Better error message for assignment to final local variables

runtime/lib/errors_patch.dart

Index: runtime/lib/errors_patch.dart
===================================================================
--- runtime/lib/errors_patch.dart	(revision 32974)
+++ runtime/lib/errors_patch.dart	(working copy)
@@ -212,11 +212,15 @@
     var level = (_invocation_type >> _InvocationMirror._CALL_SHIFT) &
          _InvocationMirror._CALL_MASK;
     var type_str =
-        (const ["method", "getter", "setter", "getter or setter"])[type];
+        (const ["method", "getter", "setter", "getter or setter", "variable"])[type];
     var args_message = args_mismatch ? " with matching arguments" : "";
     var msg;
     var memberName =
         (_memberName == null) ? "" : internal.Symbol.getName(_memberName);
+
+    if (type == _InvocationMirror._LOCAL_VAR) {
+      return "cannot assign to final variable '$memberName'.\n\n";

[Ivan Posva, 2014/02/25 07:10:07]

How about just assigning to msg here and let the rest of the function deal with
massaging the resulting string?

[hausner, 2014/02/25 18:47:05]

I'm not sure what massaging you mean. Adding the two newlines?

The problem is that I'd have to special-case the local variable case again in
the switch below, or put the entire switch statement into an else branch. Is
that really more readable?

+    }
     switch (level) {
       case _InvocationMirror._DYNAMIC: {
         if (_receiver == null) {
@@ -294,9 +298,13 @@
     }
     var memberName =
         (_memberName == null) ? "" : internal.Symbol.getName(_memberName);
-    if (!args_mismatch) {
+    var type = _invocation_type & _InvocationMirror._TYPE_MASK;
+    if (type == _InvocationMirror._LOCAL_VAR) {
       msg_buf.write(
-          "NoSuchMethodError : method not found: '$memberName'\n"
+          "NoSuchMethodError: cannot assign to final variable '$memberName'");
+    } else if (!args_mismatch) {
+      msg_buf.write(
+          "NoSuchMethodError: method not found: '$memberName'\n"
           "Receiver: $receiver_str\n"
           "Arguments: [$actual_buf]");
     } else {