[swift-evolution] Making capturing semantics of local functions explicit

[John McCall]

> On Oct 25, 2017, at 7:41 AM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
> I got bit again by a sneaky memory leak concerning local functions and would like to discuss a small language change. I vaguely remember this being discussed in the past, but can’t find the thread (if anybody could point me to it, I’d appreciate it). Basically, here’s an example of the leak:
> 
> class A {
>     func foo() {
>         func local() {
>             bar()
>         }
>     
>         methodWithEscapingClosure { [unowned self] _ in
>             self.bar()
>             local() // this leaks because local captures self
>         }
>     }
>     
>     func bar() {
>     }
> }
> 
> Its sneaky because local’s capturing of self is not obvious if you’ve trained your brain to watch out for calls prefixed with self. I would suggest having the compiler force users to make self capturing explicit, the same way it does for closures:

I think this is a good idea.  Ideally the proposal would also allow explicit capture lists in local functions.

John.

> 
> class A {
>     func foo() {
>         func local() {
>             bar() // error: Call to method ‘bar' in function ‘local' requires explicit 'self.' to make capture semantics explicit
>         }
>     
> 	// ...
>     }
> }
> 
> What do you think?
> 
> David.
> 
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