Synchronize Data.swift from the overlay (#1282)

This pulls in the fixes for:
Guard Data access to NSData._isCompact under availability guards
Ensure that hashing data with zero bytes avoids empty allocations and fix bridged empty data hashes from de-referencing null values (#12509)

Author: phausler

Foundation/Data.swift

@@ -38,6 +38,22 @@ internal func __NSDataIsCompact(_ data: NSData) -> Bool {
 import _SwiftFoundationOverlayShims
 import _SwiftCoreFoundationOverlayShims
 
+internal func __NSDataIsCompact(_ data: NSData) -> Bool {
+    if #available(OSX 10.10, iOS 9.0, tvOS 9.0, watchOS 2.0, *) {
+        return data._isCompact()
+    } else {
+        var compact = true
+        let len = data.length
+        data.enumerateBytes { (_, byteRange, stop) in
+            if byteRange.length != len {
+                compact = false
+            }
+            stop.pointee = true
+        }
+        return compact
+    }
+}
+
 @_silgen_name("__NSDataWriteToURL")
 internal func __NSDataWriteToURL(_ data: NSData, _ url: NSURL, _ options: UInt, _ error: NSErrorPointer) -> Bool
 
@@ -130,7 +146,7 @@ public final class _DataStorage {
         case .mutable:
             return try apply(UnsafeRawBufferPointer(start: _bytes?.advanced(by: range.lowerBound - _offset), count: Swift.min(range.count, _length)))
         case .customReference(let d):
-            if d._isCompact() {
+            if __NSDataIsCompact(d) {
                 let len = d.length
                 guard len > 0 else {
                     return try apply(UnsafeRawBufferPointer(start: nil, count: 0))
@@ -161,7 +177,7 @@ public final class _DataStorage {
                 return try apply(UnsafeRawBufferPointer(buffer))
             }
         case .customMutableReference(let d):
-            if d._isCompact() {
+            if __NSDataIsCompact(d) {
                 let len = d.length
                 guard len > 0 else {
                     return try apply(UnsafeRawBufferPointer(start: nil, count: 0))
@@ -505,7 +521,7 @@ public final class _DataStorage {
         case .mutable:
             return _bytes!.advanced(by: index - _offset).assumingMemoryBound(to: UInt8.self).pointee
         case .customReference(let d):
-            if d._isCompact() {
+            if __NSDataIsCompact(d) {
                 return d.bytes.advanced(by: index - _offset).assumingMemoryBound(to: UInt8.self).pointee
             } else {
                 var byte: UInt8 = 0
@@ -519,7 +535,7 @@ public final class _DataStorage {
                 return byte
             }
         case .customMutableReference(let d):
-            if d._isCompact() {
+            if __NSDataIsCompact(d) {
                 return d.bytes.advanced(by: index - _offset).assumingMemoryBound(to: UInt8.self).pointee
             } else {
                 var byte: UInt8 = 0
@@ -1626,9 +1642,11 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
     public var hashValue: Int {
         var hashValue = 0
         let hashRange: Range<Int> = _sliceRange.lowerBound..<Swift.min(_sliceRange.lowerBound + 80, _sliceRange.upperBound)
-        _withStackOrHeapBuffer(hashRange.count) { buffer in
-            _backing.withUnsafeBytes(in: hashRange) {
-                memcpy(buffer.pointee.memory, $0.baseAddress!, hashRange.count)
+        _withStackOrHeapBuffer(hashRange.count + 1) { buffer in
+            if hashRange.count > 0 {
+                _backing.withUnsafeBytes(in: hashRange) {
+                    memcpy(buffer.pointee.memory, $0.baseAddress!, hashRange.count)
+                }
             }
             hashValue = Int(bitPattern: CFHashBytes(buffer.pointee.memory.assumingMemoryBound(to: UInt8.self), hashRange.count))
         }