[Won't merge] Use stdlib random API directly via a C shim

Package.swift

@@ -43,8 +43,10 @@ let package = Package(
         targets: ["UniqueID"]),
     ],
     targets: [
+      .target(name: "Swift_UniqueID_RuntimeShims"),
       .target(
         name: "UniqueID",
+        dependencies: ["Swift_UniqueID_RuntimeShims"],
         swiftSettings: settings),
       .testTarget(
         name: "UniqueIDTests",

Sources/Swift_UniqueID_RuntimeShims/dummy.c

@@ -0,0 +1,15 @@
+// Copyright The swift-UniqueID Contributors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "shims.h"

Sources/Swift_UniqueID_RuntimeShims/include/shims.h

@@ -0,0 +1,21 @@
+// Copyright The swift-UniqueID Contributors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef Swift_UniqueID_RuntimeShims_h
+#define Swift_UniqueID_RuntimeShims_h
+
+#include "stddef.h"
+void swift_stdlib_random(void*, size_t);
+
+#endif /* Swift_UniqueID_RuntimeShims_h */

Sources/UniqueID/UniqueID+v4.swift

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+import Swift_UniqueID_RuntimeShims
+
 extension UniqueID {
 
   /// Generates a new UUID with random bits from the system's random number generator.
@@ -41,8 +43,17 @@ extension UniqueID {
   ///
   @inlinable
   public static func random() -> UniqueID {
-    var rng = SystemRandomNumberGenerator()
-    return random(using: &rng)
+    var bytes = UniqueID.null.bytes
+    withUnsafeMutableBytes(of: &bytes) { dest in
+      swift_stdlib_random(dest.baseAddress!, 16)
+    }
+    // octet 6 = time_hi_and_version (high octet).
+    // high 4 bits = version number.
+    bytes.6 = (bytes.6 & 0xF) | 0x40
+    // octet 8 = clock_seq_high_and_reserved.
+    // high 2 bits = variant (10 = standard).
+    bytes.8 = (bytes.8 & 0x3F) | 0x80
+    return UniqueID(bytes: bytes)
   }
 
   /// Generates a new UUID with random bits from the given random number generator.