Sema: BindingSet::Defaults can just be a vector and not a map

The only place where we did a lookup, we also iterated over it anyway,
and all remaining usages are simplified by downgrading it to a vector.

Author: slavapestov

include/swift/Sema/CSBindings.h

@@ -380,7 +380,7 @@ class BindingSet {
   /// before transitive ones.
   llvm::SmallMapVector<ProtocolDecl *, LiteralRequirement, 2> Literals;
 
-  llvm::SmallDenseMap<CanType, Constraint *, 2> Defaults;
+  llvm::SmallVector<Constraint *, 2> Defaults;
 
   /// The set of transitive protocol requirements inferred through
   /// subtype/conversion/equivalence relations with other type variables.
@@ -505,8 +505,8 @@ class BindingSet {
       return 1;
 
     auto numDefaultable = llvm::count_if(
-        Defaults, [](const std::pair<CanType, Constraint *> &entry) {
-          return entry.second->getKind() == ConstraintKind::Defaultable;
+        Defaults, [](Constraint *constraint) {
+          return constraint->getKind() == ConstraintKind::Defaultable;
         });
 
     // Short-circuit unviable checks if there are no defaultable bindings.
@@ -518,10 +518,12 @@ class BindingSet {
     auto unviable =
         llvm::count_if(Bindings, [&](const PotentialBinding &binding) {
           auto type = binding.BindingType->getCanonicalType();
-          auto def = Defaults.find(type);
-          return def != Defaults.end()
-                     ? def->second->getKind() == ConstraintKind::Defaultable
-                     : false;
+          for (auto *constraint : Defaults) {
+            if (constraint->getSecondType()->isEqual(type)) {
+              return constraint->getKind() == ConstraintKind::Defaultable;
+            }
+          }
+          return false;
         });
 
     assert(numDefaultable >= unviable);

lib/Sema/CSBindings.cpp

@@ -647,11 +647,11 @@ void BindingSet::inferTransitiveSupertypeBindings() {
       addLiteralRequirement(literal.second.getSource());
 
     // Infer transitive defaults.
-    for (const auto &def : bindings.Defaults) {
-      if (def.getSecond()->getKind() == ConstraintKind::FallbackType)
+    for (auto *def : bindings.Defaults) {
+      if (def->getKind() == ConstraintKind::FallbackType)
         continue;
 
-      addDefault(def.second);
+      addDefault(def);
     }
 
     // TODO: We shouldn't need this in the future.
@@ -838,11 +838,13 @@ bool BindingSet::finalizeKeyPathBindings() {
               (keyPath->getParsedRoot() ||
                (fixedRootTy && !fixedRootTy->isTypeVariableOrMember()))) {
             auto fallback = llvm::find_if(Defaults, [](const auto &entry) {
-              return entry.second->getKind() == ConstraintKind::FallbackType;
+              return entry->getKind() == ConstraintKind::FallbackType;
             });
             assert(fallback != Defaults.end());
             updatedBindings.insert(
-                {fallback->first, AllowedBindingKind::Exact, fallback->second});
+                {(*fallback)->getSecondType(),
+                 AllowedBindingKind::Exact,
+                 *fallback});
           } else {
             updatedBindings.insert(PotentialBinding::forHole(
                 TypeVar, CS.getConstraintLocator(
@@ -1106,10 +1108,10 @@ bool BindingSet::operator==(const BindingSet &other) {
   if (Defaults.size() != other.Defaults.size())
     return false;
 
-  for (auto pair : Defaults) {
-    auto found = other.Defaults.find(pair.first);
-    if (found == other.Defaults.end() ||
-        pair.second != found->second)
+  for (auto i : indices(Defaults)) {
+    auto *x = Defaults[i];
+    auto *y = other.Defaults[i];
+    if (x != y)
       return false;
   }
 
@@ -1271,8 +1273,7 @@ findInferableTypeVars(Type type,
 }
 
 void BindingSet::addDefault(Constraint *constraint) {
-  auto defaultTy = constraint->getSecondType();
-  Defaults.insert({defaultTy->getCanonicalType(), constraint});
+  Defaults.push_back(constraint);
 }
 
 bool LiteralRequirement::isCoveredBy(Type type, ConstraintSystem &CS) const {
@@ -2507,8 +2508,7 @@ void BindingSet::dump(llvm::raw_ostream &out, unsigned indent) const {
     out << " [defaults: ";
     interleave(
         Defaults,
-        [&](const auto &entry) {
-          auto *constraint = entry.second;
+        [&](Constraint *constraint) {
           auto defaultBinding =
               PrintableBinding::exact(constraint->getSecondType());
           defaultBinding.print(out, PO);

lib/Sema/ConstraintSystem.cpp

@@ -5384,8 +5384,7 @@ TypeVarBindingProducer::TypeVarBindingProducer(const BindingSet &bindings)
     if (viableBindings.size() == 1) {
       addBinding(viableBindings.front());
     } else {
-      for (const auto &entry : bindings.Defaults) {
-        auto *constraint = entry.second;
+      for (auto *constraint : bindings.Defaults) {
         Bindings.push_back(getDefaultBinding(constraint));
       }
     }
@@ -5423,8 +5422,7 @@ TypeVarBindingProducer::TypeVarBindingProducer(const BindingSet &bindings)
   {
     bool noBindings = Bindings.empty();
 
-    for (const auto &entry : bindings.Defaults) {
-      auto *constraint = entry.second;
+    for (auto *constraint : bindings.Defaults) {
       if (noBindings) {
         // If there are no direct or transitive bindings to attempt
         // let's add defaults to the list right away.