为什么可以允许从len（slice）切片？

Question

Why does the following behavior occur:

a := []int{1, 2, 3}
fmt.Println(a[0:])
fmt.Println(a[1:])
fmt.Println(a[2:])
fmt.Println(a[3:])// doesn't panic - why??
fmt.Println(a[4:])// panics as expected

Executable example

Answer 1

a[3:] builds an empty slice which, just like an empty array, is a valid and useful object (in all languages, not just in Go).

An empty slice also still points to an underlying array, a position and a capacity and can sometimes be extended:

a := []int{1, 2, 3}
emptySlice := a[1:1]
fmt.Println(emptySlice) // []
notEmpty := emptySlice[0:2]
fmt.Println(notEmpty)   // [2 3]

On the other end, a slice with a negative length is inconsistent. It means nothing and thus is forbidden.