Show mon vs mon attack messages only when target is visible

The visibility check in mattackm doesn't guarantee both the
attacker and defender can be seen by hero. Before giving
messages, check more strictly whether we could see the
message happening - either really seeing the monster or
sensing the monster by some other means, depending on
the message.  This should remove most of the "It" messages.

Also unhide mimics who get gazed by umber hulks.  We could
keep the mimic hidden and make the messages reflect the
hulk gazing at the thing the mimic is mimicing, but this
is much easier.  This fixes bz631 / H4500

src/eat.c

@@ -498,7 +498,7 @@ int *dmg_p; /* for dishing out extra damage in lieu of Int loss */
     } else if (mdef == &youmonst) {
         Your("brain is eaten!");
     } else { /* monster against monster */
-        if (visflag)
+        if (visflag && canspotmon(mdef))
             pline("%s brain is eaten!", s_suffix(Monnam(mdef)));
     }
 
@@ -512,7 +512,7 @@ int *dmg_p; /* for dishing out extra damage in lieu of Int loss */
         } else {
             /* no need to check for poly_when_stoned or Stone_resistance;
                mind flayers don't have those capabilities */
-            if (visflag)
+            if (visflag && canseemon(magr))
                 pline("%s turns to stone!", Monnam(magr));
             monstone(magr);
             if (magr->mhp > 0) {
@@ -597,7 +597,7 @@ int *dmg_p; /* for dishing out extra damage in lieu of Int loss */
          * monster mind flayer is eating another monster's brain
          */
         if (mindless(pd)) {
-            if (visflag)
+            if (visflag && canspotmon(mdef))
                 pline("%s doesn't notice.", Monnam(mdef));
             return MM_MISS;
         } else if (is_rider(pd)) {
@@ -609,7 +609,7 @@ int *dmg_p; /* for dishing out extra damage in lieu of Int loss */
         } else {
             *dmg_p += xtra_dmg;
             give_nutrit = TRUE;
-            if (*dmg_p >= mdef->mhp && visflag)
+            if (*dmg_p >= mdef->mhp && visflag && canspotmon(mdef))
                 pline("%s last thought fades away...",
                       s_suffix(Monnam(mdef)));
         }

src/mhitm.c

@@ -412,7 +412,7 @@ register struct monst *magr, *mdef;
                     && mdef->mhp > 1
                     && !mdef->mcan) {
                     if (clone_mon(mdef, 0, 0)) {
-                        if (vis) {
+                        if (vis && canspotmon(mdef)) {
                             char buf[BUFSZ];
 
                             Strcpy(buf, Monnam(mdef));
@@ -589,13 +589,17 @@ struct attack *mattk;
     char buf[BUFSZ];
 
     if (vis) {
+        if (mdef->data->mlet == S_MIMIC
+            && mdef->m_ap_type != M_AP_NOTHING)
+            seemimic(mdef);
         Sprintf(buf, "%s gazes at", Monnam(magr));
-        pline("%s %s...", buf, mon_nam(mdef));
+        pline("%s %s...", buf,
+              canspotmon(mdef) ? mon_nam(mdef) : "something");
     }
 
