Debug on packed structs causes error on non-Copy fields

[Super-Pizza]

The following code causes a compilation error.

#[repr(packed)]
#[derive(Debug)]
struct B(String);
fn main(){}

Error

error[E0507]: cannot move out of `self` which is behind a shared reference
 --> src/main.rs:4:10
  |
3 | #[derive(Debug)]
  |          ----- in this derive macro expansion
4 | struct B(String);
  |          ^^^^^^ move occurs because `self.0` has type `String`, which does not implement the `Copy` trait
  |
  = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0507`.

Meta

rustc --version --verbose:

rustc 1.73.0 (cc66ad468 2023-10-03)
binary: rustc
commit-hash: cc66ad468955717ab92600c770da8c1601a4ff33
commit-date: 2023-10-03
host: x86_64-unknown-linux-gnu
release: 1.73.0
LLVM version: 17.0.2

[Noratrieb]

This is intended, it is an attempt to make derives and repr(packed) work in more cases. The derive cannot know whether the field is copy. It cannot take a reference to the field, since it's not aligned. So it tries to copy, which at least works for Copy types.

[Super-Pizza]

Should, or can this be fixed?

[Noratrieb]

It cannot. repr(packed) sucks.

[Super-Pizza]

Atleast, we should modify the error message to make it clearer? Something like

error[E0507]: cannot move out of `self` which is behind a shared reference
 --> src/main.rs:4:10
  |
3 | #[derive(Debug)]
  |          ----- in this derive macro expansion
4 | struct B(String);
  |          ^^^^^^ move occurs because `self.0` has type `String`, which does not implement the `Copy` trait
  |
  = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
  = note: this struct is marked as `repr(packed)`, but unaligned field references are undefined behaviour.

For more information about this error, try `rustc --explain E0507`.

[Noratrieb]

that's possible.

[gurry]

@rustbot claim

[Super-Pizza]

I'll let others implement the diagnostic message.