Storing Layout for Reflect types

[makspll]

What problem does this solve or what need does it fill?

It allows for lower level manipulation, working with custom allocators in the context of dynamic types etc.

What solution would you like?

Something along the lines of:

Reflect::layout(dyn_type) -> Layout

Or:

let layout: Layout = dyn_type.get_represented_type_info().unwrap().layout()

Maybe Optional<Layout> instead if this is too restrictive

What alternative(s) have you considered?

• Storing a custom TypeData but it requires registering it for all foreign types as well, not really tractable
• Adding a Sized bound to Reflect but not sure if that's possible

Additional context

N/A

[MrGVSV]

Requiring Sized isn't ideal (not that we reflect any unsized types currently). So we'd probably want to use something like Layout::for_value, though I’m not sure if that's different from Layout::new.

And yeah Option<Layout> may be best for now. I don't know how well it will play with runtime-defined types once we add better support for those, so it's probably safer to make it optional for now.