"#[derive] can't be used on a non-Copy #[repr(packed)] struct" but struct is Copy

[RalfJung]

Reported by @retep998 at #46043 (comment), here's a minimal example:

#[repr(packed)]
#[derive(Debug)]
struct Foo {
    x: usize,
}

impl Clone for Foo {
    fn clone(&self) -> Self {
        Foo { x: self.x }
    }
}
impl Copy for Foo {}

play

[Arnavion]

The warning changed in #51143 to require the struct to derive, rather than manually impl, Copy. So technically this issue is fixed.

However #51143 (comment) says that the compiler does generate code that uses references to fields if Copy is impled manually instead of #[derive]d. Is there an issue to track fixing the compiler to emit explicit impls of Copy correctly as you mentioned in #51143 (comment) or should this one be reused for that?

[Mark-Simulacrum]

Let's just reuse this issue for that. It shouldn't be too hard of a change, I think, but I'm uncertain.

[RalfJung]

However #51143 (comment) says that the compiler does generate code that uses references to fields if Copy is impled manually instead of #[derive]d.

I don't understand. Could you post some code that doesn't behave as it should?

[retep998]

Switching from impl Copy for $name {} to #[derive(Copy)] allows winapi to avoid this issue, but also incurs a 0.2 second compile time penalty in expansion.

[steveklabnik]

Triage: this still warns today

[patrickelectric]

I'm getting this warning here:

#[derive(Derivative, Debug, Copy, Clone)]
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
#[repr(C, packed)]
pub struct QueryMenu {

warning: `#[derive]` can't be used on a `#[repr(packed)]` struct that does not derive Copy (error E0133)
   --> src/v4l2.rs:460:58
    |
460 | #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
    |                                                          ^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(safe_packed_borrows)]` on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043>
    = note: this warning originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)