But the &str at the end is an additional heap allocation and causes an additional pointer hop when using the string. The only reason the function returns a pointer to a string in the first place is so that the nil check at the beginning can return nil. The calling code always checks if the result is nil and then immediately dereferences the string pointer. A better interface would be to panic if the argument is nil, or if that's too scary then:

    func (thing *Thing) String() (string, bool) {
        if thing == nil {
            return "", false
        }
        str := ...
        return str, true
    }