[CSStep] Print changed type variables or constraints with a Changes heading. This prints type variables that changed because they involve other type variables and so may have new bindings due to this relationship. It also prints any new or changed constraints that occured as a result of acquiring these new bindings.

Author: Amritpan Kaur

lib/Sema/CSBindings.cpp

@@ -798,7 +798,7 @@ Optional<BindingSet> ConstraintSystem::determineBestBindings() {
       continue;
 
     if (isDebugMode()) {
-      bindings.dump(typeVar, llvm::errs(), solverState->depth * 2);
+      bindings.dump(typeVar, llvm::errs(), solverState->depth * 2 + 2);
     }
 
     // If these are the first bindings, or they are better than what
@@ -1983,6 +1983,10 @@ TypeVariableBinding::fixForHole(ConstraintSystem &cs) const {
 }
 
 bool TypeVariableBinding::attempt(ConstraintSystem &cs) const {
+  // Get bindings for TypeVar in scope before this attempt
+  // adds new constraints or bindings, for use in debug printing.
+  auto bindings = cs.getBindingsFor(TypeVar);
+  
   auto type = Binding.BindingType;
   auto *srcLocator = Binding.getLocator();
   auto *dstLocator = TypeVar->getImpl().getLocator();
@@ -2070,6 +2074,25 @@ bool TypeVariableBinding::attempt(ConstraintSystem &cs) const {
 
   if (cs.simplify())
     return false;
+  
+  if (cs.isDebugMode() && bindings.involvesTypeVariables()) {
+    auto &log = llvm::errs();
+    log.indent(cs.solverState->depth * 2);
+    log << "Changes: \n";
+    log.indent(cs.solverState->depth * 2 + 2);
+    log << "Constraints: \n";
+    auto gatheringKind = ConstraintGraph::GatheringKind::AllMentions;
+    auto &CG = cs.getConstraintGraph();
+    for (auto *constraint : CG.gatherConstraints(TypeVar, gatheringKind)) {
+      log.indent(cs.solverState->depth * 2 + 4);
+      log << "- ";
+      constraint->print(log, &cs.getASTContext().SourceMgr);
+      log << '\n';
+    }
+    log.indent(cs.solverState->depth * 2 + 2);
+    log << "Type Variables: \n";
+  }
+
 
   // If all of the re-activated constraints where simplified,
   // let's notify binding inference about the fact that type

lib/Sema/CSStep.cpp

@@ -508,7 +508,6 @@ bool TypeVariableStep::attempt(const TypeVariableBinding &choice) {
   if (choice.hasDefaultedProtocol())
     SawFirstLiteralConstraint = true;
 
-  // Try to solve the system with typeVar := type
   return choice.attempt(CS);
 }