Dereference error

main.rs

type Link = Option<Box<Line>>;

struct Cursor {
    line: Link,
}

struct Line {
    next: Link,
    value: String,
}

struct Buffer {
    head: Link,
    cursor: Cursor,
}

impl Buffer {
    pub fn set_cursor(&mut self) {
        self.cursor.line = self.head;
    }

    pub fn do_something_else(&self) {}
}

fn main() {
    let mut buffer = Buffer {
        head: None,
        cursor: Cursor { line: None }
    };

    buffer.set_cursor();
    buffer.do_something_else();
}

/*
This gives an error:

$ rustc main.rs
error.rs:19:28: 19:32 error: cannot move out of dereference of `&mut`-pointer
error.rs:19         self.cursor.line = self.head;
                                       ^~~~
error: aborting due to previous error
*/