[swift-evolution] Pitch: Cross-module inlining and specialization

[Slava Pestov]

> On Oct 2, 2017, at 9:15 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> 
> On Mon, Oct 2, 2017 at 22:23 Slava Pestov <spestov at apple.com <mailto:spestov at apple.com>> wrote:
>> On Oct 2, 2017, at 8:06 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>> 
>> On Mon, Oct 2, 2017 at 9:55 PM, Slava Pestov <spestov at apple.com <mailto:spestov at apple.com>> wrote:
>> 
>>> On Oct 2, 2017, at 7:52 PM, Kelvin Ma <kelvin13ma at gmail.com <mailto:kelvin13ma at gmail.com>> wrote:
>>> 
>>> Is this only a problem with fileprivate or does it extend to private members too? I feel like this would be a very valuable feature to support.
>> 
>> Private members too. Consider this example,
>> 
>> struct S {
>>   private func f() {}
>> }
>> 
>> The member S.f mangles as _T06struct1SV1f33_AB643CAAAE0894CD0BC8584D7CA3AD23LLyyF. In this case, I suppose we won’t need the private discriminator because there can only be one S.f that’s directly a member of S, and not an extension. However imagine if two different source files both defined extensions of S, with a private member f. You would need to disambiguate them somehow.
>> 
>> The simple-minded way to do this would be to require @_versioned annotations on private and fileprivate members to supply an internally unique alternative name to be used for mangling-as-though-internal (i.e. `@_versioned(my_extension_f)`). Such a function becoming public in an ABI-compatible way would require renaming the "actual" name to the unique @_versioned name.
> 
> We have _silgen_name for that, but we really don’t want to expose this more generally because people have been abusing it to make things visible to C, and they should be using @_cdecl instead.
> 
> The difference here would be that the "@_versioned name" would be subject to mangling. It's essentially equivalent to a way of specifying a custom discriminator to be hashed so that the source file name is omitted and not ABI. Not that I think it'd be elegant, but it would not be abusable like _silgen_name.

That wouldn’t solve the problem where removing @_versioned(name) and adding public would change the symbol’s name.

However, your idea of mangling versioned private symbols like internal and diagnosing conflicts might be workable.

Slava

> 
>> 
>> A more elegant refinement could be to have @_versioned private and fileprivate members mangled as though internal, erroring if two or more members with the same name are both @_versioned--would that work?
>> 
> 
> If you’re going to do that what is the value in having the capability at all?
> 
> Solely to have some way of preventing members in one file from calling members in another file at compile time.

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