Switch uninitialized Array inits to call original implementation

This strategy lets us avoid availability constraints on the new public
methods, since the original underscored version is part of the permanent
ABI.

Author: natecook1000

stdlib/public/core/Array.swift

@@ -1366,17 +1366,35 @@ extension Array {
 }
 
 extension Array {
-  @available(swift, deprecated: 5.1, renamed: "init(unsafeUninitializedCapacity:initializingWith:)")
+  /// Implementation for Array(unsafeUninitializedCapacity:initializingWith:)
+  /// and ContiguousArray(unsafeUninitializedCapacity:initializingWith:)
   @inlinable
-  public init(
+  internal init(
     _unsafeUninitializedCapacity: Int,
     initializingWith initializer: (
       _ buffer: inout UnsafeMutableBufferPointer<Element>,
       _ initializedCount: inout Int) throws -> Void
   ) rethrows {
-    try self.init(
-      unsafeUninitializedCapacity: _unsafeUninitializedCapacity,
-      initializingWith: initializer)
+    var firstElementAddress: UnsafeMutablePointer<Element>
+    (self, firstElementAddress) =
+      Array._allocateUninitialized(_unsafeUninitializedCapacity)
+
+    var initializedCount = 0
+    var buffer = UnsafeMutableBufferPointer<Element>(
+      start: firstElementAddress, count: _unsafeUninitializedCapacity)
+    defer {
+      // Update self.count even if initializer throws an error.
+      _precondition(
+        initializedCount <= _unsafeUninitializedCapacity,
+        "Initialized count set to greater than specified capacity."
+      )
+      _precondition(
+        buffer.baseAddress == firstElementAddress,
+        "Can't reassign buffer in Array(unsafeUninitializedCapacity:initializingWith:)"
+      )
+      self._buffer.count = initializedCount
+    }
+    try initializer(&buffer, &initializedCount)
   }
 
   /// Creates an array with the specified capacity, then calls the given
@@ -1404,29 +1422,16 @@ extension Array {
   ///       - initializedCount: The count of initialized elements in the array,
   ///         which begins as zero. Set `initializedCount` to the number of
   ///         elements you initialize.
-  @inlinable
+  @_alwaysEmitIntoClient @inlinable
   public init(
     unsafeUninitializedCapacity: Int,
     initializingWith initializer: (
       _ buffer: inout UnsafeMutableBufferPointer<Element>,
       _ initializedCount: inout Int) throws -> Void
   ) rethrows {
-    var firstElementAddress: UnsafeMutablePointer<Element>
-    (self, firstElementAddress) =
-      Array._allocateUninitialized(unsafeUninitializedCapacity)
-    
-    var initializedCount = 0
-    defer {
-      // Update self.count even if initializer throws an error.
-      _precondition(
-        initializedCount <= unsafeUninitializedCapacity,
-        "Initialized count set to greater than specified capacity."
-      )
-      self._buffer.count = initializedCount
-    }
-    var buffer = UnsafeMutableBufferPointer<Element>(
-      start: firstElementAddress, count: unsafeUninitializedCapacity)
-    try initializer(&buffer, &initializedCount)
+    self = try Array(
+      _unsafeUninitializedCapacity: unsafeUninitializedCapacity,
+      initializingWith: initializer)
   }
 
   /// Calls a closure with a pointer to the array's contiguous storage.

stdlib/public/core/ContiguousArray.swift

@@ -1017,22 +1017,9 @@ extension ContiguousArray {
       _ buffer: inout UnsafeMutableBufferPointer<Element>,
       _ initializedCount: inout Int) throws -> Void
   ) rethrows {
-    var firstElementAddress: UnsafeMutablePointer<Element>
-    (self, firstElementAddress) =
-      ContiguousArray._allocateUninitialized(unsafeUninitializedCapacity)
-
-    var initializedCount = 0
-    defer {
-      // Update self.count even if initializer throws an error.
-      _precondition(
-        initializedCount <= unsafeUninitializedCapacity,
-        "Initialized count set to greater than specified capacity."
-      )
-      self._buffer.count = initializedCount
-    }
-    var buffer = UnsafeMutableBufferPointer<Element>(
-      start: firstElementAddress, count: unsafeUninitializedCapacity)
-    try initializer(&buffer, &initializedCount)
+    self = try ContiguousArray(Array(
+      _unsafeUninitializedCapacity: unsafeUninitializedCapacity,
+      initializingWith: initializer))
   }
 
   /// Calls a closure with a pointer to the array's contiguous storage.