[swift-evolution] [Pitch] make @noescape the default

[Chris Lattner]

> On Dec 19, 2015, at 4:38 PM, Dmitri Gribenko via swift-evolution <swift-evolution at swift.org> wrote:
> 
> On Sat, Dec 19, 2015 at 12:10 PM, Matthew Johnson via swift-evolution
> <swift-evolution at swift.org> wrote:
>> @noescape is safer because it does not require thinking about lifetime issues for captured objects.  My hunch was that @noescape (or @autoclosure with implies @noescape) is also more common.  I had a look through the standard library and this is definitely the case there.
>> 
>> What does everyone think about making @noescape the default and introducing @escaping (or something similar) to annotate function arguments that do escape the call stack?
> 
> @noescape provides additional guarantees and the optimizer can learn
> to rely on it in future.  Thus, it effectively becomes ABI, and you
> can't remove in future versions of the library without breaking the
> ABI.  I think adding it should be an explicit decision because of
> that.

I agree.  This is a very significant API restriction that should be opted-into intentionally.  It affects subtyping relationships in protocol conformances and overrides as well.

A different objection to @noescape-by-default is that it makes closures second citizens by default, which makes teaching and learning higher-order functional programming significantly less awesome.

-Chris