[SR-3911] Evaluate ABI impact of going from opaque layouts to freezing the layout for performance

[ematejska]

Previous ID	SR-3911
Radar	rdar://problem/24057817
Original Reporter	@ematejska
Type	Bug
Status	Resolved
Resolution	Duplicate

Additional Detail from JIRA

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

md5: 4d423078b6dbeed09e52552c34924b9e

duplicates:

• SR-260 Add availability information to @_fixed_layout attribute

Issue Description:

For Library Evolution, annotations may be applied to a library's types in future versions of that library to freeze the layout for performance even if the layout of these types was opaque in the past versions of the library. In this case, the annotations are versioned, yet the library remains binary compatible. This task is to investigate the ABI impact of this.

[bob-wilson]

@swift-ci create

[bob-wilson]

This is a duplicate of SR-260: Add availability information to @_fixed_layout attribute

It is not immediately needed for ABI stability. This will first matter in some future release when an opaque type changes to have a fixed layout, so it can be done after locking down for the stable ABI.