Add exit API

Add `exit` as suggested in the roadmap.

There are a few outstanding tasks with this API

- [ ] Discuss API naming on the Swift forum.
- [ ] Availablility annotation (I would like some suggestion from
  a reviewer as opposed to guessing it myself.)
- [ ] Improve the documantations if deemed necessary.

[roadmap]: apple#16

Sources/System/CMakeLists.txt

@@ -9,6 +9,7 @@ See https://swift.org/LICENSE.txt for license information
 
 add_library(SystemPackage
   Errno.swift
+  Exit.swift
   FileDescriptor.swift
   FileHelpers.swift
   FileOperations.swift

Sources/System/Exit.swift

@@ -0,0 +1,48 @@
+/*
+ This source file is part of the Swift System open source project
+
+ Copyright (c) 2020 - 2021 Apple Inc. and the Swift System project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+*/
+
+
+#if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
+import Darwin
+#elseif os(Linux) || os(FreeBSD) || os(Android)
+import Glibc
+#elseif os(Windows)
+import ucrt
+#else
+#error("Unsupported Platform")
+#endif
+
+public struct ExitStatus: RawRepresentable, Hashable, Codable {
+  /// The raw exit status code for C standard library.
+  @_alwaysEmitIntoClient
+  public let rawValue: CInt
+
+  /// Create an exit status.
+  ///
+  /// - Parameter rawValue: The raw exit status code for C standard library.
+  @_alwaysEmitIntoClient
+  public init(rawValue: CInt) { self.rawValue = rawValue }
+
+  /// Success exit status.
+  @_alwaysEmitIntoClient
+  public static var success: Self { .init(rawValue: EXIT_SUCCESS) }
+
+  /// Failure exit status.
+  @_alwaysEmitIntoClient
+  public static var failure: Self { .init(rawValue: EXIT_FAILURE) }
+}
+
+
+/// Terminate the current process.
+///
+/// - Parameter status: The exit status for the finished process.
+@_alwaysEmitIntoClient
+public func exit(with status: ExitStatus) -> Never {
+  exit(status.rawValue)
+}

Tests/SystemTests/ExitTest.swift

@@ -0,0 +1,23 @@
+/*
+ This source file is part of the Swift System open source project
+
+ Copyright (c) 2020 Apple Inc. and the Swift System project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+*/
+
+import XCTest
+
+#if SYSTEM_PACKAGE
+import SystemPackage
+#else
+import System
+#endif
+
+final class ExitTest: XCTestCase {
+  func testConstants() {
+    XCTAssert(EXIT_SUCCESS == ExitStatus.success.rawValue)
+    XCTAssert(EXIT_FAILURE == ExitStatus.failure.rawValue)
+  }
+}

Tests/SystemTests/XCTestManifests.swift

@@ -11,6 +11,15 @@ extension ErrnoTest {
     ]
 }
 
+extension ExitTest {
+    // DO NOT MODIFY: This is autogenerated, use:
+    //   `swift test --generate-linuxmain`
+    // to regenerate.
+    static let __allTests__ExitTest = [
+        ("testConstants", testConstants),
+    ]
+}
+
 extension FileDescriptorTest {
     // DO NOT MODIFY: This is autogenerated, use:
     //   `swift test --generate-linuxmain`
@@ -27,6 +36,7 @@ extension FileOperationsTest {
     // to regenerate.
     static let __allTests__FileOperationsTest = [
         ("testAdHocOpen", testAdHocOpen),
+        ("testAdHocPipe", testAdHocPipe),
         ("testGithubIssues", testGithubIssues),
         ("testHelpers", testHelpers),
         ("testSyscalls", testSyscalls),
@@ -116,6 +126,7 @@ extension SystemStringTest {
 public func __allTests() -> [XCTestCaseEntry] {
     return [
         testCase(ErrnoTest.__allTests__ErrnoTest),
+        testCase(ExitTest.__allTests__ExitTest),
         testCase(FileDescriptorTest.__allTests__FileDescriptorTest),
         testCase(FileOperationsTest.__allTests__FileOperationsTest),
         testCase(FilePathComponentsTest.__allTests__FilePathComponentsTest),