consider changing size of empty enum to usize::MAX

spawned off of https://github.com/rust-lang/rust/issues/24590

Empty enums such as:

enum Empty { }

are a hazard in a number of ways; in particular, odd people like myself keep wanting to investigate what happens if you unsafely inject a value into Empty, e.g. via mem::transmute (or by derefing an unsafe pointer *const Empty, etc).

The resulting code you get when doing a match on such a value is pretty much guaranteed to be undefined behavior. The project developers have stated that the memory layout of an empty enum (in fact, of all non C-like enums) is undefined, and thus undefined behavior is justifiable here.

Still, it seems to easy to expose this case, and it seems like it would be really easy to prevent it, e.g. by changing the representation size of an empty enum to something semi-absurd. Like usize::MAX.

Start by reviewing the linked Rust issue #24590 and the discussion here about empty-enum layout and unsafe values. Determine whether changing the representation size to usize::MAX is technically and semantically appropriate, and document the design decision and its consequences; the issue names no implementation files or tests.