Some monsters can eat through iron bars

Any monster with rusting or corrosion attack can eat through
the bars. This includes rust monsters, grey oozes, and black puddings.

Original patch by Malcolm Ryan

src/monmove.c

@@ -1180,7 +1180,14 @@ postmov:
 			    add_damage(mtmp->mx, mtmp->my, 0L);
 		    }
 		} else if (levl[mtmp->mx][mtmp->my].typ == IRONBARS) {
-			if (flags.verbose && canseemon(mtmp))
+		    if (may_dig(mtmp->mx,mtmp->my) &&
+			(dmgtype(ptr, AD_RUST) || dmgtype(ptr, AD_CORR))) {
+			if (canseemon(mtmp))
+			    pline("%s eats through the iron bars.",
+				  Monnam(mtmp));
+			dissolve_bars(mtmp->mx, mtmp->my);
+			return(3);
+		    } else if (flags.verbose && canseemon(mtmp))
 			    Norep("%s %s %s the iron bars.", Monnam(mtmp),
 				  /* pluralization fakes verb conjugation */
 				  makeplural(locomotion(ptr, "pass")),
@@ -1266,6 +1273,14 @@ postmov:
 	return(mmoved);
 }
 
+void
+dissolve_bars(x, y)
+register int x, y;
+{
+    levl[x][y].typ = (Is_special(&u.uz) || *in_rooms(x,y,0)) ? ROOM : CORR;
+    newsym(x, y);
+}
+
 boolean
 closed_door(x, y)
 register int x, y;