...instead of the witness table’s linkage.
The WT linkage is derived from the minimum of the type’s and protocol’s visibility.
So if there is a public type conforming to a private protocol, we might not see the type’s metadata escaping (because it could be in another module).
This gives big code size wins for unused types and also for types, which are never used in a generic context.
Also it reduces the amount of symbols in the symbol table.
The size wins heavily depend on the project. I have seen binary size reductions from 0 to 20% on real world projects.
rdar://problem/30119960
