TinyBox<T: Sized> usage for no_std compatible error handling as Result<T, dyn Error>

[CPerezz]

All true rust types know their size and alignment, which dyn Trait types accomplish using their vtable.

In consequence, one could build a TinyBox<T: Sized> type that allocated when size_of_val(&T) > size_of::<usize>() but otherwise stored T internally, and then TinyBox<dyn Trait> could actually work correctly by using size_of_val(&self), which only accesses the vtable, not the actual self data pointer.

We could then make TinyBox work even without alloc, simply by making its non-alloc form panic when size_of_val(&T) > size_of::<usize>().

At this point, if one has a no_std compatable Error trait then one could rebuild the entire trait infrastructure of std::io on top of this trait. This gives you Result<T,dyn Error> which works even without alloc for small enums, OS ernos, etc., but not large complex error types with backtrace allocations, etc.

I've zero time to implement this, but if anyone gets interested then I'm happy to give some pointers.

cc rust-lang/project-error-handling#20

Originally posted by @burdges in #343 (comment)

[marta-belles]

Closing it for now.