[swift-evolution] [Pitch] Scoped @available

[Xiaodi Wu]

This would be very useful, but the spelling needs dramatic improvement.

"Available unavailable" is already challenging to read, but at least it is
learnable with time. The example where "@available(*, deprecated, access:
internal) open" means "fileprivate" is entirely unreasonable.

On Sun, Jul 9, 2017 at 22:40 rintaro ishizaki via swift-evolution <
swift-evolution at swift.org> wrote:

> Hi evolution community,
>
> I would like to propose "Scoped @available" attribute.
>
> What I want to achieve is to declare something that is publicly
> unavailable, but still usable from narrower scope. A concrete example is
> IndexableBase in the standard library:
>
> https://github.com/apple/swift/blob/master/stdlib/public/core/Collection.swift#L18-L20
> Workaround for this problem in stdlib is to use typealias to underscored
> declaration. However, we can't use this technique in our module because
> underscored declarations are still visible and usable from outside.
>
> As a solution, I propose to add "access" parameter to @available
> attribute, which limits the effect of @available attribute to the
> specified value or outer.
>
> ---
> // Module: Library
>
> /// This protocol is available for internal, but deprecated for public.
> @available(*, deprecated, access: public, message: "it will be removed in
> future")
> public protocol OldProtocol {
>     /* ... */
> }
>
> public Foo: OldProtocol { // No diagnostics
> }
>
> ---
> // Module: main
>
> import Library
>
> public Bar: OldProtocol { // warning: 'OldProtocol' is deprecated: it will
> be removed in future
> }
>
> ---
>
> I think this is useful when you want to stop exposing declarations, but
> want to keep using them internally.
>
> More examples:
>
> // is `open`, going to be `filerprivate`
> @available(*, deprecated, access: internal)
> open class Foo {}
>
> // was `internal`, now `private`
> @available(*, unavailable, access: fileprivate)
> var value: Int
>
> // No effect (invisible from public anyway): emit a warning
> @available(*, unavailable, access: public)
> internal struct Foo {}
>
> What do you think?
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>
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