Make unpredictableSeed use CryptGenRandom (CryptoAPI) on Windows

std/random.d

@@ -1772,6 +1772,49 @@ else
     }
 }
 
+version (Windows)
+{
+    pragma(lib, "advapi32.lib"); // `std.registry` does so, too.
+
+    private bool wincryptGenRandom(T)(out T result) @trusted
+    {
+        import core.sys.windows.windef : DWORD, PBYTE;
+        import core.sys.windows.wincrypt :
+            CryptAcquireContext,
+            CryptGenRandom,
+            CryptReleaseContext,
+            CRYPT_VERIFYCONTEXT,
+            HCRYPTPROV,
+            MS_STRONG_PROV,
+            PROV_RSA_FULL;
+
+        HCRYPTPROV wincryptHandle;
+
+        const gotHandle = CryptAcquireContext(
+            &wincryptHandle,
+            null,
+            MS_STRONG_PROV.ptr,
+            PROV_RSA_FULL,
+            CRYPT_VERIFYCONTEXT,
+        );
+        if (!gotHandle)
+            return false;
+
+        scope (exit)
+            if (!CryptReleaseContext(wincryptHandle, 0)) { /* ignore */ }
+
+        const gotRandom = CryptGenRandom(
+            wincryptHandle,
+            DWORD(T.sizeof),
+            cast(PBYTE) &result,
+        );
+        if (!gotRandom)
+            return false;
+
+        return true;
+    }
+}
+
 /**
 A "good" seed for initializing random number engines. Initializing
 with $(D_PARAM unpredictableSeed) makes engines generate different
@@ -1788,7 +1831,19 @@ how excellent the source of entropy is.
 */
 @property uint unpredictableSeed() @trusted nothrow @nogc
 {
-    version (AnyARC4Random)
+    version (Windows)
+    {
+        uint result;
+        if (!wincryptGenRandom!uint(result))
+        {
+            version (none)
+                return fallbackSeed();
+            else
+                assert(false, "CryptAcquireContext() or CryptGenRandom() failed.");
+        }
+        return result;
+    }
+    else version (AnyARC4Random)
     {
         return arc4random();
     }
@@ -1837,7 +1892,19 @@ if (isUnsigned!UIntType)
         /// ditto
         @property UIntType unpredictableSeed() @nogc nothrow @trusted
         {
-            version (AnyARC4Random)
+            version (Windows)
+            {
+                UIntType result;
+                if (!wincryptGenRandom!UIntType(result))
+                {
+                    version (none)
+                        return fallbackSeed();
+                    else
+                        assert(false, "CryptAcquireContext() or CryptGenRandom() failed.");
+                }
+                return result;
+            }
+            else version (AnyARC4Random)
             {
                 static if (UIntType.sizeof <= uint.sizeof)
                 {