Hello everyone, can someone please explain how lifetime assignment exactly works here and how all this weirdness leads to the compile error that we have here...?

fn strtok<'a>(haystack: &'a mut &'a str, c: char) -> &'a str {
    if let Some(v) = haystack.find(c) {
        let first = &haystack[..v];
        *haystack = &haystack[(v + c.len_utf8())..];
        first
    } else {
        return *haystack;
    }
}

fn main() {
    let mut x = "hello world";
    // &'a mut &'a str -> what the function expects
    // &'x mut &'static str -> what I am providing
    let hello = strtok(&mut x, ' ');
    assert_eq!(hello, "hello");
    assert_eq!(x, "world");
}

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