Sema: Avoid copying BindingSets

They're stored inside the ConstraintGraphNode now, so returning a
`const BindingSet *` from within determineBestBindings() should
be safe.

include/swift/Sema/CSBindings.h

@@ -385,6 +385,10 @@ public:
   BindingSet(ConstraintSystem &CS, TypeVariableType *TypeVar,
              const PotentialBindings &info);
 
+  BindingSet(BindingSet &&other) = default;
+
+  BindingSet(const BindingSet &other) = delete;
+
   ConstraintSystem &getConstraintSystem() const { return CS; }
 
   TypeVariableType *getTypeVariable() const { return TypeVar; }

include/swift/Sema/ConstraintSystem.h

@@ -5270,7 +5270,7 @@ public:
 
   /// Determine whether given type variable with its set of bindings is viable
   /// to be attempted on the next step of the solver.
-  std::optional<BindingSet> determineBestBindings(
+  const BindingSet *determineBestBindings(
       llvm::function_ref<void(const BindingSet &)> onCandidate);
 
   /// Get bindings for the given type variable based on current
@@ -6200,7 +6200,7 @@ class TypeVarBindingProducer : public BindingProducer<TypeVariableBinding> {
 public:
   using Element = TypeVariableBinding;
 
-  TypeVarBindingProducer(BindingSet &bindings);
+  TypeVarBindingProducer(const BindingSet &bindings);
 
   /// Retrieve a set of bindings available in the current state.
   ArrayRef<Binding> getCurrentBindings() const { return Bindings; }

lib/Sema/CSBindings.cpp

@@ -1180,7 +1180,7 @@ bool BindingSet::operator<(const BindingSet &other) {
   return isPotentiallyIncomplete() < other.isPotentiallyIncomplete();
 }
 
-std::optional<BindingSet> ConstraintSystem::determineBestBindings(
+const BindingSet *ConstraintSystem::determineBestBindings(
     llvm::function_ref<void(const BindingSet &)> onCandidate) {
   // Look for potential type variable bindings.
   BindingSet *bestBindings = nullptr;
@@ -1238,10 +1238,7 @@ std::optional<BindingSet> ConstraintSystem::determineBestBindings(
       bestBindings = &bindings;
   }
 
-  if (!bestBindings)
-    return std::nullopt;
-
-  return std::optional(*bestBindings);
+  return bestBindings;
 }
 
 /// Find the set of type variables that are inferable from the given type.

lib/Sema/CSStep.cpp

@@ -263,7 +263,7 @@ StepResult ComponentStep::take(bool prevFailed) {
   SmallString<64> potentialBindings;
   llvm::raw_svector_ostream bos(potentialBindings);
 
-  auto bestBindings = CS.determineBestBindings([&](const BindingSet &bindings) {
+  const auto *bestBindings = CS.determineBestBindings([&](const BindingSet &bindings) {
     if (CS.isDebugMode() && bindings.hasViableBindings()) {
       bos.indent(CS.solverState->getCurrentIndent() + 2);
       bos << "(";

lib/Sema/CSStep.h

@@ -540,7 +540,7 @@ class TypeVariableStep final : public BindingStep<TypeVarBindingProducer> {
   bool SawFirstLiteralConstraint = false;
 
 public:
-  TypeVariableStep(BindingContainer &bindings,
+  TypeVariableStep(const BindingContainer &bindings,
                    SmallVectorImpl<Solution> &solutions)
       : BindingStep(bindings.getConstraintSystem(), {bindings}, solutions),
         TypeVar(bindings.getTypeVariable()) {}

lib/Sema/ConstraintSystem.cpp

@@ -5309,7 +5309,7 @@ ConstraintSystem::inferKeyPathLiteralCapability(KeyPathExpr *keyPath) {
   return success(mutability, isSendable);
 }
 
-TypeVarBindingProducer::TypeVarBindingProducer(BindingSet &bindings)
+TypeVarBindingProducer::TypeVarBindingProducer(const BindingSet &bindings)
     : BindingProducer(bindings.getConstraintSystem(),
                       bindings.getTypeVariable()->getImpl().getLocator()),
       TypeVar(bindings.getTypeVariable()), CanBeNil(bindings.canBeNil()) {

unittests/Sema/BindingInferenceTests.cpp

@@ -196,7 +196,7 @@ TEST_F(SemaTest, TestTransitiveProtocolInference) {
                      cs.getConstraintLocator({}, LocatorPathElt::ContextualType(
                                                      CTP_Initialization)));
 
