WarnLimit == 0 || elapsedMS < WarnLimit)
     return;
 
-  ASTNode anchor;
-  if (auto *locator = Anchor.dyn_cast<ConstraintLocator *>()) {
-    anchor = simplifyLocatorToAnchor(locator);
-    // If locator couldn't be simplified down to a single AST
-    // element, let's warn about its root.
-    if (!anchor)
-      anchor = locator->getAnchor();
-  } else {
-    anchor = Anchor.get<Expr *>();
-  }
+  auto sourceRange = getAffectedRange();
 
-  if (anchor.getStartLoc().isValid()) {
+  if (sourceRange.Start.isValid()) {
     Context.Diags
-        .diagnose(anchor.getStartLoc(), diag::debug_long_expression, elapsedMS,
+        .diagnose(sourceRange.Start, diag::debug_long_expression, elapsedMS,
                   WarnLimit)
-        .highlight(anchor.getSourceRange());
+        .highlight(sourceRange);
   }
 }