error[E0740]: field must implement `Copy` or be wrapped in `ManuallyDrop<...>` to be used in a union
  --> $DIR/issue-41073.rs:2:5
   |
LL |     a: A,
   |     ^^^^
   |
   = note: union fields must not have drop side-effects, which is currently enforced via either `Copy` or `ManuallyDrop<...>`
help: wrap the field type in `ManuallyDrop<...>`
   |
LL |     a: std::mem::ManuallyDrop<A>,
   |        +++++++++++++++++++++++ +

error: aborting due to previous error

For more information about this error, try `rustc --explain E0740`.