Rust error E0740: field must implement `Copy` or be wrapped in `ManuallyDrop<...>` to be used in a union

What rustc prints

error[E0740]: field must implement `Copy` or be wrapped in `ManuallyDrop<...>` to be used in a union
 --> main.rs:2:5
  |
2 |     a: A, // error!
  |     ^^^^
  |
  = 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<...>`
  |
2 |     a: std::mem::ManuallyDrop<A>, // error!
  |        +++++++++++++++++++++++ +

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

union Test {
    a: A, // error!
}

#[derive(Debug)]
struct A(i32);

impl Drop for A {
    fn drop(&mut self) { println!("A"); }
}

Related errors

• E0308mismatched types
• E0277the trait bound is not satisfied
• E0599no method named X found in the current scope
• E0433failed to resolve
• E0735generic parameters cannot use `Self` in their defaults
• E0737`#[track_caller]` can only be used with the Rust ABI
• E0741`A` must implement `ConstParamTy` to be used as the type of a const generic parameter
• E0743C-variadic type `...` may not be nested inside another type

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