must_be.h: Add must_be() macro

It's like static_assert(3), but can be used in more places.  It's
necessary for writing a must_be_array() macro.

Link: <https://stackoverflow.com/a/57537491>
Cc: Christian Göttsche <cgzones@googlemail.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/Makefile.am

@@ -88,6 +88,7 @@ libshadow_la_SOURCES = \
 	memzero.c \
 	memzero.h \
 	motd.c \
+	must_be.h \
 	myname.c \
 	nss.c \
 	nscd.c \

lib/must_be.h

@@ -0,0 +1,61 @@
+/*
+ * SPDX-FileCopyrightText: 2019-2023, Alejandro Colomar <alx@kernel.org>
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+
+#ifndef SHADOW_INCLUDE_LIBMISC_MUST_BE_H_
+#define SHADOW_INCLUDE_LIBMISC_MUST_BE_H_
+
+
+#include <config.h>
+
+#include <assert.h>
+
+
+/*
+ * SYNOPSIS
+ *	int must_be(bool e);
+ *
+ * ARGUMENTS
+ *	e	Expression to be asserted.
+ *
+ * DESCRIPTION
+ *	This macro fails compilation if 'e' is false.  If 'e' is true,
+ *	it returns (int) 0, so it doesn't affect the expression in which
+ *	it is contained.
+ *
+ *	This macro is similar to static_assert(3).  While
+ *	static_assert(3) can only be used where a statement is allowed,
+ *	this must_be() macro can be used wherever an expression is
+ *	allowed.
+ *
+ * RETURN VALUE
+ *	0
+ *
+ * ERRORS
+ *	If 'e' is false, the compilation will fail, as when using
+ *	static_assert(3).
+ *
+ * EXAMPLES
+ *	#define must_be_array(a)  must_be(is_array(a))
+ *
+ *	#define NITEMS(a)  (sizeof(a) / sizeof(*(a)) + must_be_array(a))
+ *
+ *	int foo[42];
+ *	int bar[NITEMS(foo)];
+ */
+
+
+#define must_be(e)                                                            \
+(                                                                             \
+	0 * (int) sizeof(                                                     \
+		struct {                                                      \
+			static_assert(e, "");                                 \
+			int ISO_C_forbids_a_struct_with_no_members_;          \
+		}                                                             \
+	)                                                                     \
+)
+
+
+#endif  // include guard