error[E0502]: cannot borrow `u.z.c` as immutable because it is also borrowed as mutable
  --> $DIR/issue-45157.rs:37:20
   |
LL |         let mref = &mut u.s.a;
   |                    ---------- mutable borrow occurs here
...
LL |         let nref = &u.z.c;
   |                    ^^^^^^ immutable borrow occurs here
LL |         //~^ ERROR cannot borrow `u.z.c` as immutable because it is also borrowed as mutable [E0502]
LL |         println!("{} {}", mref, nref)
   |                           ---- borrow later used here

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0502"