Go string appears shorter than it's first rune

I was running some fuzzing on my code and it found a bug. I have reduced it down to the following code snippet and I cannot see what is wrong.

Given the string

s := string("\xc0")

The len(s) function returns 1. However, if you loop through the string the first rune is length 3.

    for _, r := range s {
        fmt.Println("len of rune:", utf8.RuneLen(r)) // Will print 3
    }

My assumptions are:

• len(string) is returning the number of bytes in the string
• utf8.RuneLen(r) is returning the number of bytes in the rune

I assume I am misunderstanding something, but how can the length of a string be less than the length of one of it’s runes?

Playground here: https://go.dev/play/p/SH3ZI2IZyrL

>Solution :

The explanation is simple: your input is not valid UTF-8 encoded string.

fmt.Println(utf8.ValidString(s))

This outputs: false.

If you go with a valid string holding a rune of \xc0:

s = string([]rune{'\xc0'})

Then output is:

len of s: 2
runs in s: 1
len of rune: 2

Try it on the Go Playground.