/*
error: lifetime may not live long enough
 --> src/lib.rs:8:12
  |
7 | fn foo1<'a, 'b>(a: &'a mut (), b: &'b mut ()) -> &'b mut () {
  |         --  -- lifetime `'b` defined here
  |         |
  |         lifetime `'a` defined here
8 |     let x: &'a mut () = b;
  |            ^^^^^^^^^^ type annotation requires that `'b` must outlive `'a`
  |
  = help: consider adding the following bound: `'b: 'a`

error: lifetime may not live long enough
 --> src/lib.rs:9:5
  |
7 | fn foo1<'a, 'b>(a: &'a mut (), b: &'b mut ()) -> &'b mut () {
  |         --  -- lifetime `'b` defined here
  |         |
  |         lifetime `'a` defined here
8 |     let x: &'a mut () = b;
9 |     x
  |     ^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
  |
  = help: consider adding the following bound: `'a: 'b`

help: `'a` and `'b` must be the same: replace one with the other
*/
fn foo1<'a, 'b>(a: &'a mut (), b: &'b mut ()) -> &'b mut () {
    let x: &'a mut () = b;
    x
}
/*error: lifetime may not live long enough
 --> src/lib.rs:4:12
  |
1 | fn foo2<'a, 'b>(a: &'b mut &'a mut ()) -> &'b mut () {
  |         --  -- lifetime `'b` defined here
  |         |
  |         lifetime `'a` defined here
...
4 |     let x: &'a mut () = &mut *a;
  |            ^^^^^^^^^^ type annotation requires that `'b` must outlive `'a`
  |
  = help: consider adding the following bound: `'b: 'a`
*/
fn foo2<'a, 'b>(a: &'b mut &'a mut ()) -> &'b mut () {
    // the error is that the user's return type is wrong:
    // this should be `&'b mut ()`
    let x: &'a mut () = &mut *a;
    x
}