Macro define_opaque_storage_ffi 

macro_rules! define_opaque_storage_ffi {
    (
        $(#[$meta:meta])*
        $vis:vis struct $name:ident(
            $size:expr,
            $align_const:expr,
            $align_literal:literal,
            $ffi_fn:path
            $(, $arg:ident : $arg_ty:ty)* $(,)?
        );
    ) => { ... };
}

Defines an opaque C++ storage type with explicit size, alignment, and an FFI init function.