Add AnyHashableSendable (#36)

* Add `AnyHashableSendable`

* conformances

* Tests

* Update AnyHashableSendable.swift

* Update AnyHashableSendableTests.swift

* Update AnyHashableSendable.swift

README.md

@@ -35,6 +35,7 @@ This library comes with a number of tools that make working with Swift concurren
 testable.
 
   * [`LockIsolated`](#lockisolated)
+  * [`AnyHashableSendable`](#anyhashablesendable)
   * [Streams](#streams)
   * [Tasks](#tasks)
   * [Serial execution](#serial-execution)
@@ -44,6 +45,11 @@ testable.
 The `LockIsolated` type helps wrap other values in an isolated context. It wraps the value in a 
 class with a lock, which allows you to read and write the value with a synchronous interface.
 
+### `AnyHashableSendable`
+
+The `AnyHashableSendable` type is a type-erased wrapper like `AnyHashable` that preserves the
+sendability of the underlying value.
+
 ### Streams
 
 The library comes with numerous helper APIs spread across the two Swift stream types:

Sources/ConcurrencyExtras/AnyHashableSendable.swift

@@ -0,0 +1,42 @@
+/// A type-erased hashable, sendable value.
+///
+/// A sendable version of `AnyHashable` that is useful in working around the limitation that an
+/// existential `any Hashable` does not conform to `Hashable`.
+public struct AnyHashableSendable: Hashable, Sendable {
+  public let base: any Hashable & Sendable
+
+  /// Creates a type-erased hashable, sendable value that wraps the given instance.
+  public init(_ base: some Hashable & Sendable) {
+    if let base = base as? AnyHashableSendable {
+      self = base
+    } else {
+      self.base = base
+    }
+  }
+
+  public static func == (lhs: Self, rhs: Self) -> Bool {
+    AnyHashable(lhs.base) == AnyHashable(rhs.base)
+  }
+
+  public func hash(into hasher: inout Hasher) {
+    hasher.combine(base)
+  }
+}
+
+extension AnyHashableSendable: CustomDebugStringConvertible {
+  public var debugDescription: String {
+    "AnyHashableSendable(" + String(reflecting: base) + ")"
+  }
+}
+
+extension AnyHashableSendable: CustomReflectable {
+  public var customMirror: Mirror {
+    Mirror(self, children: ["value": base])
+  }
+}
+
+extension AnyHashableSendable: CustomStringConvertible {
+  public var description: String {
+    String(describing: base)
+  }
+}

Sources/ConcurrencyExtras/Documentation.docc/ConcurrencyExtras.md

@@ -236,6 +236,7 @@ need to make weaker assertions due to non-determinism, but can still assert on s
 ### Data races
 
 - ``LockIsolated``
+- ``AnyHashableSendable``
 
 ### Serial execution
 

Tests/ConcurrencyExtrasTests/AnyHashableSendableTests.swift

@@ -0,0 +1,18 @@
+import ConcurrencyExtras
+import XCTest
+
+final class AnyHashableSendableTests: XCTestCase {
+  func testBasics() {
+    XCTAssertEqual(AnyHashableSendable(1), AnyHashableSendable(1))
+    XCTAssertNotEqual(AnyHashableSendable(1), AnyHashableSendable(2))
+
+    func make(_ base: some Hashable & Sendable) -> AnyHashableSendable {
+      AnyHashableSendable(base)
+    }
+
+    let flat = make(1)
+    let nested = make(flat)
+
+    XCTAssertEqual(flat, nested)
+  }
+}