Implement Fracpack definition_will_not_change in rust and fixed arrays

[sparkplug0025]

No description provided.

[tbfleming]

Very unfortunate. I hope they fix the need to use Vec someday.

[sparkplug0025]

Agreed. 😢

This was the closest that I could try: https://doc.rust-lang.org/std/mem/union.MaybeUninit.html#initializing-an-array-element-by-element

But it does not work....

        let data = {
            let mut data: [MaybeUninit<T>; N] = unsafe { MaybeUninit::uninit().assume_init() };
            for elem in &mut data[..] {
                *elem = MaybeUninit::new(T::embedded_unpack(src, pos, &mut heap_pos)?);
            }
            unsafe { mem::transmute::<_, [T; N]>(data) }
        };

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
   --> fracpack/src/fracpack.rs:675:22
    |
675 |             unsafe { mem::transmute::<_, [T; N]>(data) }
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: source type: `[MaybeUninit<T>; N]` (this type does not have a fixed size)
    = note: target type: `[T; N]` (this type does not have a fixed size)

Issue: rust-lang/rust#61956

[tbfleming]

The llvm backend has an optimization pass which sometimes eliminates heap usage. It's really tough to predict when it does it though.

[tbfleming]

This change will be unnecessary after fix to [T;N]'s FIXED_SIZE

[sparkplug0025]

nice catch. I leaked the responsibility of the nested type, thanks for bringing me back here!

[tbfleming]

Should always be true when !definition_will_not_change

[sparkplug0025]

Maybe the name of this variable is misleading but the Fracpackable::USE_HEAP actually reflects this exact function of cpp: https://github.com/gofractally/psibase/blob/sparkplug0025/fracpack-arrays/libraries/psio/include/psio/fracpack.hpp#L317-L329

What you are referring to is to the following blocks, where we dont write to (or unpack from) the heap:

    let pack_heap = if !opts.definition_will_not_change {
        quote! { <u16 as fracpack::Packable>::pack(&(heap as u16), dest); }
    } else {
        quote! {}
    };
    let unpack_heap_size = if !opts.definition_will_not_change {
        quote! { let heap_size = <u16 as fracpack::Packable>::unpack(src, pos)?; }
    } else {
        quote! { let heap_size = #fixed_size; }
    };

[tbfleming]

Look at line 309

[sparkplug0025]

oh, nice catch! fixed!