Make opsem for all enums consistent, independent of number of variants and #[non_exhaustive] #151786
Description
The way we generate MIR currently means that enum opsem distinguished two classes of enums
- Those with more than 1 variant or a
#[non_exhaustive]attribute - Those with 1 or 0 variants and no
#[non_exhaustive]
For the latter, we sometimes omit reading the discriminant since we "already know the result". Also see #150681 where this is point 1.
I think we should treat all enums consistently. This example should be considered UB. In particular, I think #[non_exhaustive] should not affect the opsem.
This change was separated out of #150681 at the request of @traviscross, so that it can be discussed separately. So, let's discuss. :) @scottmcm and @nikomatsakis already expressed agreement with this next step in that PR.
Cc @meithecatte -- IIRC this had a non-zero crater fallout; where are the results of that again?