可以在这里找到答案http://clang-developer.42468.n3.nabble.com/Adding-nodes-to-Clang-s-AST-td4054800.html

然而，节点被添加到编译 AST - 例如，如果有人想向代码注入命名空间，他不能直接通过代码使用命名空间，因为词法分析器不知道.

However, the nodes are added to the compiled AST - for instance, in case one wants to inject a namespace to the code, he can't use the namespace directly via the code because the lexer don't know about it.