Matching on an enum should read the entire enum

[matthewjasper]

Enum layout optimizations mean that the discriminant of an enum may not be stored in a tag disjoint from the rest of the fields of the enum. Run this example with MIRI to see this is a problem. https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=4ea0e31adbb3fe1893be528277060248

This is fixed by #56790, but this issue is being raised for tracking any backports.

cc @nikomatsakis

[goffrie]

err... maybe I'm missing something, but doesn't the linked code sample already get rejected by NLL?

[matthewjasper]

Yes, the example has been corrected now.