编译器提供了内建的功能在编译时求得一个数组的长度(如len (a))。
The compiler provides a built-in function, which is evaluated at compile time, to determine the length of an array (e.g. len (a)).
切片s的容量是数组从s[0]到数组尾端的大小，并由内建的cap函数返回。
The capacity of a slice s is the size of the array from s [0] to the end of the array, and is returned by the cap built-in function.
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