Monsters can see player resistances

If monsters see you resist something, generally elemental or magical
attack, or if they see you reflect an attack, they learn that and
will adjust their attack accordingly.

Originally from SporkHack, but this version comes via EvilHack with
some minor changes.

src/trap.c

@@ -1251,6 +1251,7 @@ trapeffect_slp_gas_trap(
         seetrap(trap);
         if (Sleep_resistance || breathless(g.youmonst.data)) {
             You("are enveloped in a cloud of gas!");
+            monstseesu(M_SEEN_SLEEP);
         } else {
             pline("A cloud of gas puts you to sleep!");
             fall_asleep(-rnd(25), TRUE);
@@ -3525,6 +3526,7 @@ dofiretrap(
           the(box ? xname(box) : surface(u.ux, u.uy)));
     if (Fire_resistance) {
         shieldeff(u.ux, u.uy);
+        monstseesu(M_SEEN_FIRE);
         num = rn2(2);
     } else if (Upolyd) {
         num = d(2, 4);
@@ -5331,6 +5333,7 @@ chest_trap(
             if (Shock_resistance) {
                 shieldeff(u.ux, u.uy);
                 You("don't seem to be affected.");
+                monstseesu(M_SEEN_ELEC);
                 dmg = 0;
             } else
                 dmg = d(4, 4);
@@ -5763,6 +5766,8 @@ lava_effects(void)
         You("sink into the %s%s!", hliquid("lava"),
             !boil_away ? ", but it only burns slightly"
                        : " and are about to be immolated");
+        if (Fire_resistance)
+            monstseesu(M_SEEN_FIRE);
         if (u.uhp > 1)
             losehp(!boil_away ? 1 : (u.uhp / 2), lava_killer,
                    KILLED_BY); /* lava damage */