[SR-4341] Review existential type metadata format for ABI stability

[bob-wilson]

Previous ID	SR-4341
Radar	rdar://problem/31409498
Original Reporter	@bob-wilson
Type	Task
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Task, AffectsABI
Assignee	None
Priority	Medium

md5: 40d6fa97ab2a611294fa629e2e6d4d45

Issue Description:

From the Existential Metadata section of the ABI Stability manifesto:

Existential type metadata contains the number of witness tables present, whether the type is class-constrained, and a protocol descriptor for each protocol constraint. A protocol descriptor describes an individual protocol constraint, such as whether it is class-constrained, the size of conforming witness tables, and protocol descriptors for any protocols it refines. Protocol descriptors are layout compatible with the Objective-C runtime's protocol records on Apple platforms. The format of the existential type metadata needs to be reviewed as part of the ABI definition.

[bob-wilson]

@swift-ci create

[bob-wilson]

Joe Groff reviewed this: "The details of existential type metadata for the most part can be private to the runtime. We should be able to ensure it's fully opaque and adjust the format in the future without breaking ABI."