Your handwritten one has a major performance bug:

    found := false
    for _, v := range s {
        if v == needle {
            found = true
            break
        }
    }

Do you see it? It copies the v into a local variable, which could be tremendously wasteful if it's a large struct. You should instead be taking a pointer to s[i] and comparing the value there with `needle`.

If you'd used `slices.Contains(s, needle)`, on the other hand, it could have such a performance bug in it and you'd never know.