error[E0658]: unions with non-`Copy` fields are unstable
  --> $DIR/feature-gate-untagged_unions.rs:9:1
   |
LL | / union U3 {
LL | |     a: String,
LL | | }
   | |_^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/55149
   = help: add `#![feature(untagged_unions)]` to the crate attributes to enable

error[E0658]: unions with non-`Copy` fields are unstable
  --> $DIR/feature-gate-untagged_unions.rs:13:1
   |
LL | / union U4<T> {
LL | |     a: T,
LL | | }
   | |_^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/55149
   = help: add `#![feature(untagged_unions)]` to the crate attributes to enable

error[E0658]: unions with `Drop` implementations are unstable
  --> $DIR/feature-gate-untagged_unions.rs:17:1
   |
LL | / union U5 {
LL | |     a: u8,
LL | | }
   | |_^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/55149
   = help: add `#![feature(untagged_unions)]` to the crate attributes to enable

error[E0740]: unions may not contain fields that need dropping
  --> $DIR/feature-gate-untagged_unions.rs:10:5
   |
LL |     a: String,
   |     ^^^^^^^^^
   |
note: `std::mem::ManuallyDrop` can be used to wrap the type
  --> $DIR/feature-gate-untagged_unions.rs:10:5
   |
LL |     a: String,
   |     ^^^^^^^^^

error[E0740]: unions may not contain fields that need dropping
  --> $DIR/feature-gate-untagged_unions.rs:14:5
   |
LL |     a: T,
   |     ^^^^
   |
note: `std::mem::ManuallyDrop` can be used to wrap the type
  --> $DIR/feature-gate-untagged_unions.rs:14:5
   |
LL |     a: T,
   |     ^^^^

error: aborting due to 5 previous errors

Some errors have detailed explanations: E0658, E0740.
For more information about an error, try `rustc --explain E0658`.