Now that struct initializers "just" fall into the delegating case when
they're made inlinable, the only interesting case is class
initializers, which can be checked in a more direct way than what we
were doing before.
(and when the struct in question is non-fixed-layout, which was
already implemented)
This ensures that these initializers are never fieldwise in Swift 5
mode, which makes it safe for library authors to add new fields.
Initializers for non-fixed-layout structs that are inlinable or
are defined in a different module are treated as delegating
initializers.
Previously, only initializers containing a 'self.init' call were
delegating; initializers that assigned to 'self' were not, which
resulted in DI treating them as a root initializer where the
stored 'self' value was exploded into a series of stores to each
stored property member.
They were not resilient as a result.
Fixes <https://bugs.swift.org/browse/SR-5649>,
<rdar://problem/33767516>.
Value type initializers must initialize stored properties directly
if they do not delegate to another initializer via self.init().
Since direct stored property access is not permitted for resilient
value types from outside their resilience domain, this means that
such initializers are prohibited in two cases:
- If the initializer is defined in an extension from outside the
value type's resilience domain
- If the initializer is public and @_inlineable, since it might get
inlined outside the value type's resilience domain
Right now, such initializers cannot *assign* to self either;
I filed <https://bugs.swift.org/browse/SR-3686> to track the issue.
