error[E0596]: cannot borrow `*z.0.0` as mutable, as it is behind a `&` reference
  --> $DIR/cant-mutate-imm-borrow.rs:13:17
   |
LL |     let mut c = || {
   |                 ^^ cannot borrow as mutable
LL |
LL |         z.0.0.0 = format!("X1");
   |         ------- mutable borrow occurs due to use of `*z.0.0` in closure

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0596`.