lib/cast.h: const_cast(): Add macro for dropping 'const'

Uses of this macro indicate a code smell, but in some cases, libc
functions require breaking const correctness.  Use this macro to wrap
casts in such cases, so that we limit the danger of the cast.

It only permits discarding const.  Discarding any other qualifiers, or
doing other type changes should result in a compile-time error.

Link: <https://software.codidact.com/posts/286575/287345#answer-287345>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/Makefile.am

@@ -37,6 +37,7 @@ libshadow_la_SOURCES = \
 	basename.c \
 	bit.c \
 	bit.h \
+	cast.h \
 	chkname.c \
 	chkname.h \
 	chowndir.c \

lib/cast.h

@@ -0,0 +1,21 @@
+// SPDX-FileCopyrightText: 2022-2024, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#ifndef SHADOW_INCLUDE_LIB_CAST_H_
+#define SHADOW_INCLUDE_LIB_CAST_H_
+
+
+#include <config.h>
+
+#include "must_be.h"
+
+
+#define const_cast(T, p)                                                      \
+({                                                                            \
+	static_assert(is_same_type(typeof(&*(p)), const T), "");              \
+	(T) (p);                                                              \
+})
+
+
+#endif  // include guard