[swift-dev] Reason for call only non-escaping parameter

[John McCall]

> On May 31, 2017, at 12:21 PM, Dimitri Racordon via swift-dev <swift-dev at swift.org> wrote:
> Hi everyone,
> 
> I failed to find the reason why Swift does not allows a non-escaping parameter to be assigned to a local variable. Here is a minimal example:
> 
> func f(_ closure: () -> Int) {
>     let a = closure
> }
> 
> I do understand that assigning a non-escaping closure to a variable whose lifetime exceeds that of the function would (by definition) violate the non-escaping property. For instance, doing that is understandably illegal:
> 
> var global = { 0 }
> func f(_ closure: () -> Int) {
>     global = closure
> }
> 
> But in my first example, since `a` is stack allocated, there’s no risk that `closure` escapes the scope of `f`.
> 
> Is there some use case I’m missing, where such assignment could be problematic?
> Or is this a limitation of the compiler, which wouldn't go all the way to check whether the lifetime of the assignee is compatible with that of the non-escaping parameter may exceed that of the variable it is assigned to?
> 
> Thank you very much for your time and your answer.

Examples like yours, where a non-escaping closure parameter has a new constant name bound to it, are supportable but rather pointless — as a programmer, why have two names for the same value?  Examples that would be more useful, like assigning the closure into a local variable or allowing it to be used in a more complex expression (like ? :), complicate the analysis for non-escaping closures in a way that would significantly subvert their purpose.

John.
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