[Proposal] SF-NNNN Extending Calendar.RecurrenceRule.End

This proposal concerns adding couple properties to `Caledar.RecurrenceRule.End`,
first introduced in SF-0009.

Author: hristost

Proposals/NNNN-calendar-recurrence-rule-end-count-and-date.md

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+# Extending `Calendar.RecurrenceRule.End`
+
+* Proposal: SF-NNNN
+* Author(s): Hristo Staykov <hstaykov@apple.com>
+* Review Manager: [Tina Liu](https://github.com/itingliu)
+* Status: **Active review**
+* Bugs: <rdar://134294130>
+* Implementation: [apple/swift-foundation#888](https://github.com/apple/swift-foundation/pull/888)
+* Previous Proposal: [SF-0009](0009-calendar-recurrence-rule.md)
+
+## Revision history
+
+* **v1** Initial version
+
+## Introduction
+
+In [SF-0009](0009-calendar-recurrence-rule.md) we introduced `Calendar.RecurrenceRule`. In this API, we represent the end of a recurrence rule with the struct `Calendar.RecurrenceRule.End`:
+
+```swift
+/// When a recurring event stops recurring
+public struct End: Sendable, Equatable {
+    /// The event stops repeating after a given number of times
+    /// - Parameter count: how many times to repeat the event, including
+    ///                    the first occurrence. `count` must be greater
+    ///                    than `0`
+    public static func afterOccurrences(_ count: Int) -> Self
+    /// The event stops repeating after a given date
+    /// - Parameter date: the date on which the event may last occur. No
+    ///                   further occurrences will be found after that
+    public static func afterDate(_ date: Date) -> Self
+    /// The event repeats indefinitely
+    public static var never: Self
+    
+}
+```
+
+This is de-facto an enum that was declared as struct to be future-proof. However, the original API only allowed construction of the recurrence rule end, and did not allow any introspection afterwards. This proposal adds a few properties to `Calendar.RecurrenceRule.End` to remedy this.
+
+## Detailed design
+
+```swift
+public extension Calendar.RecurrenceRule.End {
+    /// At most many times the event may occur
+    /// This value is set when the struct was initialized with `.afterOccurrences()`
+    @available(FoundationPreview 6.0.2, *)
+    public var count: Int? { get }
+
+    /// The latest date when the event may occur
+    /// This value is set when the struct was initialized with `.afterDate()`
+    @available(FoundationPreview 6.0.2, *)
+    public var date: Date? { get }
+}
+```
+
+where `FoundationPreview 6.0.2` is equivalent to `macOS 15.2, iOS 18.2, tvOS 18.2, watchOS 11.2`.
+
+## Impact on existing code
+
+None.