Make the API for iterating packs in an opened environment callback-based

This is generally easier for clients to work with.

Author: rjmccall

include/swift/AST/GenericEnvironment.h

@@ -195,15 +195,19 @@ class alignas(1 << DeclAlignInBits) GenericEnvironment final
   /// Retrieve the UUID for an opened element environment.
   UUID getOpenedElementUUID() const;
 
-  using PackElementBinding =
-      std::pair<ElementArchetypeType *, PackArchetypeType *>;
-
-  /// Retrieve the bindings for the opened pack element archetypes in this
-  /// generic environment to the pack archetypes that contain them.
-  ///
-  /// \param bindings The vector to populate with the pack element bindings.
-  void getPackElementBindings(
-      SmallVectorImpl<PackElementBinding> &bindings) const;
+  void forEachPackElementArchetype(
+          llvm::function_ref<void(ElementArchetypeType*)> function) const;
+
+  using PackElementBindingCallback =
+    llvm::function_ref<void(ElementArchetypeType *elementType,
+                            PackType *packSubstitution)>;
+
+  /// Given that this is an opened element environment, iterate the
+  /// opened pack element bindings: the pack archetype that's been opened
+  /// (which may not be meaningful in the surrounding context), the element
+  /// archetype that it has been opened as, and the pack type whose elements
+  /// are opened.
+  void forEachPackElementBinding(PackElementBindingCallback function) const;
 
   /// Create a new, primary generic environment.
   static GenericEnvironment *forPrimary(GenericSignature signature);

lib/AST/GenericEnvironment.cpp

@@ -131,8 +131,18 @@ UUID GenericEnvironment::getOpenedElementUUID() const {
   return getTrailingObjects<OpenedElementEnvironmentData>()->uuid;
 }
 
-void GenericEnvironment::getPackElementBindings(
-    SmallVectorImpl<PackElementBinding> &bindings) const {
+void GenericEnvironment::forEachPackElementArchetype(
+          llvm::function_ref<void(ElementArchetypeType *)> function) const {
+  auto packElements = getGenericSignature().getInnermostGenericParams();
+  for (auto eltInterfaceType: packElements) {
+    auto *elementArchetype =
+      mapTypeIntoContext(eltInterfaceType)->castTo<ElementArchetypeType>();
+    function(elementArchetype);
+  }
+}
+
+void GenericEnvironment::forEachPackElementBinding(
+    PackElementBindingCallback function) const {
   auto sig = getGenericSignature();
   auto shapeClass = getOpenedElementShapeClass();
   auto packElements = sig.getInnermostGenericParams();
@@ -156,13 +166,13 @@ void GenericEnvironment::getPackElementBindings(
 
     assert(elementIt != packElements.end());
     auto *elementArchetype =
-        mapTypeIntoContext(*elementIt++)->getAs<ElementArchetypeType>();
-    auto *packArchetype =
-        mapTypeIntoContext(genericParam)->getAs<PackArchetypeType>();
-
-    assert(elementArchetype && packArchetype);
-    bindings.emplace_back(elementArchetype, packArchetype);
+        mapTypeIntoContext(*elementIt++)->castTo<ElementArchetypeType>();
+    auto *packSubstitution =
+        maybeApplyOuterContextSubstitutions(genericParam)->castTo<PackType>();
+    function(elementArchetype, packSubstitution);
   }
+
+  assert(elementIt == packElements.end());
 }
 
 GenericEnvironment::GenericEnvironment(GenericSignature signature)