[AST] Add Pattern::hasAnyMutableBindings

Author: hamishknight

include/swift/AST/Pattern.h

@@ -202,6 +202,9 @@ class alignas(8) Pattern {
   /// Does this binding declare something that requires storage?
   bool hasStorage() const;
 
+  /// Does this pattern have any mutable 'var' bindings?
+  bool hasAnyMutableBindings() const;
+
   static bool classof(const Pattern *P) { return true; }
 
   //*** Allocation Routines ************************************************/

lib/AST/Pattern.cpp

@@ -291,6 +291,15 @@ bool Pattern::hasStorage() const {
   return HasStorage;
 }
 
+bool Pattern::hasAnyMutableBindings() const {
+  auto HasMutable = false;
+  forEachVariable([&](VarDecl *VD) {
+    if (!VD->isLet())
+      HasMutable = true;
+  });
+  return HasMutable;
+}
+
 /// Return true if this is a non-resolved ExprPattern which is syntactically
 /// irrefutable.
 static bool isIrrefutableExprPattern(const ExprPattern *EP) {