Sema: Remove BindingSet::getConstraintSystem()

We shouldn't store a pointer to the ConstraintSystem inside every
BindingSet, but there are some annoying things to untangle before
we can do that.

As a starting point toward that, remove the getConstraintSystem()
getter so that at least we can't reach up to the ConstraintSystem
from the outside.

Author: slavapestov

include/swift/Sema/CSBindings.h

@@ -410,8 +410,6 @@ class BindingSet {
 
   BindingSet(const BindingSet &other) = delete;
 
-  ConstraintSystem &getConstraintSystem() const { return CS; }
-
   TypeVariableType *getTypeVariable() const { return TypeVar; }
 
   /// Check whether this binding set belongs to a type variable

include/swift/Sema/ConstraintSystem.h

@@ -6196,7 +6196,9 @@ class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
 public:
   using Element = TypeVariableBinding;
 
-  TypeVarBindingProducer(const BindingSet &bindings);
+  TypeVarBindingProducer(ConstraintSystem &cs,
+                         TypeVariableType *typeVar,
+                         const BindingSet &bindings);
 
   /// Retrieve a set of bindings available in the current state.
   ArrayRef<Binding> getCurrentBindings() const { return Bindings; }

lib/Sema/CSStep.cpp

@@ -335,7 +335,8 @@ StepResult ComponentStep::take(bool prevFailed) {
     switch (*step) {
     case StepKind::Binding:
       return suspend(
-          std::make_unique<TypeVariableStep>(*bestBindings, Solutions));
+          std::make_unique<TypeVariableStep>(CS, bestBindings->getTypeVariable(),
+                                             *bestBindings, Solutions));
     case StepKind::Disjunction: {
       CS.retireConstraint(disjunction->first);
       return suspend(

lib/Sema/CSStep.h

@@ -540,10 +540,12 @@ class TypeVariableStep final : public BindingStep<TypeVarBindingProducer> {
   bool SawFirstLiteralConstraint = false;
 
 public:
-  TypeVariableStep(const BindingContainer &bindings,
+  TypeVariableStep(ConstraintSystem &cs,
+                   TypeVariableType *typeVar,
+                   const BindingContainer &bindings,
                    SmallVectorImpl<Solution> &solutions)
-      : BindingStep(bindings.getConstraintSystem(), {bindings}, solutions),
-        TypeVar(bindings.getTypeVariable()) {}
+      : BindingStep(cs, {cs, typeVar, bindings}, solutions),
+        TypeVar(typeVar) {}
 
   void setup() override;
 

lib/Sema/ConstraintSystem.cpp

@@ -5323,10 +5323,12 @@ ConstraintSystem::inferKeyPathLiteralCapability(KeyPathExpr *keyPath) {
   return success(mutability, isSendable);
 }
 
-TypeVarBindingProducer::TypeVarBindingProducer(const BindingSet &bindings)
-    : BindingProducer(bindings.getConstraintSystem(),
-                      bindings.getTypeVariable()->getImpl().getLocator()),
-      TypeVar(bindings.getTypeVariable()), CanBeNil(bindings.canBeNil()) {
+TypeVarBindingProducer::TypeVarBindingProducer(
+    ConstraintSystem &cs,
+    TypeVariableType *typeVar,
+    const BindingSet &bindings)
+    : BindingProducer(cs, typeVar->getImpl().getLocator()),
+      TypeVar(typeVar), CanBeNil(bindings.canBeNil()) {
   if (bindings.isDirectHole()) {
     auto *locator = getLocator();
     // If this type variable is associated with a code completion token