[swift-dev] Help needed: SE-0035 design detail

[Joe Groff]

> On Apr 10, 2016, at 12:46 PM, Daniel Duan via swift-dev <swift-dev at swift.org> wrote:
> 
> Hi all,
> 
> I'm in the process of implementing SE-0035, which limits capturing inout
> parameter to @noescape contexts. The proposal is clear on capture behavior for
> the following:
> 
> 1. closure literals
> 2. nested function passed as arguments.
> 
> But I'm not sure what to do with this case, in which 'x' escapes.
> 
> func captureAndEscape(inout x: Int) -> () -> Void { func foo()
> { _ = x } return foo }
> 
> The most obvious answer is it should be considered with the same rule as
> a closure literal, but a nested function can not have @noescape in its type
> (for now anyways).
> 
> So, should this be legal, then? If not, where/how should the error be? 

Ideally IMO, we would consider a reference to a local function to be `@noescape` or not based on how the reference is used, rather than the type of the function declaration itself:

func escapes(_: () -> ())
func noescapes(_: @noescape () -> ())

func foo(inout x: Int) -> () -> () {
  func local() { _ = x }

  local() // full application doesn't form a closure, ref is @noescape here

  noescapes(local) // parameter is noescape, so ref is noescape

  escapes(local) // parameter is escapable, so ref is escapable

  var x = local // assigning to var forms a closure, so ref is escapabale

  return local // returning forms a closure, so ref is escapable
}

-Joe