Rust string manipulation, flow control improvements

Question

I am new to the language and currently learning from the rust programming book. I am wondering if there are any better way to improve my code. Especially the 2 pop (not sure if its the best practice)

Here is the code in question:

fn converttemp() {
    //user input
    println!("Please input temp (ex: 60F): ");
    let mut temp = String::new();
    io::stdin()
        .read_line(&mut temp)
        .expect("Failed to read line");

    //getting the  unit at the end of the string
    let unit = temp.trim().chars().last().unwrap();

    //removing \n and unit character
    temp.pop();
    temp.pop();

    //parse string to float
    let mut val: c_float = temp.parse().unwrap();

    //temp conversion control flow
    if unit == 'f' || unit == 'F' {
        val = (val - 32.0) * 5.0 / 9.0;
        println!("{temp}F to C = {val}C");
    } else if unit == 'c' || unit == 'C' {
        val = val * 9.0 / 5.0 + 32.0;
        println!("{temp}C to F = {val}F");
    } else {
        println!("error");
    }
}

I am a little confused at &str when I store the popped string but it retains String datatype when it is not stored. I am also a little confused on Option, for my case particularly Option. I had to. find workarounds when the data ended up being these.

edit: when I store the new popped string

let val = temp.pop();

val is Option<Char>

let unit = temp.trim().chars().last();

unit is Option<Char>

What is the major difference between String and &str

All I know is that one is mutable and one is immutable