Adjust from backdeploy to bit casting as always emit into client

Author: phausler

stdlib/public/Concurrency/ContinuousClock.swift

@@ -143,8 +143,8 @@ extension ContinuousClock: Clock {
 @_unavailableInEmbedded
 extension ContinuousClock {
   @available(SwiftStdlib 5.7, *)
-  @backDeployed(before: SwiftStdlib 9999)
-  public var systemEpoch: Instant { Instant(_value: .seconds(0)) }
+  @_alwaysEmitIntoClient
+  public var systemEpoch: Instant { unsafeBitCast(Duration.seconds(0), to: Instant.self) }
 }
 
 @available(SwiftStdlib 5.7, *)

stdlib/public/Concurrency/SuspendingClock.swift

@@ -131,8 +131,8 @@ extension SuspendingClock: Clock {
 @_unavailableInEmbedded
 extension SuspendingClock {
   @available(SwiftStdlib 5.7, *)
-  @backDeployed(before: SwiftStdlib 9999)
-  public var systemEpoch: Instant { Instant(_value: .seconds(0)) }
+  @_alwaysEmitIntoClient
+  public var systemEpoch: Instant { unsafeBitCast(Duration.seconds(0), to: Instant.self) }
 }
 
 @available(SwiftStdlib 5.7, *)

test/Concurrency/Runtime/clock.swift

@@ -158,6 +158,13 @@ import StdlibUnittest
       }
     }
 
+    tests.test("Ensure abi layout size of Instant") {
+      // If this test fails it means the ABI of ContinuousClock.Instant has been broken!
+      // it MUST be the same laoyut of that of Duration
+      expectEqual(MemoryLayout<ContinuousClock.Instant>.size, MemoryLayout<Duration>.size)
+      expectEqual(MemoryLayout<SuspendingClock.Instant>.size, MemoryLayout<Duration>.size)
+    }
+
     await runAllTestsAsync()
   }
 }