问如何在结构内容上返回对迭代器的适当的生命周期引用？

EN

struct Grid {
    width: usize,
    height: usize,
}

impl Grid {
    fn new(width: usize, height: usize) -> Grid {
        Grid { width, height }
    }

    fn iter<'a>(&'a self) -> &'a impl Iterator<Item = (usize, usize)> {
        let i = (0..self.height).flat_map(|y: usize| (0..self.width).map(move |x| (x, y)));
        &i
    }
}

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
  --> src/lib.rs:12:43
   |
12 |         let i = (0..self.height).flat_map(|y: usize| (0..self.width).map(move |x| (x, y)));
   |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime 'a as defined on the method body at 11:13...
  --> src/lib.rs:11:13
   |
11 |     fn iter<'a>(&'a self) -> &'a impl Iterator<Item = (usize, usize)> {
   |             ^^
   = note: ...so that the types are compatible:
           expected &&Grid
              found &&'a Grid
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that return value is valid for the call
  --> src/lib.rs:11:34
   |
11 |     fn iter<'a>(&'a self) -> &'a impl Iterator<Item = (usize, usize)> {
   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^