Accessing the last element of a Vec or a slice

I have some code that looks like this:

trait Stack {
    fn top(&mut self) -> Option<f64>;
}

impl Stack for Vec<f64> {
    fn top(&mut self) -> Option<f64> {
        match self.pop() {
            None => None,
            Some(v) => {
                self.push(v);
                Some(v)
            }
        }
    }
}

fn main() {
    let mut stack: Vec<f64> = Vec::new();
    stack.push(5.3);
    stack.push(2.3);
    stack.push(1.3);

    match stack.top() {
        Some(v) => println!("Top of the stack: {}", v),
        None => println!("The stack is empty"),
    }
}

Right now, the top() method is modifying self, but I think that this should not be necessary. The obvious way to do it didn't really work:

fn top(&mut self) -> Option<f64> {
    match self.len() {
        0 => None,
        n => self[n - 1],
    }
}

I've toyed around a bit with converting usize to i32 and back, but none of what I'm writing looks as short and readable as I think it should.

And just after posting the question, the answer appears to be obvious:

fn top (&mut self) -> Option<&f64> {
    match self.len() {
        0 => None,
        n => Some(&self[n-1])
    }
}

I.e. the usize was never the problem - the return type of top() was.