AST: Introduce a getFirstElement() utility on EnumCaseDecl.

This utility helps codify the practice of using the attributes of the first element of a case decl as the attributes for the entire case.

Author: tshortli

include/swift/AST/Decl.h

@@ -7479,7 +7479,16 @@ class EnumCaseDecl final : public Decl,
             Bits.EnumCaseDecl.NumElements};
   }
   SourceRange getSourceRange() const;
-  
+
+  /// Returns the first of the member elements or null if there are no elements.
+  /// The attributes written with an EnumCaseDecl will be attached to each of
+  /// the elements instead so inspecting the attributes of the first element is
+  /// often useful.
+  EnumElementDecl *getFirstElement() const {
+    auto elements = getElements();
+    return elements.empty() ? nullptr : elements.front();
+  }
+
   static bool classof(const Decl *D) {
     return D->getKind() == DeclKind::EnumCase;
   }

lib/AST/ASTPrinter.cpp

@@ -278,15 +278,12 @@ PrintOptions PrintOptions::printSwiftInterfaceFile(ModuleDecl *ModuleToPrint,
 
       // Skip enum cases containing enum elements we wouldn't print.
       if (auto *ECD = dyn_cast<EnumCaseDecl>(D)) {
-        auto elements = ECD->getElements();
-        if (!elements.empty()) {
+        if (auto *element = ECD->getFirstElement()) {
           // Enum elements are usually not printed, so we have to override the
-          // print option controlling that. We only check the first element
-          // because all the elements in a single case decl should have the same
-          // characteristics.
+          // print option controlling that.
           PrintOptions optionsCopy = options;
           optionsCopy.ExplodeEnumCaseDecls = true;
-          if (!shouldPrint(elements[0], optionsCopy))
+          if (!shouldPrint(element, optionsCopy))
             return false;
         }
       }
@@ -4219,14 +4216,14 @@ void PrintAST::printEnumElement(EnumElementDecl *elt) {
 }
 
 void PrintAST::visitEnumCaseDecl(EnumCaseDecl *decl) {
-  auto elems = decl->getElements();
-  if (!elems.empty()) {
+  if (auto *element = decl->getFirstElement()) {
     // Documentation comments over the case are attached to the enum elements.
-    printDocumentationComment(elems[0]);
-    printAttributes(elems[0]);
+    printDocumentationComment(element);
+    printAttributes(element);
   }
   Printer.printIntroducerKeyword("case", Options, " ");
 
+  auto elems = decl->getElements();
   llvm::interleave(elems.begin(), elems.end(),
     [&](EnumElementDecl *elt) {
       printEnumElement(elt);

lib/IDE/SyntaxModel.cpp

@@ -1077,8 +1077,8 @@ ASTWalker::PreWalkAction ModelASTWalker::walkToDeclPre(Decl *D) {
 
     // We need to handle the special case where attributes semantically
     // attach to enum element decls while syntactically locate before enum case decl.
-    if (!EnumCaseD->getElements().empty()) {
-      if (!handleAttrs(EnumCaseD->getElements().front()->getAttrs()))
+    if (auto *element = EnumCaseD->getFirstElement()) {
+      if (!handleAttrs(element->getAttrs()))
         return Action::SkipChildren();
     }
     if (pushStructureNode(SN, D)) {

lib/Sema/TypeCheckAvailability.cpp

@@ -1609,9 +1609,8 @@ abstractSyntaxDeclForAvailableAttribute(const Decl *ConcreteSyntaxDecl) {
   } else if (auto *ECD = dyn_cast<EnumCaseDecl>(ConcreteSyntaxDecl)) {
     // Similar to the PatternBindingDecl case above, we return the
     // first EnumElementDecl.
-    ArrayRef<EnumElementDecl *> Elems = ECD->getElements();
-    if (!Elems.empty()) {
-      return Elems.front();
+    if (auto *Elem = ECD->getFirstElement()) {
+      return Elem;
     }
   }