[swift-evolution] [Proposal idea] Support for pure functions

[Joe Groff]

> On Dec 21, 2015, at 1:49 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Dec 21, 2015, at 12:05 PM, Jacob Bandes-Storch <jtbandes at gmail.com <mailto:jtbandes at gmail.com>> wrote:
>> 
>> On Mon, Dec 21, 2015 at 11:55 AM, Chris Lattner via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> …there probably has to be some way to unsafely “force a call to a non-pure function to be allowed in a pure one”, both because of type system limitations as well as interoperability with C and other languages.  Even ignoring issues around errno, it would be sad for a pure function to not be able to call “sin(x)” just because it weren’t marked __attribute__((const)).
>> 
>> Minor tangent, but should the same apply to @noescape? 
> 
> Yes, it should.  I believe you can currently use an unsafe cast to remove @noescape, and that the stdlib does it in a few places.  Dmitri, do you know where?

The runtime does this using unsafeBitCast, but don't follow its example—I don't want to promise this will always be possible. There are representation optimizations we can do with @noescape closures that would be blocked if they always had to be bitcastable to refcounted escapable closures. I'd like to introduce a `Builtin.makeEscapable` operation specifically to go from @noescape to escapable, introducing a refcounting shim if necessary in the future.

-Joe
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