Use nint for native-sized integers in Marshal (#118335)

* Fix CA2020: Prevent behavioral change

Author: xtqqczze

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshal.Unix.cs

@@ -56,19 +56,19 @@ internal static unsafe void GetAnsiStringBytes(ReadOnlySpan<char> chars, Span<by
             bytes[actualByteLength] = 0;
         }
 
-        public static unsafe IntPtr AllocHGlobal(IntPtr cb)
+        public static unsafe IntPtr AllocHGlobal(nint cb)
         {
-            return (nint)NativeMemory.Alloc((nuint)(nint)cb);
+            return (nint)NativeMemory.Alloc((nuint)cb);
         }
 
         public static unsafe void FreeHGlobal(IntPtr hglobal)
         {
             NativeMemory.Free((void*)(nint)hglobal);
         }
 
-        public static unsafe IntPtr ReAllocHGlobal(IntPtr pv, IntPtr cb)
+        public static unsafe IntPtr ReAllocHGlobal(IntPtr pv, nint cb)
         {
-            return (nint)NativeMemory.Realloc((void*)(nint)pv, (nuint)(nint)cb);
+            return (nint)NativeMemory.Realloc((void*)(nint)pv, (nuint)cb);
         }
 
         public static IntPtr AllocCoTaskMem(int cb) => AllocHGlobal((nint)(uint)cb);

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshal.Windows.cs

@@ -144,7 +144,7 @@ public static unsafe void FreeHGlobal(IntPtr hglobal)
             }
         }
 
-        public static unsafe IntPtr ReAllocHGlobal(IntPtr pv, IntPtr cb)
+        public static unsafe IntPtr ReAllocHGlobal(IntPtr pv, nint cb)
         {
             if (pv == IntPtr.Zero)
             {

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshal.cs

@@ -351,7 +351,7 @@ public static unsafe int ReadInt32(IntPtr ptr, int ofs)
         [RequiresDynamicCode("Marshalling code for the object might not be available")]
         [EditorBrowsable(EditorBrowsableState.Never)]
         [Obsolete("ReadIntPtr(Object, Int32) may be unavailable in future releases.")]
-        public static IntPtr ReadIntPtr(object ptr, int ofs)
+        public static nint ReadIntPtr(object ptr, int ofs)
         {
 #if TARGET_64BIT
             return (nint)ReadInt64(ptr, ofs);
@@ -360,7 +360,7 @@ public static IntPtr ReadIntPtr(object ptr, int ofs)
 #endif
         }
 
-        public static IntPtr ReadIntPtr(IntPtr ptr, int ofs)
+        public static nint ReadIntPtr(IntPtr ptr, int ofs)
         {
 #if TARGET_64BIT
             return (nint)ReadInt64(ptr, ofs);
@@ -369,7 +369,7 @@ public static IntPtr ReadIntPtr(IntPtr ptr, int ofs)
 #endif
         }
 
-        public static IntPtr ReadIntPtr(IntPtr ptr) => ReadIntPtr(ptr, 0);
+        public static nint ReadIntPtr(IntPtr ptr) => ReadIntPtr(ptr, 0);
 
         public static unsafe long ReadInt64(IntPtr ptr, int ofs)
         {
@@ -468,32 +468,28 @@ public static unsafe void WriteInt32(IntPtr ptr, int ofs, int val)
 
         public static void WriteInt32(IntPtr ptr, int val) => WriteInt32(ptr, 0, val);
 
-        public static void WriteIntPtr(IntPtr ptr, int ofs, IntPtr val)
+        public static void WriteIntPtr(IntPtr ptr, int ofs, nint val)
         {
 #if TARGET_64BIT
             WriteInt64(ptr, ofs, (long)val);
 #else // 32
-#pragma warning disable CA2020 // Prevent from behavioral change
             WriteInt32(ptr, ofs, (int)val);
-#pragma warning restore CA2020
 #endif
         }
 
         [RequiresDynamicCode("Marshalling code for the object might not be available")]
         [EditorBrowsable(EditorBrowsableState.Never)]
         [Obsolete("WriteIntPtr(Object, Int32, IntPtr) may be unavailable in future releases.")]
-        public static void WriteIntPtr(object ptr, int ofs, IntPtr val)
+        public static void WriteIntPtr(object ptr, int ofs, nint val)
         {
 #if TARGET_64BIT
             WriteInt64(ptr, ofs, (long)val);
 #else // 32
-#pragma warning disable CA2020 // Prevent from behavioral change
             WriteInt32(ptr, ofs, (int)val);
-#pragma warning restore CA2020
 #endif
         }
 
-        public static void WriteIntPtr(IntPtr ptr, IntPtr val) => WriteIntPtr(ptr, 0, val);
+        public static void WriteIntPtr(IntPtr ptr, nint val) => WriteIntPtr(ptr, 0, val);
 
         public static unsafe void WriteInt64(IntPtr ptr, int ofs, long val)
         {