[CSBindings] Move binding printing out of determineBestBindings and check that bindings exist before printing.

Author: Amritpan Kaur

include/swift/Sema/ConstraintSystem.h

@@ -5557,7 +5557,10 @@ class ConstraintSystem {
       Type wrapperType, Type paramType, ParamDecl *param, Identifier argLabel,
       ConstraintKind matchKind, ConstraintLocatorBuilder locator);
 
-  Optional<BindingSet> determineBestBindings();
+  /// Determine whether given type variable with its set of bindings is viable
+  /// to be attempted on the next step of the solver.
+  Optional<BindingSet> determineBestBindings(
+      llvm::function_ref<void(const BindingSet &)> onCandidate);
 
   /// Get bindings for the given type variable based on current
   /// state of the constraint system.

lib/Sema/CSBindings.cpp

@@ -742,7 +742,8 @@ BindingSet::BindingScore BindingSet::formBindingScore(const BindingSet &b) {
                          -numNonDefaultableBindings);
 }
 
-Optional<BindingSet> ConstraintSystem::determineBestBindings() {
+Optional<BindingSet> ConstraintSystem::determineBestBindings(
+    llvm::function_ref<void(const BindingSet &)> onCandidate) {
   // Look for potential type variable bindings.
   Optional<BindingSet> bestBindings;
   llvm::SmallDenseMap<TypeVariableType *, BindingSet> cache;
@@ -804,9 +805,7 @@ Optional<BindingSet> ConstraintSystem::determineBestBindings() {
     if (!bindings || !isViable)
       continue;
 
-    if (isDebugMode()) {
-      bindings.dump(llvm::errs(), solverState->getCurrentIndent());
-    }
+    onCandidate(bindings);
 
     // If these are the first bindings, or they are better than what
     // we saw before, use them instead.

lib/Sema/CSStep.cpp

@@ -346,30 +346,44 @@ StepResult ComponentStep::take(bool prevFailed) {
 
   /// Try to figure out what this step is going to be,
   /// after the scope has been established.
-  auto bestBindings = CS.determineBestBindings();
+  SmallString<64> potentialBindings;
+  llvm::raw_svector_ostream bos(potentialBindings);
+
+  auto bestBindings = CS.determineBestBindings([&](const BindingSet &bindings) {
+    if (CS.isDebugMode() && bindings.hasViableBindings()) {
+      bindings.dump(bos, CS.solverState->getCurrentIndent() + 2);
+    }
+  });
+
   auto *disjunction = CS.selectDisjunction();
   auto *conjunction = CS.selectConjunction();
 
   if (CS.isDebugMode()) {
     PrintOptions PO;
     PO.PrintTypesForDebugging = true;
 
+    auto &log = getDebugLogger();
+    if (!potentialBindings.empty()) {
+      log << "(Potential Binding(s): " << '\n';
+      log << potentialBindings;
+    }
+    log.indent(CS.solverState->getCurrentIndent());
+
     if (disjunction) {
-      auto &log = getDebugLogger();
       log.indent(2);
       log << "Disjunction(s) = [";
       auto constraints = disjunction->getNestedConstraints();
       log << constraints[0]->getFirstType()->getString(PO);
-      log << "])\n";
+      log << "]";
     }
     if (conjunction) {
-      auto &log = getDebugLogger();
       log.indent(2);
       log << "Conjunction(s) = [";
       auto constraints = conjunction->getNestedConstraints();
       log << constraints[0]->getFirstType()->getString(PO);
-      log << "])\n";
+      log << "]";
     }
+    log << ")\n";
   }
 
   if (CS.shouldAttemptFixes()) {