The map key can be a value from any built-in or struct type as long as the value can be used in an expression with the == operator. Slices, functions, and struct types that contain slices can’t be used as a map key.
map key可以使用任何内置类型或结构类型的值，只要这个值可以使用在==表达式中。slice，函数，和包含slice的结构体不能用作key。
参考资料：
Go in Action。