[4.2][stdlib] Update default generator to SystemRandomNumberGenerator (#17990)

* Rename SystemRandomNumberGenerator, remove .default

Author: airspeedswift

stdlib/public/core/Bool.swift

@@ -124,15 +124,16 @@ public struct Bool {
   ///         print("Maybe another try?")
   ///     }
   ///
-  /// `Bool.random()` uses the default random generator, `Random.default`. The
-  /// call in the example above is equivalent to
-  /// `Bool.random(using: &Random.default)`.
+  /// `Bool.random()` uses the default random generator,
+  /// `SystemRandomNumberGenerator`. To supply a non-default generator, call the
+  /// equivalent method that takes one as an argument.
   ///
   /// - Returns: Either `true` or `false`, randomly chosen with equal
   ///   probability.
   @inlinable
   public static func random() -> Bool {
-    return Bool.random(using: &Random.default)
+    var g = SystemRandomNumberGenerator()
+    return Bool.random(using: &g)
   }
 }
 

stdlib/public/core/Collection.swift

@@ -1061,7 +1061,8 @@ extension Collection {
   ///   empty, the method returns `nil`.
   @inlinable
   public func randomElement() -> Element? {
-    return randomElement(using: &Random.default)
+    var g = SystemRandomNumberGenerator()
+    return randomElement(using: &g)
   }
 
   /// Do not use this method directly; call advanced(by: n) instead.

stdlib/public/core/CollectionAlgorithms.swift

@@ -416,7 +416,8 @@ extension Sequence {
   /// - Complexity: O(*n*)
   @inlinable
   public func shuffled() -> [Element] {
-    return shuffled(using: &Random.default)
+    var g = SystemRandomNumberGenerator()
+    return shuffled(using: &g)
   }
 }
 
@@ -471,7 +472,8 @@ extension MutableCollection where Self : RandomAccessCollection {
   /// - Complexity: O(*n*)
   @inlinable
   public mutating func shuffle() {
-    shuffle(using: &Random.default)
+    var g = SystemRandomNumberGenerator()
+    shuffle(using: &g)
   }
 }
 

stdlib/public/core/FloatingPoint.swift.gyb

@@ -2515,7 +2515,8 @@ where Self.RawSignificand : FixedWidthInteger {
   /// - Returns: A random value within the bounds of `range`.
   @inlinable
   public static func random(in range: ${Range}<Self>) -> Self {
-    return Self.random(in: range, using: &Random.default)
+    var g = SystemRandomNumberGenerator()
+    return Self.random(in: range, using: &g)
   }
 }
 

stdlib/public/core/Integers.swift.gyb

@@ -2629,7 +2629,8 @@ extension FixedWidthInteger {
   /// - Returns: A random value within the bounds of `range`.
   @inlinable
   public static func random(in range: ${Range}<Self>) -> Self {
-    return Self.random(in: range, using: &Random.default)
+    var g = SystemRandomNumberGenerator()
+    return Self.random(in: range, using: &g)
   }
 }
 

stdlib/public/core/Random.swift

@@ -32,7 +32,7 @@ import SwiftShims
 ///         }
 ///
 ///         static func random() -> Weekday {
-///             return Weekday.randomWeekday(using: &Random.default)
+///             return Weekday.random(using: &Random.default)
 ///         }
 ///     }
 ///
@@ -60,6 +60,7 @@ public protocol RandomNumberGenerator {
 }
 
 extension RandomNumberGenerator {
+  @inlinable
   public mutating func _fill(bytes buffer: UnsafeMutableRawBufferPointer) {
     // FIXME: Optimize
     var chunk: UInt64 = 0
@@ -112,46 +113,45 @@ extension RandomNumberGenerator {
 /// The default source of random data.
 ///
 /// When you generate random values, shuffle a collection, or perform another
-/// operation that depends on random data, this type's `default` property is
-/// the generator used by default. For example, the two method calls in this
-/// example are equivalent:
+/// operation that depends on random data, this type's `default` property is the
+/// generator used by default. For example, the two method calls in this example
+/// are equivalent:
 ///
 ///     let x = Int.random(in: 1...100)
-///     let y = Int.random(in: 1...100, using: &Random.default)
+///     var g = SystemRandomNumberGenerator()
+///     let y = Int.random(in: 1...100, using: &g)
 ///
-/// `Random.default` is automatically seeded, is safe to use in multiple
-/// threads, and uses a cryptographically secure algorithm whenever possible.
+/// `SystemRandomNumberGenerator` is automatically seeded, is safe to use in
+/// multiple threads, and uses a cryptographically secure algorithm whenever
+/// possible.
 ///
 /// Platform Implementation of `Random`
 /// ===================================
 ///
-/// While the `Random.default` generator is automatically seeded and
-/// thread-safe on every platform, the cryptographic quality of the stream of
-/// random data produced by the generator may vary. For more detail, see the
+/// While the `SystemRandomNumberGenerator` generator is automatically seeded
+/// and thread-safe on every platform, the cryptographic quality of the stream
+/// of random data produced by the generator may vary. For more detail, see the
 /// documentation for the APIs used by each platform.
 ///
 /// - Apple platforms use `arc4random_buf(3)`.
 /// - Linux platforms use `getrandom(2)` when available; otherwise, they read
 ///   from `/dev/urandom`.
-public struct Random : RandomNumberGenerator {
-  /// The default instance of the `Random` random number generator.
-  public static var `default`: Random {
-    get { return Random() }
-    set { /* Discard */ }
-  }
-
-  private init() {}
+@_fixed_layout
+public struct SystemRandomNumberGenerator : RandomNumberGenerator {
+  @inlinable
+  public init() { }
 
   /// Returns a value from a uniform, independent distribution of binary data.
   ///
   /// - Returns: An unsigned 64-bit random value.
-  @effects(releasenone)
+  @inlinable
   public mutating func next() -> UInt64 {
     var random: UInt64 = 0
     _stdlib_random(&random, MemoryLayout<UInt64>.size)
     return random
   }
 
+  @inlinable
   public mutating func _fill(bytes buffer: UnsafeMutableRawBufferPointer) {
     if !buffer.isEmpty {
       _stdlib_random(buffer.baseAddress!, buffer.count)

validation-test/stdlib/Random.swift

@@ -17,11 +17,12 @@ RandomTests.test("_fill(bytes:)") {
     expectEqual(bytes1, zeros)
     expectEqual(bytes2, zeros)
 
-    bytes1.withUnsafeMutableBytes { Random.default._fill(bytes: $0) }
+    var g = SystemRandomNumberGenerator()
+    bytes1.withUnsafeMutableBytes { g._fill(bytes: $0) }
     expectNotEqual(bytes1, bytes2)
     expectNotEqual(bytes1, zeros)
 
-    bytes2.withUnsafeMutableBytes { Random.default._fill(bytes: $0) }
+    bytes2.withUnsafeMutableBytes { g._fill(bytes: $0) }
     expectNotEqual(bytes1, bytes2)
     expectNotEqual(bytes2, zeros)
   }