[ruby/etc] Deprecate names under `Struct`

https://github.com/ruby/etc/commit/25c538120c

ext/etc/etc.c

@@ -54,6 +54,12 @@ char *getlogin();
 
 #define RUBY_ETC_VERSION "1.1.0"
 
+#ifdef HAVE_RB_DEPRECATE_CONSTANT
+void rb_deprecate_constant(VALUE mod, const char *name);
+#else
+# define rb_deprecate_constant(mod,name) ((void)(mod),(void)(name))
+#endif
+
 #include "constdefs.h"
 
 /* call-seq:
@@ -1165,6 +1171,7 @@ Init_etc(void)
     rb_define_const(mEtc, "Passwd", sPasswd);
 #endif
     rb_define_const(rb_cStruct, "Passwd", sPasswd); /* deprecated name */
+    rb_deprecate_constant(rb_cStruct, "Passwd");
     rb_extend_object(sPasswd, rb_mEnumerable);
     rb_define_singleton_method(sPasswd, "each", etc_each_passwd, 0);
 
@@ -1200,6 +1207,7 @@ Init_etc(void)
     rb_define_const(mEtc, "Group", sGroup);
 #endif
     rb_define_const(rb_cStruct, "Group", sGroup); /* deprecated name */
+    rb_deprecate_constant(rb_cStruct, "Group");
     rb_extend_object(sGroup, rb_mEnumerable);
     rb_define_singleton_method(sGroup, "each", etc_each_group, 0);
 #endif

ext/etc/extconf.rb

@@ -47,6 +47,11 @@ if !File.exist?("#{srcdir}/depend")
   %x[#{RbConfig.ruby} #{srcdir}/mkconstants.rb -o #{srcdir}/constdefs.h]
 end
 
+decl = [
+  "void rb_deprecate_constant(VALUE, const char *);",
+]
+have_func('rb_deprecate_constant(Qnil, "None")', [decl])
+
 $distcleanfiles << "constdefs.h"
 
 create_makefile("etc")