error[E0499]: cannot borrow `l.0` as mutable more than once at a time
  --> $DIR/issue-25579.rs:24:32
   |
LL |         &mut Sexpression::Cons(ref mut expr) => { //[ast]~ ERROR [E0499]
   |                                ^^^^^^^^^^^^ mutable borrow starts here in previous iteration of loop
...
LL | }
   | - mutable borrow ends here

error[E0506]: cannot assign to `l` because it is borrowed
  --> $DIR/issue-25579.rs:25:13
   |
LL |         &mut Sexpression::Cons(ref mut expr) => { //[ast]~ ERROR [E0499]
   |                                ------------ borrow of `l` occurs here
LL |             l = &mut **expr; //[ast]~ ERROR [E0506]
   |             ^^^^^^^^^^^^^^^ assignment to borrowed `l` occurs here

error: aborting due to 2 previous errors

Some errors occurred: E0499, E0506.
For more information about an error, try `rustc --explain E0499`.