Do not derive Clone/Copy/etc for opaque types

[VeaaC]

Currently bindgen will generate code that derives for opaque types, e.g.:

typedef struct MyOpaqueStruct MyOpaqueStruct;

#[derive(Copy, Clone, Debug)]
pub struct MyOpaqueStruct {
    _unused: [u8; 0],
}

This has several problems:

• If a user can obtain a reference to a type they can clone it, even though that is normally only possible through FFI calls, and will almost always result in UB
• It prevents a user from implementing those traits on their own

The only current solution seems to be to turn of those derivations completely, forcing a user to implement them manually on the non-opaque types.

I would suggest to just not derive any of these in case of an opaque struct (since I cannot come up with a use-case were they would actually work)

[emilio]

This makes sense... I can see a few cases where it'd work, but yeah, it's sketchy at best.

[jschwe]

#[derive(Copy, Clone, Debug)]
pub struct MyOpaqueStruct {
    _unused: [u8; 0],
}

Probably worth mentioning that in the bindgen code, these are "forward declared types". E.g. the opaque annotation does not cause bindgen to generate a struct like the above.