1 error[E0658]: unions with non-`Copy` fields other than `ManuallyDrop<T>` are unstable
2 --> $DIR/feature-gate-untagged_unions.rs:20:5
4 LL | a: std::cell::RefCell<i32>,
5 | ^^^^^^^^^^^^^^^^^^^^^^^^^^
7 = note: see issue #55149 <https://github.com/rust-lang/rust/issues/55149> for more information
8 = help: add `#![feature(untagged_unions)]` to the crate attributes to enable
10 error[E0740]: unions cannot contain fields that may need dropping
11 --> $DIR/feature-gate-untagged_unions.rs:16:5
16 = note: a type is guaranteed not to need dropping when it implements `Copy`, or when it is the special `ManuallyDrop<_>` type
17 help: when the type does not implement `Copy`, wrap it inside a `ManuallyDrop<_>` and ensure it is manually dropped
19 LL | a: std::mem::ManuallyDrop<String>,
20 | +++++++++++++++++++++++ +
22 error[E0740]: unions cannot contain fields that may need dropping
23 --> $DIR/feature-gate-untagged_unions.rs:24:5
28 = note: a type is guaranteed not to need dropping when it implements `Copy`, or when it is the special `ManuallyDrop<_>` type
29 help: when the type does not implement `Copy`, wrap it inside a `ManuallyDrop<_>` and ensure it is manually dropped
31 LL | a: std::mem::ManuallyDrop<T>,
32 | +++++++++++++++++++++++ +
34 error: aborting due to 3 previous errors
36 Some errors have detailed explanations: E0658, E0740.
37 For more information about an error, try `rustc --explain E0658`.