support for nested rulesets inside dynamic mixins, with lexical scoping

lib/less/tree/mixin.js

@@ -45,7 +45,13 @@ tree.mixin.Definition.prototype = {
         context = { frames: [this, frame].concat(env.frames) };
 
         return new(tree.Ruleset)(null, this.rules.map(function (rule) {
-            return new(tree.Rule)(rule.name, rule.value.eval(context));
+            if (rule.rules) {
+                return new(tree.Ruleset)(rule.selectors, rule.rules.map(function (r) {
+                    return new(tree.Rule)(r.name, r.value.eval(context));
+                }));
+            } else {
+                return new(tree.Rule)(rule.name, rule.value.eval(context));
+            }
         }));
     }
 };

test/css/mixins-args.css

@@ -33,3 +33,9 @@ body {
 .scope-mix {
   width: 8;
 }
+.content {
+  width: 600px;
+}
+.content .column {
+  margin: 600px;
+}

test/less/mixins-args.less

@@ -62,3 +62,11 @@ body {
 .scope-mix {
   .global-mixin(3);
 }
+
+.nested-ruleset (@width: 200px) {
+    width: @width;
+    .column { margin: @width; }
+}
+.content {
+    .nested-ruleset(600px);
+}