How Golang solve the invalid UTF-8 character?

revalid.go

package main

import (
    "fmt"
    "unicode/utf8"
)

func main() {
    s := "a\xc5z"
    fmt.Printf("%q\n", s)
    if !utf8.ValidString(s) {
        v := make([]rune, 0, len(s))
        for i, r := range s {
            if r == utf8.RuneError {
                _, size := utf8.DecodeRuneInString(s[i:])
                if size == 1 {
                    continue
                }
            }
            v = append(v, r)
        }
        s = string(v)
    }
    fmt.Printf("%q\n", s)
}