它定义一个具有单一参数credentials的函数，供auth - constraints宏使用。
It defines a function with a single argument, credentials, which will be used by the auth-constraints macro.
给定参数(如果有且定义为宏)的展开。
The expansion of the given arguments (if any and if these are defined as macros).
不能将具有参数的预处理器宏用作值定义。
You cannot use preprocessor macros with arguments as value definitions.
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