[swift-evolution] [Pitch] Scoped @available

[rintaro ishizaki]

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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