[swift-evolution] [Proposal] Partial initializers

[Slava Pestov]

> On Jan 21, 2016, at 9:13 PM, Chris Lattner <clattner at apple.com> wrote:
> 
>> 
>> On Jan 20, 2016, at 8:51 PM, Slava Pestov via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Adding more details…
>> 
>>> On Jan 20, 2016, at 8:46 PM, Slava Pestov via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> I'm a little concerned about exposing these things publicly: there's no point if the struct or class has non-public fields (because new non-delegating initializers can't initialize the non-public fields), and by default every struct and class is allowed to add non-public fields in new versions of a library. (See the LibraryEvolution.rst doc for more info.) I can kind of see making these private vs. internal, but that has other issues (see below).
>>>> 
>>>> Even in the case where all fields are public and the struct promises not to change any fields, you'd also have to promise that the partial initializer never changes which properties it's initializing; that's now part of the binary interface of the file. That's unusual for something that behaves like a function; usually the body is not part of the ABI.
>>>> 
>>>> I'd much rather just ban 'public' and never have partial inits be part of a library's public interface. Public initializer methods would be presented as plain methods, with no hint of their initializer origins.
>> 
>> If we just go with @_transparent functions for partial initialization, there are no new rules to add regarding ABI resilience. :-)
> 
> Ignoring the fact that @_transparent isn’t a user feature, I don’t see how this helps.  Mandatory inlining of @_transparent happens after DI runs.

I’m of the opinion that making it a user feature and running inlining before DI would solve the ‘partial initializers’ use-case. :-)

> 
> -Chris

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