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

[Taylor Swift]

again, i should reiterate, most users aren’t compiler engineers and so most
people use access modifiers as a means of code organization. being able to
diagnose when a “private” symbol is being referenced from somewhere it
shouldn’t be is very important; the linking and mangling details should be
handled by the compiler underneath all of that.

On Mon, Oct 2, 2017 at 11:45 PM, Slava Pestov <spestov at apple.com> wrote:

>
> 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> wrote:
>
>> On Oct 2, 2017, at 8:06 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>> On Mon, Oct 2, 2017 at 9:55 PM, Slava Pestov <spestov at apple.com> wrote:
>>
>>>
>>> On Oct 2, 2017, at 7:52 PM, Kelvin Ma <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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