error[E0524]: two closures require unique access to `x` at the same time
  --> $DIR/borrowck-closures-mut-of-mut.rs:14:18
   |
LL |     let mut c1 = || set(&mut *x);
   |                  --           - first borrow occurs due to use of `x` in closure
   |                  |
   |                  first closure is constructed here
LL |     let mut c2 = || set(&mut *x);
   |                  ^^           - second borrow occurs due to use of `x` in closure
   |                  |
   |                  second closure is constructed here
LL |
LL |     c2(); c1();
   |           -- first borrow later used here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0524`.