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