Requires Copy constraint on parametrized struct that is already known to #[derive(Copy)]

[marcianx]

pub trait MetaAssocTypes {
    type AssocType;
}

#[derive(Clone, Copy)]
struct IterBase<'a, Meta: MetaAssocTypes>
    where Meta::AssocType: 'a
{
    val: &'a Meta::AssocType
}

#[derive(Clone, Copy)]
pub struct FwdIter<'a, Meta: MetaAssocTypes>
    where Meta::AssocType: 'a
{
    iter: IterBase<'a, Meta>
}

fn main() {}

Playground

Private struct IterBase is known to implement Copy for all types it supports. However, #[derive(Copy)] for public struct FwdIter (which simply wraps IterBase) gives the following error:

<anon>:12:17: 12:21 error: the trait `Copy` may not be implemented for this type; field `iter` does not implement `Copy` [E0204]
<anon>:12 #[derive(Clone, Copy)]
                          ^~~~
<anon>:12:17: 12:21 note: in this expansion of #[derive_Copy] (defined in <anon>)
<anon>:12:17: 12:21 help: see the detailed explanation for E0204
error: aborting due to previous error
playpen: application terminated with error code 101

One workaround is to make IterBase public and explicitly add IterBase<'a, Meta>: Copy to the where clause of FwdIter (and all its impls).

[Aatch]

This is a limitation of derive, you can just do impl Copy for FwdIter (with the appropriate type params added) instead.

[marcianx]

I just tried that:

impl<'a, Meta: MetaAssocTypes> Copy for FwdIter<'a, Meta>
    where Meta::AssocType: 'a {}

but I received the same error on this impl line.

[Aatch]

Ah, right, that's because the same limitation affects IterBase, it's requiring that Meta be Copy too.

[bluss]

And both Copy and Clone need manual implementations here. (Clone can reuse Copy though).

[marcianx]

It's strange that IterBase requires Meta to be copy as IterBase it only contains a shared reference.

But sure enough, as it says https://doc.rust-lang.org/std/marker/trait.Copy.html

There is a small difference between the two: the derive strategy will also place a Copy bound on type parameters, which isn't always desired.

Thanks for the clarification.

[huonw]

Duplicate of #26925, thanks for filing!

[marcianx]

@huonw My last message is related to the bug you marked this as a dupe of. But isn't the original bug reported here different? The error says

`Copy` may not be implemented for this type; field `iter` does not implement `Copy`

But it is implement right above it in my example up top. So some information is not being propagated correctly.

[huonw]

@marcianx, oh, sorry, you are correct that this seems different!

[Mark-Simulacrum]

So looking at the expanded code, it looks like the problem is with the impl of Copy for FwdIter. The current implementation only requires that Meta: Copy, but that doesn't mean that Meta::AssocType: Copy, which leads to this error. I think this is a bug in the derive code, but I'm not sure it's all that easy to fix--if even possible--because in order to know whether Meta::AssocType: Copy is required, you need to inspect the IterBase type to see if it uses AssocType as a field, which I don't think is possible.

I'm thinking we should close this as not really possible to fix, but since I'm uncertain, I'll leave open for the time being. Perhaps someone more familiar with this code could comment? Otherwise, I'll close in a week or two.

// current impl
impl<'a, Meta: Copy + MyTrait> Copy for FwdIter<'a, Meta>
    where Meta::AssocType: 'a { }

// required impl
impl<'a, Meta: Copy + MyTrait> Copy for FwdIter<'a, Meta>
    where Meta::AssocType: Copy + 'a { }

[Mark-Simulacrum]

Closing in favor of #26925.