-    auto bindings = inferBindings(cs, typeVar);
+    auto &bindings = inferBindings(cs, typeVar);
     ASSERT_TRUE(bindings.getConformanceRequirements().empty());
     ASSERT_TRUE(bool(bindings.TransitiveProtocols));
     verifyProtocolInferenceResults(*bindings.TransitiveProtocols,
@@ -218,7 +218,7 @@ TEST_F(SemaTest, TestTransitiveProtocolInference) {
     cs.addConstraint(ConstraintKind::Conversion, typeVar, GPT1,
                      cs.getConstraintLocator({}));
 
-    auto bindings = inferBindings(cs, typeVar);
+    auto &bindings = inferBindings(cs, typeVar);
     ASSERT_TRUE(bindings.getConformanceRequirements().empty());
     ASSERT_TRUE(bool(bindings.TransitiveProtocols));
     verifyProtocolInferenceResults(*bindings.TransitiveProtocols,
@@ -281,10 +281,10 @@ TEST_F(SemaTest, TestComplexTransitiveProtocolInference) {
   cs.addConstraint(ConstraintKind::Equal, typeVar1, typeVar5, nilLocator);
   cs.addConstraint(ConstraintKind::Conversion, typeVar5, typeVar6, nilLocator);
 
-  auto bindingsForT1 = inferBindings(cs, typeVar1);
-  auto bindingsForT2 = inferBindings(cs, typeVar2);
-  auto bindingsForT3 = inferBindings(cs, typeVar3);
-  auto bindingsForT5 = inferBindings(cs, typeVar5);
+  auto &bindingsForT1 = inferBindings(cs, typeVar1);
+  auto &bindingsForT2 = inferBindings(cs, typeVar2);
+  auto &bindingsForT3 = inferBindings(cs, typeVar3);
+  auto &bindingsForT5 = inferBindings(cs, typeVar5);
 
   ASSERT_TRUE(bool(bindingsForT1.TransitiveProtocols));
   verifyProtocolInferenceResults(*bindingsForT1.TransitiveProtocols,
@@ -335,7 +335,7 @@ TEST_F(SemaTest, TestTransitiveProtocolInferenceThroughEquivalenceChains) {
   cs.addConstraint(ConstraintKind::ConformsTo, typeVar2, protocolTy0, nilLocator);
   cs.addConstraint(ConstraintKind::ConformsTo, typeVar3, protocolTy1, nilLocator);
 
-  auto bindings = inferBindings(cs, typeVar0);
+  auto &bindings = inferBindings(cs, typeVar0);
 
   ASSERT_TRUE(bool(bindings.TransitiveProtocols));
   verifyProtocolInferenceResults(*bindings.TransitiveProtocols,

unittests/Sema/SemaFixture.cpp

@@ -124,8 +124,8 @@ ProtocolType *SemaTest::createProtocol(llvm::StringRef protocolName,
   return ProtocolType::get(PD, parent, Context);
 }
 
-BindingSet SemaTest::inferBindings(ConstraintSystem &cs,
-                                   TypeVariableType *typeVar) {
+const BindingSet &SemaTest::inferBindings(ConstraintSystem &cs,
+                                          TypeVariableType *typeVar) {
   for (auto *typeVar : cs.getTypeVariables()) {
     auto &node = cs.getConstraintGraph()[typeVar];
     node.resetBindingSet();

unittests/Sema/SemaFixture.h

@@ -80,8 +80,8 @@ protected:
   ProtocolType *createProtocol(llvm::StringRef protocolName,
                                Type parent = Type());
 
-  static BindingSet inferBindings(ConstraintSystem &cs,
-                                  TypeVariableType *typeVar);
+  static const BindingSet &inferBindings(ConstraintSystem &cs,
+                                         TypeVariableType *typeVar);
 };
 
 } // end namespace unittest