[swift-evolution] Add wrappers for allocWithTailElems_{n} routines

[Joe Groff]

> On Dec 13, 2016, at 1:37 PM, Karl via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On 13 Dec 2016, at 20:33, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>> on Tue Dec 13 2016, Alexey Komnin <swift-evolution at swift.org> wrote:
>> 
>>> Hi everyone!
>>> 
>>> I really don't know whether you have discussed it before. So, here is the deal.
>>> 
>>> Currently, Builtin routines like allocWithTailElems_{n} not available
>>> outside of stdlib. On the other hand, those routines are widely used
>>> in implementation of containers: Array, Set, Dictionary and others.
>>> 
>>> On the other hand, some Builtin routines are wrapped by publicly
>>> available operators and functions like unsafeBitCast, (==).
>>> ManagedBuffer.create(minimumCapacity:) class method is just a wrapper
>>> around call to allocWithTailElems_1 builtin function. So, it is
>>> possible to implement some containers, like Deque or linked-list using
>>> ManagedBuffer, but it's not enough if developer is going to implement
>>> advanced data structures like OrderedSet, LRU-cache or something more
>>> complex.
>>> 
>>> Instead, the developers are forced to use complex pointer arithmetic
>>> with UnsafeRawPtr. My point is: I think the language must provide all
>>> the available tools to build algorithms and data structures. First of
>>> all, it should provide the tools used for stdlib implementation.
>>> 
>>> Could we make some ManagedBuffer-alike wrappers around
>>> allocWithTailElems_{n} routines?
>> 
>> Feel free to design and propose these interfaces (in phase 2 of course).
>> 
>> -- 
>> -Dave
>> 
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> 
> Would the optimiser hints also possibly be up for making public (_onFastPath() and so)?
> 
> You can use them anyway; I don’t know why they’re underscored…

Underscored APIs are not guaranteed to exist in the future. The underscoring is a hack because we aren't able to use our normal access control mechanisms with the standard library yet for technical reasons.

-Joe