error[E0506]: cannot assign to `self.container_field` because it is borrowed
  --> $DIR/issue-81365-6.rs:18:9
   |
LL |         let first = &self[0];
   |                      ---- `self.container_field` is borrowed here
LL |         self.container_field = true;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ `self.container_field` is assigned to here but it was already borrowed
LL |         first;
   |         ----- borrow later used here
   |
   = note: borrow occurs due to deref coercion to `[()]`
note: deref defined here
  --> $DIR/issue-81365-6.rs:9:5
   |
LL |     type Target = [()];
   |     ^^^^^^^^^^^

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0506`.