error[E0382]: assign of moved value: `t`
  --> $DIR/borrowck-partial-reinit-2.rs:15:5
   |
LL |     let mut t = Test { a: 1, b: None};
   |         ----- move occurs because `t` has type `Test`, which does not implement the `Copy` trait
LL |     let mut u = Test { a: 2, b: Some(Box::new(t))};
   |                                               - value moved here
LL |     t.b = Some(Box::new(u));
   |     ^^^ value assigned here after move

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0382`.