error[E0308]: mismatched types
  --> $DIR/region-lifetime-bounds-on-fns-where-clause.rs:20:43
   |
LL |     let _: fn(&mut &isize, &mut &isize) = a;
   |            ----------------------------   ^ one type is more general than the other
   |            |
   |            expected due to this
   |
   = note: expected fn pointer `for<'a, 'b, 'c, 'd> fn(&'a mut &'b _, &'c mut &'d _)`
                 found fn item `for<'a, 'b> fn(&'a mut &_, &'b mut &_) {a::<'_, '_>}`

error: lifetime may not live long enough
  --> $DIR/region-lifetime-bounds-on-fns-where-clause.rs:8:5
   |
LL | fn b<'a, 'b>(x: &mut &'a isize, y: &mut &'b isize) {
   |      --  -- lifetime `'b` defined here
   |      |
   |      lifetime `'a` defined here
LL |     // Illegal now because there is no `'b:'a` declaration.
LL |     *x = *y;
   |     ^^^^^^^ assignment requires that `'b` must outlive `'a`
   |
   = help: consider adding the following bound: `'b: 'a`

error: lifetime may not live long enough
  --> $DIR/region-lifetime-bounds-on-fns-where-clause.rs:14:5
   |
LL | fn c<'a,'b>(x: &mut &'a isize, y: &mut &'b isize) {
   |      -- -- lifetime `'b` defined here
   |      |
   |      lifetime `'a` defined here
...
LL |     a(x, y);
   |     ^^^^^^^ argument requires that `'b` must outlive `'a`
   |
   = help: consider adding the following bound: `'b: 'a`
   = note: requirement occurs because of a mutable reference to `&isize`
   = note: mutable references are invariant over their type parameter
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0308`.