We should imho leave dyn Trait unsized but instead use small box optimizations, although our current smallbox crate wastes one usize unnecessarily.
You instead want a SmallBox<T,Size> type that determines whether its internal Size holds T, or whether the first usize contains a Box, by invoking mem::size_of_val with the vtable provided by dyn Trait and ptr::null, ala https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=9f175095c2f072317c31d3e5c68e291b
It works because mem::size_of_val never dereferences its data pointer. All dyn Trait record their size n their vtable. We cannot create dyn Traits from unsized types, but even those keep their size in the fat pointer.
We'd make alloc an optional crate feature, so this SmallBox crate still works without std or even alloc. At this point, you define an error type SmallBox<dyn Error,usize> that exists without without std or alloc, provided your Error trait avoids std too.