error[E0501]: cannot borrow `*f` as mutable because previous closure requires unique access
  --> $DIR/borrowck-insert-during-each.rs:16:5
   |
LL |       f.foo(
   |       ^ --- first borrow later used by call
   |  _____|
   | |
LL | |
LL | |         |a| {
   | |         --- closure construction occurs here
LL | |             f.n.insert(*a);
   | |             --- first borrow occurs due to use of `f` in closure
LL | |         })
   | |__________^ second borrow occurs here

error[E0500]: closure requires unique access to `f` but it is already borrowed
  --> $DIR/borrowck-insert-during-each.rs:18:9
   |
LL |     f.foo(
   |     - --- first borrow later used by call
   |     |
   |     borrow occurs here
LL |
LL |         |a| {
   |         ^^^ closure construction occurs here
LL |             f.n.insert(*a);
   |             --- second borrow occurs due to use of `f` in closure

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0500, E0501.
For more information about an error, try `rustc --explain E0500`.