intrusive_adapter! for ?Sized types

[tower120]

I want to use intrusive_adapter! with dyn_struct's DynStruct :

type SparseComponentPtrs = DynStruct<LinkedListLink, Option<NonNull<u8>>>;
intrusive_adapter!(SparseAdapter = Box<SparseComponentPtrs>: SparseComponentPtrs { header: LinkedListLink });

But have following error:

error[E0599]: the function or associated item `uninit` exists for union `MaybeUninit<DynStruct<LinkedListLink, Option<NonNull<u8>>>>`, but its trait bounds were not satisfied
  --> crates\ecs_rs\src\component_archetype_cache.rs:25:1
   |
25 | intrusive_adapter!(SparseAdapter = Box<DynStruct<LinkedListLink, Option<NonNull<u8>>>>: DynStruct<LinkedListLink, Option<NonNull<u8>>> { header: LinkedListLink ...
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ function or associated item cannot be called on `MaybeUninit<DynStruct<LinkedListLink, Option<NonNull<u8>>>>` due to unsatisfied trait bounds
   |
  ::: C:\Users\Andy\.cargo\registry\src\github.com-1ecc6299db9ec823\dyn_struct-0.2.0\src\lib.rs:96:1
   |
96 | pub struct DynStruct<Header, Tail> {
   | ---------------------------------- doesn't satisfy `_: Sized`
   |
   = note: the following trait bounds were not satisfied:
           `DynStruct<LinkedListLink, Option<NonNull<u8>>>: Sized`
   = note: this error originates in the macro `_memoffset__let_base_ptr` (in Nightly builds, run with -Z macro-backtrace for more info)

Can I do something about this?

[Amanieu d'Antras]

You can try implementing the Adapter trait manually instead of using the macro. The reason it doesn't work is that memoffset doesn't support unsized types since those don't have a fixed layout at compile-time.

[tower120]

I see, thanks!