error[E0499]: cannot borrow `v` as mutable more than once at a time
  --> $DIR/two-phase-sneaky.rs:10:9
   |
LL |     v[0].push_str({
   |     -    -------- first borrow later used by call
   |     |
   |     first mutable borrow occurs here
LL |
LL |         v.push(format!("foo"));
   |         ^ second mutable borrow occurs here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0499`.