Record and walk the semantic initializer for a custom attribute.

Author: DougGregor

include/swift/AST/Attr.h

@@ -1428,6 +1428,7 @@ class CustomAttr final : public DeclAttribute,
   TypeLoc type;
   Expr *arg;
   PatternBindingInitializer *initContext;
+  Expr *semanticInit = nullptr;
 
   unsigned hasArgLabelLocs : 1;
   unsigned numArgLabels : 16;
@@ -1463,6 +1464,9 @@ class CustomAttr final : public DeclAttribute,
   Expr *getArg() const { return arg; }
   void setArg(Expr *newArg) { arg = newArg; }
 
+  Expr *getSemanticInit() const { return semanticInit; }
+  void setSemanticInit(Expr *expr) { semanticInit = expr; }
+
   PatternBindingInitializer *getInitContext() const { return initContext; }
 
   static bool classof(const DeclAttribute *DA) {

lib/AST/ASTWalker.cpp

@@ -131,7 +131,12 @@ class Traversal : public ASTVisitor<Traversal, Expr*, Stmt*,
       if (doIt(mutableCustomAttr->getTypeLoc()))
         return true;
 
-      if (auto arg = customAttr->getArg()) {
+      if (auto semanticInit = customAttr->getSemanticInit()) {
+        if (auto newSemanticInit = doIt(semanticInit))
+          mutableCustomAttr->setSemanticInit(newSemanticInit);
+        else
+          return true;
+      } else if (auto arg = customAttr->getArg()) {
         if (auto newArg = doIt(arg))
           mutableCustomAttr->setArg(newArg);
         else

lib/Sema/TypeCheckConstraints.cpp

@@ -2557,6 +2557,9 @@ bool TypeChecker::typeCheckBinding(Pattern *&pattern, Expr *&initializer,
         var->getParentPatternBinding()->setInitializerSubsumed(0);
         tc.Context.setSideCachedPropertyDelegateBackingPropertyType(
             var, initType->mapTypeOutOfContext());
+
+        // Record the semantic initializer.
+        var->getAttachedPropertyDelegate()->setSemanticInit(expr);
       }
 
       initializer = expr;