Why is rune in golang an alias for int32 and not uint32?

Question

The type rune in Go is defined as

an alias for int32 and is equivalent to int32 in all ways. It is used, by conv

Answer 1

The fact that it's allowed a negative value lets you define your own rune sentinel values.

For example:

const EOF rune = -1

func (l *lexer) next() (r rune) {
    if l.pos >= len(l.input) {
        l.width = 0
        return EOF
    }
    r, l.width = utf8.DecodeRuneInString(l.input[l.pos:])
    l.pos += l.width
    return r
}

Seen here in a talk by Rob Pike: Lexical Scanning in Go.