This works fine:

package main

var foo string

func main() {
    fn := func() {
        foo = "AAA"
    }
    fn()
}

But if we move the variable declaration inside main():

package main

func main() {
    var foo string
    fn := func() {
        foo = "AAA"
    }
    fn()
}

— then it rants “foo declared and not used”.

Why so?