[Concurrency] Rename TaskPriority members.

Based on feedback from the second review, we decided to go with
high/default/low/background, with aliases for the Dispatch-inspired
names. While here, make TaskPriority be backed by a UInt8 to better
describe the actual restrictions, and start removing userInteractive,
because clients shouldn't be able to specify it.

(cherry picked from commit 8750f20)

Author: DougGregor

stdlib/public/Concurrency/SourceCompatibilityShims.swift

@@ -34,7 +34,16 @@ extension Task where Success == Never, Failure == Never {
 @available(SwiftStdlib 5.5, *)
 extension TaskPriority {
   @available(*, deprecated, message: "unspecified priority will be removed; use nil")
-  public static let unspecified: TaskPriority = .init(rawValue: 0x00)
+  @_alwaysEmitIntoClient
+  public static var unspecified: TaskPriority {
+    .init(rawValue: 0x00)
+  }
+
+  @available(*, deprecated, message: "userInteractive priority will be removed")
+  @_alwaysEmitIntoClient
+  public static var userInteractive: TaskPriority {
+    .init(rawValue: 0x21)
+  }
 }
 
 @available(SwiftStdlib 5.5, *)
@@ -55,7 +64,7 @@ extension Task where Failure == Error {
   @_alwaysEmitIntoClient
   @available(*, deprecated, message: "`Task.runDetached` was replaced by `Task.detached` and will be removed shortly.")
   public static func runDetached(
-    priority: TaskPriority = .unspecified,
+    priority: TaskPriority? = nil,
     operation: __owned @Sendable @escaping () async throws -> Success
   ) -> Task<Success, Failure> {
     detached(priority: priority, operation: operation)

stdlib/public/Concurrency/Task.swift

@@ -198,17 +198,18 @@ extension Task: Equatable {
 ///       much of a thing on other platforms (i.e. server side Linux systems).
 @available(SwiftStdlib 5.5, *)
 public struct TaskPriority: RawRepresentable, Sendable {
-  public typealias RawValue = Int
-  public var rawValue: Int
+  public typealias RawValue = UInt8
+  public var rawValue: UInt8
 
-  public init(rawValue: Int) {
+  public init(rawValue: UInt8) {
     self.rawValue = rawValue
   }
 
-  public static let userInteractive: TaskPriority = .init(rawValue: 0x21)
-  public static let userInitiated: TaskPriority = .init(rawValue: 0x19)
+  public static let high: TaskPriority = .init(rawValue: 0x19)
+  public static let userInitiated: TaskPriority = high
   public static let `default`: TaskPriority = .init(rawValue: 0x15)
-  public static let utility: TaskPriority = .init(rawValue: 0x11)
+  public static let low: TaskPriority = .init(rawValue: 0x11)
+  public static let utility: TaskPriority = low
   public static let background: TaskPriority = .init(rawValue: 0x09)
 }
 
@@ -263,7 +264,7 @@ extension Task where Success == Never, Failure == Never {
       }
 
       // Otherwise, query the system.
-      return TaskPriority(rawValue: _getCurrentThreadPriority())
+      return TaskPriority(rawValue: UInt8(_getCurrentThreadPriority()))
     }
   }
 }
@@ -318,11 +319,11 @@ struct JobFlags {
         return nil
       }
 
-      return TaskPriority(rawValue: value)
+      return TaskPriority(rawValue: UInt8(value))
     }
 
     set {
-      bits = (bits & ~0xFF00) | Int32(((newValue?.rawValue ?? 0) << 8))
+      bits = (bits & ~0xFF00) | Int32((Int(newValue?.rawValue ?? 0) << 8))
     }
   }
 
@@ -620,7 +621,7 @@ extension Task where Success == Never, Failure == Never {
     let priority = getJobFlags(currentTask).priority ?? Task.currentPriority._downgradeUserInteractive
 
     return await Builtin.withUnsafeContinuation { (continuation: Builtin.RawUnsafeContinuation) -> Void in
-      let job = _taskCreateNullaryContinuationJob(priority: priority.rawValue, continuation: continuation)
+      let job = _taskCreateNullaryContinuationJob(priority: Int(priority.rawValue), continuation: continuation)
       _enqueueJobGlobalWithDelay(duration, job)
     }
   }
@@ -648,7 +649,7 @@ extension Task where Success == Never, Failure == Never {
     let priority = getJobFlags(currentTask).priority ?? Task.currentPriority._downgradeUserInteractive
 
     return await Builtin.withUnsafeContinuation { (continuation: Builtin.RawUnsafeContinuation) -> Void in
-      let job = _taskCreateNullaryContinuationJob(priority: priority.rawValue, continuation: continuation)
+      let job = _taskCreateNullaryContinuationJob(priority: Int(priority.rawValue), continuation: continuation)
       _enqueueJobGlobal(job)
     }
   }