[swift-dev] No return functions and program exit

[John McCall]

> On Feb 6, 2017, at 2:25 PM, Joe Groff via swift-dev <swift-dev at swift.org> wrote:
>> On Feb 6, 2017, at 11:22 AM, Michael Gottesman <mgottesman at apple.com> wrote:
>> 
>> Here is my suggestion:
>> 
>> 1. We assume by default the leaking case.
>> 2. We change noreturn functions from C to maybe have a special semantic tag on them that says that cleanups should occur before them (i.e. UIApplicationMain).
> 
> I feel that "clean up before" is the safer ground case, and if we do any work to whitelist a group, it should be for the common "leakable" noreturns, like exit/_exit/abort/fatalError. That way, we momentarily burn some pointless cycles in the case we get it "wrong" rather than permanently leak memory.

I think I agree with this. :)

One reasonable approach to fixing the user problem Michael is identifying without providing special-case behavior to noreturn calls would be to simply make a stronger language guarantee about value liveness in general.  For example, we could say that if a value is "obviously" not used in or after a particular statement, then it is guaranteed to have been destroyed before that statement executes.  We can use the same definition of "obvious" that DI uses, which gives us a clear implementation path: we just run a lifetime-shortening pass as a mandatory optimization, possibly even as part of DI.

Of course, that's not necessarily great for debugging.

John.