error[E0499]: cannot borrow `v` as mutable more than once at a time
  --> $DIR/borrowck-vec-pattern-loan-from-mut.rs:6:13
   |
LL |     let vb: &mut [isize] = &mut v;
   |                            ------ first mutable borrow occurs here
...
LL |             v.push(tail[0] + tail[1]);
   |             ^      ------- first borrow later used here
   |             |
   |             second mutable borrow occurs here

error: aborting due to previous error

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