error[E0308]: method not compatible with trait
  --> $DIR/issue-37884.rs:6:5
   |
LL |     fn next(&'a mut self) -> Option<Self::Item>
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected fn pointer `fn(&mut RepeatMut<'a, T>) -> Option<_>`
              found fn pointer `fn(&'a mut RepeatMut<'a, T>) -> Option<_>`
note: the anonymous lifetime as defined here...
  --> $DIR/issue-37884.rs:6:5
   |
LL |     fn next(&'a mut self) -> Option<Self::Item>
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...does not necessarily outlive the lifetime `'a` as defined here
  --> $DIR/issue-37884.rs:3:6
   |
LL | impl<'a, T: 'a> Iterator for RepeatMut<'a, T> {
   |      ^^

error: aborting due to previous error

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