[Runtime] Extract a function object for demangling using base metadata.

The field metadata translation has a great little lambda for extracting
generic arguments from metadata when demangling. Extract it into a
reusable function object.

Author: DougGregor

stdlib/public/runtime/MetadataLookup.cpp

@@ -1223,5 +1223,44 @@ swift_getTypeByMangledNameImpl(const char *typeNameStart, size_t typeNameLength,
   return swift_checkMetadataState(MetadataState::Complete, metadata).Value;
 }
 
+const Metadata *
+SubstGenericParametersFromMetadata::operator()(
+                                        unsigned depth, unsigned index) const {
+  // On first access, compute the descriptor path.
+  if (descriptorPath.empty()) {
+    if (auto *baseDesc = base->getTypeContextDescriptor()) {
+      const auto *parent = reinterpret_cast<
+                              const ContextDescriptor *>(baseDesc);
+      while (parent) {
+        if (parent->isGeneric())
+          descriptorPath.push_back(parent);
+
+        parent = parent->Parent.get();
+      }
+    }
+  }
+
+  if (depth >= descriptorPath.size())
+    return nullptr;
+
+  unsigned currentDepth = 0;
+  unsigned flatIndex = index;
+  const ContextDescriptor *currentContext = descriptorPath.back();
+
+  for (const auto *context : llvm::reverse(descriptorPath)) {
+    if (currentDepth >= depth)
+      break;
+
+    flatIndex += context->getNumGenericParams();
+    currentContext = context;
+    ++currentDepth;
+  }
+
+  if (index >= currentContext->getNumGenericParams())
+    return nullptr;
+
+  return base->getGenericArgs()[flatIndex];
+}
+
 #define OVERRIDE_METADATALOOKUP COMPATIBILITY_OVERRIDE
 #include "CompatibilityOverride.def"

stdlib/public/runtime/Private.h

@@ -235,6 +235,22 @@ class TypeInfo {
   using SubstGenericParameterFn =
     llvm::function_ref<const Metadata *(unsigned depth, unsigned index)>;
 
+  /// Function object that produces substitutions for the generic parameters
+  /// that occur within a mangled name, using the generic arguments from
+  /// the given metadata.
+  ///
+  /// Use with \c _getTypeByMangledName to decode potentially-generic types.
+  class SWIFT_RUNTIME_LIBRARY_VISIBILITY SubstGenericParametersFromMetadata {
+    const Metadata *base;
+    mutable std::vector<const ContextDescriptor *> descriptorPath;
+
+  public:
+    explicit SubstGenericParametersFromMetadata(const Metadata *base)
+      : base(base) { }
+
+    const Metadata *operator()(unsigned depth, unsigned index) const;
+  };
+
   /// Retrieve the type metadata described by the given type name.
   ///
   /// \p substGenericParam Function that provides generic argument metadata

stdlib/public/runtime/ReflectionMirror.mm

@@ -332,44 +332,10 @@ static bool _shouldReportMissingReflectionMetadataWarnings() {
   if (!field.hasMangledTypeName())
     return {name, FieldType().withIndirect(field.isIndirectCase())};
 
-  std::vector<const ContextDescriptor *> descriptorPath;
-  {
-    const auto *parent = reinterpret_cast<
-                            const ContextDescriptor *>(baseDesc);
-    while (parent) {
-      if (parent->isGeneric())
-        descriptorPath.push_back(parent);
-
-      parent = parent->Parent.get();
-    }
-  }
-
   auto typeName = field.getMangledTypeName(0);
 
-  auto typeInfo = _getTypeByMangledName(
-      typeName,
-      [&](unsigned depth, unsigned index) -> const Metadata * {
-        if (depth >= descriptorPath.size())
-          return nullptr;
-
-        unsigned currentDepth = 0;
-        unsigned flatIndex = index;
-        const ContextDescriptor *currentContext = descriptorPath.back();
-
-        for (const auto *context : llvm::reverse(descriptorPath)) {
-          if (currentDepth >= depth)
-            break;
-
-          flatIndex += context->getNumGenericParams();
-          currentContext = context;
-          ++currentDepth;
-        }
-
-        if (index >= currentContext->getNumGenericParams())
-          return nullptr;
-
-        return base->getGenericArgs()[flatIndex];
-      });
+  SubstGenericParametersFromMetadata substitutions(base);
+  auto typeInfo = _getTypeByMangledName(typeName, substitutions);
 
   // Complete the type metadata before returning it to the caller.
   if (typeInfo) {