perf($rootScope): mark the $watchCollection interceptor as $pure for literals

... to allow shallow watching literal inputs of any type

Author: jbedard

src/ng/parse.js

@@ -1944,6 +1944,7 @@ function $ParseProvider() {
         return second(first(value));
       }
       chainedInterceptor.$stateful = first.$stateful || second.$stateful;
+      chainedInterceptor.$pure = first.$pure && second.$pure;
 
       return chainedInterceptor;
     }
@@ -1979,14 +1980,18 @@ function $ParseProvider() {
       // If the expression itself has no inputs then use the full expression as an input.
       if (!interceptorFn.$stateful) {
         useInputs = !parsedExpression.inputs;
-        fn.inputs = (parsedExpression.inputs ? parsedExpression.inputs : [parsedExpression]).map(function(e) {
+        fn.inputs = parsedExpression.inputs ? parsedExpression.inputs : [parsedExpression];
+
+        if (!interceptorFn.$pure) {
+          fn.inputs = fn.inputs.map(function(e) {
               // Remove the isPure flag of inputs when it is not absolute because they are now wrapped in a
-              // potentially non-pure interceptor function.
+              // non-pure interceptor function.
               if (e.isPure === PURITY_RELATIVE) {
                 return function depurifier(s) { return e(s); };
               }
               return e;
             });
+        }
       }
 
       return addWatchDelegate(fn);

src/ng/rootScope.js

@@ -581,10 +581,10 @@ function $RootScopeProvider() {
        *    de-registration function is executed, the internal watch operation is terminated.
        */
       $watchCollection: function(obj, listener) {
-        // Mark it as $stateful to avoid any input-watching and always invoke the interceptor
+        // Mark literals as $pure to watch literal inputs of any type.
+        // Mark non literals as $stateful to avoid input-watching and always invoke the interceptor
         // since the inputs will most likely be non-simple (like collections!).
-        // ... unless it is a literal and input-watching may be sufficient.
-        $watchCollectionInterceptor.$stateful = !$parse(obj).literal;
+        $watchCollectionInterceptor.$stateful = !($watchCollectionInterceptor.$pure = $parse(obj).literal);
 
         var self = this;
         // the current value, updated on each dirty-check run