Add loadDynamicallyReplacedFunctionDecl

To support lazy resolution of the cross-referenced function in a serialized @_dynamicReplacement(for: ...) attribute, add a utility to the LazyMemberLoader and plumb it through.  This is a more general utility than the current resolver, which relies on the type checker to strip the attribute off of VarDecls and fan it back out onto accessors, which means serialization has only ever seen AbstractFunctionDecls.

Author: CodaFi

include/swift/AST/LazyResolver.h

@@ -103,6 +103,11 @@ class alignas(void*) LazyMemberLoader {
   virtual void
   loadRequirementSignature(const ProtocolDecl *proto, uint64_t contextData,
                            SmallVectorImpl<Requirement> &requirements) = 0;
+
+  /// Returns the replaced decl for a given @_dynamicReplacement(for:) attribute.
+  virtual ValueDecl *
+  loadDynamicallyReplacedFunctionDecl(const DynamicReplacementAttr *DRA,
+                                      uint64_t contextData) = 0;
 };
 
 /// A class that can lazily load conformances from a serialized format.

lib/ClangImporter/ImporterImpl.h

@@ -1230,6 +1230,12 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
     llvm_unreachable("unimplemented for ClangImporter");
   }
 
+  ValueDecl *
+  loadDynamicallyReplacedFunctionDecl(const DynamicReplacementAttr *DRA,
+                                      uint64_t contextData) override {
+    llvm_unreachable("unimplemented for ClangImporter");
+  }
+
   void loadRequirementSignature(const ProtocolDecl *decl, uint64_t contextData,
                                 SmallVectorImpl<Requirement> &reqs) override {
     llvm_unreachable("unimplemented for ClangImporter");

lib/Serialization/Deserialization.cpp

@@ -5457,6 +5457,11 @@ ModuleFile::loadAssociatedTypeDefault(const swift::AssociatedTypeDecl *ATD,
   return getType(contextData);
 }
 
+ValueDecl *ModuleFile::loadDynamicallyReplacedFunctionDecl(
+    const DynamicReplacementAttr *DRA, uint64_t contextData) {
+  return cast<ValueDecl>(getDecl(contextData));
+}
+
 void ModuleFile::finishNormalConformance(NormalProtocolConformance *conformance,
                                          uint64_t contextData) {
   using namespace decls_block;

lib/Serialization/ModuleFile.h

@@ -839,6 +839,10 @@ class ModuleFile
   virtual Type loadAssociatedTypeDefault(const AssociatedTypeDecl *ATD,
                                          uint64_t contextData) override;
 
+  virtual ValueDecl *
+  loadDynamicallyReplacedFunctionDecl(const DynamicReplacementAttr *DRA,
+                                      uint64_t contextData) override;
+
   virtual void finishNormalConformance(NormalProtocolConformance *conformance,
                                        uint64_t contextData) override;