Parameters

fn bump(mut x: i32) -> i32 {
    x += 1
    return x
}

Borrowed parameters:

type Counter struct {
    value: i32
}

fn Counter::Inc(&mut self) {
    self.value += 1
}

fn Read(c: &Counter) -> i32 {
    return c.value
}

Use &T for read-only borrow and &mut T for mutable borrow.