[BUG] derive Introspect doesn't work in embedded generics

[zsluedem]

Describe the bug A clear and concise description of what the bug is. derive Introspect doesn't work in embedded generics

To Reproduce Steps to reproduce the behavior:

#[derive(Introspect, Serde, Copy, Drop)]
struct A<T>{
    a: T,
    b: u64
}

#[derive(Introspect, Serde, Copy, Drop)]
struct B<T>{
    a: A<T>,
    b: u64
}

failed with

error: Variable not dropped.
 --> gg.cairo[impls]:8:16
            c: core::serde::Serde::deserialize(ref serialized)?,
               ^**********************************************^
note: the variable needs to be dropped due to the divergence here:
  --> gg.cairo[impls]:9:16
            d: core::serde::Serde::deserialize(ref serialized)?,
               ^**********************************************^
note: Trait has no implementation in context: core::traits::Drop::<kingdom_lord::gg::A::<T>>
note: Trait has no implementation in context: core::traits::Destruct::<kingdom_lord::gg::A::<T>>

error: Variable not dropped.
 -->gg.cairo[impls]:8:16
            c: core::serde::Serde::deserialize(ref serialized)?,
               ^**********************************************^
note: the variable needs to be dropped due to the potential panic here:
  -->gg.cairo[impls]:9:16
            d: core::serde::Serde::deserialize(ref serialized)?,
               ^*********************************************^
note: Trait has no implementation in context: core::traits::Drop::<kingdom_lord::gg::A::<T>>
note: Trait has no implementation in context: core::traits::Destruct::<kingdom_lord::gg::A::<T>>
note: Trait has no implementation in context: core::traits::PanicDestruct::<kingdom_lord::gg::A::<T>>

Expected behavior Build should work,

Screenshots If applicable, add screenshots to help explain your problem.

Additional context Add any other context about the problem here.

[remybar]

Hi @zsluedem !

Thanks for the bug report !

After some investigations, this is not a Dojo bug but a strange behaviour in Cairo. I tried to compile the following Cairo code outside Dojo and I get the same error:

#[derive(Serde, Copy, Drop)]
struct A<T> {
    a: T,
}

#[derive(Serde, Copy, Drop)]
struct B<T> {
    a: A<T>,
    b: u8
}

But, if I move the a field of struct B at the end of the struct, it compiles properly.

On a same idea, this code does not compile:

#[derive(Serde, Copy, Drop)]
struct A<T> {
    a: T,
}

#[derive(Serde, Copy, Drop)]
struct C<T> {
    c: T,
}

#[derive(Serde, Copy, Drop)]
struct B<T> {
    b: u8
    a: A<T>,
    c: C<T>
}

I continue the investigation to know if it's a known limitation, a bug to fix or if there is a specific syntax for these cases.

@tarrencev could you assign this issue to me please ?

[remybar]

After some investigations, here is the solution: your struct A must implement the Destruct trait as follow:

#[derive(Serde, Copy, Drop, Introspect, Destruct)]
struct A<T> {
    a: T
}

#[derive(Serde, Copy, Drop, Introspect)]
struct B<T> {
    a: A<T>,
    b: u8
}

It also works for the example with the struct C but this struct C has also to implement the Destruct trait.

Tested with the spawn-and-move example and it compiles !

[zsluedem]

#[derive(Introspect, Serde, Copy, Drop)] struct A<T>{ a: T, b: u64 }

#[derive(Introspect, Serde, Copy, Drop)] struct B<T>{ a: A<T>, b: u64 }

I thought derive Drop would also implement Destruct. Interesting solution.