error[E0499]: cannot borrow `c.shared` as mutable more than once at a time
  --> ui/lockall_borrow_shared.rs:20:9
   |
20 |           c.shared.lock(|_| {
   |           ^        ---- --- first mutable borrow occurs here
   |           |        |
   |  _________|        first borrow later used by call
   | |
21 | |             c.shared.lock(|_| {}); // borrow error
   | |             -------- first borrow occurs due to use of `c` in closure
22 | |         });
   | |__________^ second mutable borrow occurs here

error[E0499]: cannot borrow `c` as mutable more than once at a time
  --> ui/lockall_borrow_shared.rs:20:23
   |
20 |         c.shared.lock(|_| {
   |         -------- ---- ^^^ second mutable borrow occurs here
   |         |        |
   |         |        first borrow later used by call
   |         first mutable borrow occurs here
21 |             c.shared.lock(|_| {}); // borrow error
   |             -------- second borrow occurs due to use of `c` in closure