[swift-evolution] [Proposal] Scoped resources (like C# using statement)

[Chris Lattner]

On Dec 29, 2015, at 8:02 PM, Trent Nadeau via swift-evolution <swift-evolution at swift.org> wrote:
> Doing this manually is possible using `defer` statements among other options, but this is error prone as a `defer` can be forgotten, `lock`/`unlock` calls for two locks can be switched due to a typo, etc. Having a dedicated language construct for this common case makes it easier to read and write while making code shorter and clearer.
> 
> ```swift
> do {
>     lock.enterScope()
>     defer { lock.exitScope() }
> 
>     let file = try getFileHandle()
>     file.enterScope()
>     defer { file.exitScope() }
> 
>     // statements
> }

We have another pattern that types can use, which is:

lock.withThisLockHeld {
  … stuff ...
}

This can be done today with trailing closures.  Other examples of this are “autoreleasepool” and withUnsafePointer (for other reasons).

-Chris