[SR-4332] Review interacting with enum discriminator through the witness table

[bob-wilson]

Previous ID	SR-4332
Radar	rdar://problem/31408033
Original Reporter	@bob-wilson
Type	Task
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Task, AffectsABI, Runtime
Assignee	@aschwaighofer
Priority	Medium

md5: e442adb143a8f1f5e41463146abc2a5a

Issue Description:

As part of ABI stability, we should review the efficiency of interacting with the enum discriminator through the witness table

[bob-wilson]

From the ABI Stability manifesto: "The value witness table stores whether a type is trivial and/or bitwise movable, whether there are extra inhabitants and if so how to store and retrieve them, etc. For enums, the value witness table will also provide functionality for interacting with the discriminator. There may be more efficient ways of representing enums that simplify this functionality (or provide a fast path), and that's under investigation"

[bob-wilson]

@swift-ci create

[aschwaighofer]

This is done
#12697