fix: can not find the variable that be created in plugin.

packages/less/src/less/tree/ruleset.js

@@ -189,6 +189,9 @@ Ruleset.prototype = Object.assign(new Node(), {
         for (i = 0; (rule = rsRules[i]); i++) {
             if (!rule.evalFirst) {
                 rsRules[i] = rule = rule.eval ? rule.eval(context) : rule;
+                if (rule instanceof Declaration) {
+                    ruleset.resetCache();
+                }
             }
         }
 

packages/less/test/index.js

@@ -81,7 +81,8 @@ var testMap = [
     [{plugin: 'test/plugins/visitor/'}, 'visitorPlugin/'],
     [{plugin: 'test/plugins/filemanager/'}, 'filemanagerPlugin/'],
     [{math: 0}, '3rd-party/'],
-    [{ processImports: false }, 'process-imports/']
+    [{ processImports: false }, 'process-imports/'],
+    [{}, 'define-var/']
 ];
 testMap.forEach(function(args) {
     lessTester.runTestSet.apply(lessTester, args)

packages/test-data/css/define-var/define-var.css

@@ -0,0 +1,4 @@
+div {
+  height: 1px;
+  width: 33px;
+}

packages/test-data/less/define-var/define-var.less

@@ -0,0 +1,9 @@
+@plugin "../../plugin/plugin-define-var.js";
+
+@a: 1px;
+
+div {
+    height: @a;
+    x();
+    width: @x;
+}

packages/test-data/plugin/plugin-define-var.js

@@ -0,0 +1,11 @@
+module.exports = class LessPluginBestMixin {
+    install(less, pluginManager, functions) {
+        functions.add("x", () => {
+            return new less.tree.Declaration(
+                "@x",
+                new less.tree.Value(new less.tree.Keyword("33px"))
+            );
+        });
+        functions.add("define_var", (key) => {});
+    }
+};