I’m just getting up to speed with rust. As an exercise I’m trying to recreate strtok() in rust. However I’m stuck with correcting the types..

Here’s what I have (Live Demo):

fn strtok(str: Option<&'static str>, delimiter: char) -> &'static str {
    // Remember the string slice for next time
    let temp_str = "";
    let mut static_view: &mut &'static str = temp_str;
    if str.is_some() {
        static_view = str.unwrap();
    }
    // Get token from start to delimiter 
    let index = static_view.find(delimiter).unwrap();
    let token = &static_view[..index];
    static_view = &static_view[index..];
    token
}

fn main() {
    let str: &str = "Hello World, you son of a bitch!";
    let token1 = strtok(Some(str), ',');
    let token2 = strtok(None, ',');

    println!("{}", token1);
    println!("{}", token2);
}

I wanted to create a mutable static reference to a static string (slice), so I could switch the string slice for a different one if provided. If not, the function should go off the last known string slice.

However, I get this error:

    error[E0308]: mismatched types
 --> src/main.rs:5:46
  |
5 |     let mut static_view: &mut &'static str = temp_str;
  |                          -----------------   ^^^^^^^^ types differ in mutability
  |                          |
  |                          expected due to this
  |
  = note: expected mutable reference `&mut &'static _`
                     found reference `&_`

Which I don’t really understand. I never asked for the slice to be mutable, but I want the reference to it be mutable so I can reassign it. How should