NFC: Refactor conveniences for filtering out unavailable decls.

Introduce `AvailableDuringLoweringDeclFilter` which can be composed with
`OptionalTransformRange` to implement iterators that filter out unavailable
decls.

Author: tshortli

include/swift/AST/Decl.h

@@ -1288,6 +1288,17 @@ void *allocateMemoryForDecl(AllocatorTy &allocator, size_t baseSize,
   return mem;
 }
 
+/// A convenience for filtering out decls that are not available during
+/// lowering.
+struct AvailableDuringLoweringDeclFilter {
+  AvailableDuringLoweringDeclFilter() {}
+  llvm::Optional<Decl *> operator()(Decl *D) const {
+    if (D->isAvailableDuringLowering())
+      return D;
+    return llvm::None;
+  }
+};
+
 // A private class for forcing exact field layout.
 class alignas(8) _GenericContext {
 // Not really public. See GenericContext.

include/swift/AST/DeclContext.h

@@ -84,6 +84,7 @@ namespace swift {
   class SerializedTopLevelCodeDecl;
   class StructDecl;
   class AccessorDecl;
+  struct AvailableDuringLoweringDeclFilter;
 
 namespace serialization {
 using DeclID = llvm::PointerEmbeddedInt<unsigned, 31>;
@@ -865,12 +866,11 @@ class IterableDeclContext {
   /// on.
   DeclRange getMembers() const;
 
-  /// Retrieve the current set of members in this context, without triggering the
-  /// creation of new members via code synthesis, macro expansion, etc.
+  /// Retrieve the current set of members in this context, without triggering
+  /// the creation of new members via code synthesis, macro expansion, etc.
   ///
   /// This operation should only be used in narrow places where any side-effect
-  /// producing operations have been done earlier. For the most part, this means that
-  /// it should only be used in the implementation of
+  /// producing operations have been done earlier.
   DeclRange getCurrentMembers() const;
 
   /// Get the members that were syntactically present in the source code,
@@ -884,6 +884,15 @@ class IterableDeclContext {
   /// The resulting list of members will be stable across translation units.
   ArrayRef<Decl *> getABIMembers() const;
 
+  using DeclsForLowering =
+      OptionalTransformRange<ArrayRef<Decl *>,
+                             AvailableDuringLoweringDeclFilter>;
+
+  /// Get all of the members within this context that should be included when
+  /// lowering to SIL/IR, including any implicitly-synthesized members. The
+  /// decls returned by \c getABIMembers() are a superset of these decls.
+  DeclsForLowering getMembersForLowering() const;
+
   /// Get all of the members within this context, including any
   /// implicitly-synthesized members.
   ///

include/swift/SIL/SILModule.h

@@ -1079,16 +1079,6 @@ namespace Lowering {
 /// Objective-C runtime, i.e., +alloc and -dealloc.
 LLVM_LIBRARY_VISIBILITY bool usesObjCAllocator(ClassDecl *theClass);
 } // namespace Lowering
-
-/// Apply the given function to each ABI member of \c D skipping the members
-/// that should be skipped according to \c shouldSkipLowering()
-template <typename F>
-void forEachMemberToLower(IterableDeclContext *D, F &&f) {
-  for (auto *member : D->getABIMembers()) {
-    if (member->isAvailableDuringLowering())
-      f(member);
-  }
-}
 } // namespace swift
 
 #endif

include/swift/SIL/SILVTableVisitor.h

@@ -141,9 +141,9 @@ template <class T> class SILVTableVisitor {
     if (!theClass->hasKnownSwiftImplementation())
       return;
 
-    forEachMemberToLower(theClass, [&](Decl *member) {
+    for (Decl *member : theClass->getMembersForLowering()) {
       maybeAddMember(member);
-    });
+    }
   }
 };
 

lib/AST/DeclContext.cpp

@@ -930,6 +930,11 @@ ArrayRef<Decl *> IterableDeclContext::getABIMembers() const {
       ArrayRef<Decl *>());
 }
 
+IterableDeclContext::DeclsForLowering
+IterableDeclContext::getMembersForLowering() const {
+  return DeclsForLowering(getABIMembers(), AvailableDuringLoweringDeclFilter());
+}
+
 ArrayRef<Decl *> IterableDeclContext::getAllMembers() const {
   ASTContext &ctx = getASTContext();
   return evaluateOrDefault(

lib/SILGen/SILGenType.cpp

@@ -1098,9 +1098,9 @@ class SILGenType : public TypeMemberVisitor<SILGenType> {
   void emitType() {
     SGM.emitLazyConformancesForType(theType);
 
-    forEachMemberToLower(theType, [&](Decl *member) {
+    for (Decl *member : theType->getMembersForLowering()) {
       visit(member);
-    });
+    }
 
     // Build a vtable if this is a class.
     if (auto theClass = dyn_cast<ClassDecl>(theType)) {
@@ -1276,9 +1276,9 @@ class SILGenExtension : public TypeMemberVisitor<SILGenExtension> {
     // @_objcImplementation extension, but we don't actually need to do any of
     // the stuff that it currently does.
 
-    forEachMemberToLower(e, [&](Decl *member) {
+    for (Decl *member : e->getMembersForLowering()) {
       visit(member);
-    });
+    }
 
     // If this is a main-interface @_objcImplementation extension and the class
     // has a synthesized destructor, emit it now.