error[E0740]: unions cannot contain fields that may need dropping
  --> $DIR/field_checks.rs:24:5
   |
LL |     a: String,
   |     ^^^^^^^^^
   |
   = note: a type is guaranteed not to need dropping when it implements `Copy`, or when it is the special `ManuallyDrop<_>` type
help: when the type does not implement `Copy`, wrap it inside a `ManuallyDrop<_>` and ensure it is manually dropped
   |
LL |     a: std::mem::ManuallyDrop<String>,
   |        +++++++++++++++++++++++      +

error[E0740]: unions cannot contain fields that may need dropping
  --> $DIR/field_checks.rs:28:5
   |
LL |     a: std::cell::RefCell<i32>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: a type is guaranteed not to need dropping when it implements `Copy`, or when it is the special `ManuallyDrop<_>` type
help: when the type does not implement `Copy`, wrap it inside a `ManuallyDrop<_>` and ensure it is manually dropped
   |
LL |     a: std::mem::ManuallyDrop<std::cell::RefCell<i32>>,
   |        +++++++++++++++++++++++                       +

error[E0740]: unions cannot contain fields that may need dropping
  --> $DIR/field_checks.rs:32:5
   |
LL |     a: T,
   |     ^^^^
   |
   = note: a type is guaranteed not to need dropping when it implements `Copy`, or when it is the special `ManuallyDrop<_>` type
help: when the type does not implement `Copy`, wrap it inside a `ManuallyDrop<_>` and ensure it is manually dropped
   |
LL |     a: std::mem::ManuallyDrop<T>,
   |        +++++++++++++++++++++++ +

error[E0740]: unions cannot contain fields that may need dropping
  --> $DIR/field_checks.rs:44:5
   |
LL |     nest: U5,
   |     ^^^^^^^^
   |
   = note: a type is guaranteed not to need dropping when it implements `Copy`, or when it is the special `ManuallyDrop<_>` type
help: when the type does not implement `Copy`, wrap it inside a `ManuallyDrop<_>` and ensure it is manually dropped
   |
LL |     nest: std::mem::ManuallyDrop<U5>,
   |           +++++++++++++++++++++++  +

error[E0740]: unions cannot contain fields that may need dropping
  --> $DIR/field_checks.rs:48:5
   |
LL |     nest: [U5; 0],
   |     ^^^^^^^^^^^^^
   |
   = note: a type is guaranteed not to need dropping when it implements `Copy`, or when it is the special `ManuallyDrop<_>` type
help: when the type does not implement `Copy`, wrap it inside a `ManuallyDrop<_>` and ensure it is manually dropped
   |
LL |     nest: std::mem::ManuallyDrop<[U5; 0]>,
   |           +++++++++++++++++++++++       +

error: aborting due to 5 previous errors

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