Golang的内置函数实现

Question

我无法找到内置函数的实现。

在builtin.go中，这是我发现的：

// The copy built-in function copies elements from a source slice 
into a
// destination slice. (As a special case, it also will copy bytes 
from a
// string to a slice of bytes.) The source and destination may 
overlap. Copy
// returns the number of elements copied, which will be the minimum 
of
// len(src) and len(dst).
func copy(dst, src []Type) int

内置函数的实际实现在哪里？