[]byte("string") created in for loops are "persisted" on iteration

[thehowl]

Another issue pertaining to byte slices generated as a result of converting from a string. See-also: #1569, and #1135 for a possibly related bug about for loop retaining the same block on iteration.

package main

func main() {
	for i := 0; i < 2; i++ {
		l := []byte("lead")
		if i == 0 {
			l[0] = 'f'
		}
		println(string(l))
	}
}

// Output:
// fead
// fead

The output should be:

fead
lead

as l is modified only in the first iteration.

This does not happen if the byte slice is "explicitly" initialized from its bytes:

package main

func main() {
	for i := 0; i < 2; i++ {
		l := []byte{'a', 'b', 'c', 'd'}
		if i == 0 {
			l[0] = 10
		}
		println(l)
	}
}

// Output:
// fead
// lead

Workaround: use append to initialize a new byte slice.

package main

func main() {
	for i := 0; i < 2; i++ {
		l := append([]byte(nil), []byte("lead")...)
		if i == 0 {
			l[0] = 'f'
		}
		println(string(l))
	}
}

// Output:
// fead
// lead