enum discriminant should be picked to make enum NonZero

[RalfJung]

I would expect the following program to demonstrate that Test is NonZero:

#![allow(dead_code)]
use std::mem;

enum Test { A(i32), B(i32) }

fn main() {
    println!("{} {}", mem::size_of::<Test>(), mem::size_of::<Option<Test>>());
}

The reasoning is that Test itself can never use the NonZero discriminant optimization itself, so it should use discriminants 1 and 2 to guarantee that it is itself never all-zero. This should be backwards-compatible as AFAIK safe code cannot observe, and unsafe code is not allowed to rely on, the concrete values of the discriminants.

(Not sure if an issue is enough for this or if it needs an RFC, so I started with the simpler-to-do one. ;)

[kennytm]

I don't think it should use the NonZero trick, but "compress nested enum variant IDs" i.e.

    Option<Test>
==  enum {
        Some(enum {
            A(i32),   // <- discriminant = 0
            B(i32),   // <- discriminant = 1
        }),
        None,  // <- discriminant = 2
    }

[eddyb]

Closing as duplicate of rust-lang/rfcs#1230.