What is the simplest way to expand a slice to its capacity?

"Extending" a slice to its capacity is simply a slice expression, and specify the capacity as the high index. The high index does not need to be less than the length. The restriction is:

For arrays or strings, the indices are in range if 0 <= low <= high <= len(a), otherwise they are out of range. For slices, the upper index bound is the slice capacity cap(a) rather than the length.

Example:

b := make([]byte, 10, 20)
fmt.Println(len(b), cap(b), b)

b = b[:cap(b)]
fmt.Println(len(b), cap(b), b)

Output (try it on the Go Playground):

10 20 [0 0 0 0 0 0 0 0 0 0]
20 20 [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]

You can expand a slice to its capacity with slicing:

s = s[:cap(s)]