[AST] Add a helper method to PackExpansionExpr that gathers all packs

that are expanded within the pattern.

Author: hborla

include/swift/AST/Expr.h

@@ -3619,9 +3619,7 @@ class PackExpansionExpr final : public Expr,
     return {getTrailingObjects<PackElementExpr *>(), getNumBindings()};
   }
 
-  void setBinding(unsigned i, PackElementExpr *e) {
-    getMutableBindings()[i] = e;
-  }
+  void getExpandedPacks(SmallVectorImpl<ASTNode> &packs);
 
   GenericEnvironment *getGenericEnvironment() {
     return Environment;

lib/AST/Expr.cpp

@@ -1259,6 +1259,41 @@ PackExpansionExpr::create(ASTContext &ctx, Expr *patternExpr,
                                        implicit, type);
 }
 
+void PackExpansionExpr::getExpandedPacks(SmallVectorImpl<ASTNode> &packs) {
+  struct PackCollector : public ASTWalker {
+    llvm::SmallVector<ASTNode, 2> packs;
+
+    virtual PreWalkResult<Expr *> walkToExprPre(Expr *E) override {
+      // Don't walk into nested pack expansions
+      if (isa<PackExpansionExpr>(E)) {
+        return Action::SkipChildren(E);
+      }
+
+      if (isa<PackElementExpr>(E)) {
+        packs.push_back(E);
+      }
+
+      return Action::Continue(E);
+    }
+
+    virtual PreWalkAction walkToTypeReprPre(TypeRepr *T) override {
+      // Don't walk into nested pack expansions
+      if (isa<PackExpansionTypeRepr>(T)) {
+        return Action::SkipChildren();
+      }
+
+      if (isa<PackReferenceTypeRepr>(T)) {
+        packs.push_back(T);
+      }
+
+      return Action::Continue();
+    }
+  } packCollector;
+
+  getPatternExpr()->walk(packCollector);
+  packs.append(packCollector.packs.begin(), packCollector.packs.end());
+}
+
 PackElementExpr *
 PackElementExpr::create(ASTContext &ctx, SourceLoc eachLoc, Expr *packRefExpr,
                         bool implicit, Type type) {