Fix issue 18084 - tempCString type should not change layout when used in

unittests.

std/internal/cstring.d

@@ -218,17 +218,20 @@ private:
 
     To* _ptr;
     size_t _length;        // length of the string
+
+    // the 'small string optimization'
     version (unittest)
     {
-        enum buffLength = 16 / To.sizeof;   // smaller size to trigger reallocations
+        // smaller size to trigger reallocations. Padding is to account for
+        // unittest/non-unittest cross-compilation (to avoid corruption)
+        To[16 / To.sizeof] _buff;
+        To[(256 - 16) / To.sizeof] _unittest_pad;
     }
     else
     {
-        enum buffLength = 256 / To.sizeof;   // production size
+        To[256 / To.sizeof] _buff; // production size
     }
 
-    To[buffLength] _buff;  // the 'small string optimization'
-
     static TempCStringBuffer trustedVoidInit() { TempCStringBuffer res = void; return res; }
 }