     if (magr->mcan || !magr->mcansee || !mdef->mcansee
         || (magr->minvis && !perceives(mdef->data)) || mdef->msleeping) {
-        if (vis)
+        if (vis && canspotmon(mdef))
             pline("but nothing happens.");
         return MM_MISS;
     }
@@ -691,8 +695,10 @@ register struct attack *mattk;
         if (vis) {
             /* 'it' -- previous form is no longer available and
                using that would be excessively verbose */
-            pline("%s expels it.", Monnam(magr));
+            pline("%s expels %s.", Monnam(magr),
-            pline("It turns into %s.", a_monnam(mdef));
+                  canspotmon(mdef) ? "it" : something);
+            if (canspotmon(mdef))
+                pline("It turns into %s.", a_monnam(mdef));
         }
         return MM_HIT; /* bypass mdamagem() */
     }
@@ -808,7 +814,7 @@ register struct attack *mattk;
                 mon_to_stone(magr);
                 return MM_HIT; /* no damage during the polymorph */
             }
-            if (vis)
+            if (vis && canspotmon(magr))
                 pline("%s turns to stone!", Monnam(magr));
             monstone(magr);
             if (magr->mhp > 0)
@@ -827,7 +833,7 @@ register struct attack *mattk;
     case AD_DGST:
         /* eating a Rider or its corpse is fatal */
         if (is_rider(pd)) {
-            if (vis)
+            if (vis && canseemon(magr))
                 pline("%s %s!", Monnam(magr),
                       (pd == &mons[PM_FAMINE])
                           ? "belches feebly, shrivels up and dies"
@@ -923,10 +929,10 @@ register struct attack *mattk;
             tmp = 0;
             break;
         }
-        if (vis)
+        if (vis && canseemon(mdef))
             pline("%s is %s!", Monnam(mdef), on_fire(pd, mattk));
         if (pd == &mons[PM_STRAW_GOLEM] || pd == &mons[PM_PAPER_GOLEM]) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline("%s burns completely!", Monnam(mdef));
             mondied(mdef);
             if (mdef->mhp > 0)
@@ -938,7 +944,7 @@ register struct attack *mattk;
         tmp += destroy_mitem(mdef, SCROLL_CLASS, AD_FIRE);
         tmp += destroy_mitem(mdef, SPBOOK_CLASS, AD_FIRE);
         if (resists_fire(mdef)) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline_The("fire doesn't seem to burn %s!", mon_nam(mdef));
             shieldeff(mdef->mx, mdef->my);
             golemeffects(mdef, AD_FIRE, tmp);
@@ -952,10 +958,10 @@ register struct attack *mattk;
             tmp = 0;
             break;
         }
-        if (vis)
+        if (vis && canseemon(mdef))
             pline("%s is covered in frost!", Monnam(mdef));
         if (resists_cold(mdef)) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline_The("frost doesn't seem to chill %s!", mon_nam(mdef));
             shieldeff(mdef->mx, mdef->my);
             golemeffects(mdef, AD_COLD, tmp);
@@ -968,11 +974,11 @@ register struct attack *mattk;
             tmp = 0;
             break;
         }
-        if (vis)
+        if (vis && canseemon(mdef))
             pline("%s gets zapped!", Monnam(mdef));
         tmp += destroy_mitem(mdef, WAND_CLASS, AD_ELEC);
         if (resists_elec(mdef)) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline_The("zap doesn't shock %s!", mon_nam(mdef));
             shieldeff(mdef->mx, mdef->my);
             golemeffects(mdef, AD_ELEC, tmp);
@@ -987,11 +993,11 @@ register struct attack *mattk;
             break;
         }
         if (resists_acid(mdef)) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline("%s is covered in %s, but it seems harmless.",
                       Monnam(mdef), hliquid("acid"));
             tmp = 0;
-        } else if (vis) {
+        } else if (vis && canseemon(mdef)) {
             pline("%s is covered in %s!", Monnam(mdef), hliquid("acid"));
             pline("It burns %s!", mon_nam(mdef));
         }
@@ -1004,7 +1010,7 @@ register struct attack *mattk;
         if (magr->mcan)
             break;
         if (pd == &mons[PM_IRON_GOLEM]) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline("%s falls to pieces!", Monnam(mdef));
             mondied(mdef);
             if (mdef->mhp > 0)
@@ -1028,7 +1034,7 @@ register struct attack *mattk;
         if (magr->mcan)
             break;
         if (pd == &mons[PM_WOOD_GOLEM] || pd == &mons[PM_LEATHER_GOLEM]) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline("%s falls to pieces!", Monnam(mdef));
             mondied(mdef);
             if (mdef->mhp > 0)
@@ -1054,7 +1060,7 @@ register struct attack *mattk;
             break;
         }
         if (!resists_ston(mdef)) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline("%s turns to stone!", Monnam(mdef));
             monstone(mdef);
         post_stone:
@@ -1069,20 +1075,21 @@ register struct attack *mattk;
     case AD_TLPT:
         if (!cancelled && tmp < mdef->mhp && !tele_restrict(mdef)) {
             char mdef_Monnam[BUFSZ];
+            boolean wasseen = canspotmon(mdef);
             /* save the name before monster teleports, otherwise
                we'll get "it" in the suddenly disappears message */
-            if (vis)
+            if (vis && wasseen)
                 Strcpy(mdef_Monnam, Monnam(mdef));
             mdef->mstrategy &= ~STRAT_WAITFORU;
             (void) rloc(mdef, TRUE);
-            if (vis && !canspotmon(mdef) && mdef != u.usteed)
+            if (vis && wasseen && !canspotmon(mdef) && mdef != u.usteed)
                 pline("%s suddenly disappears!", mdef_Monnam);
         }
         break;
     case AD_SLEE:
         if (!cancelled && !mdef->msleeping
             && sleep_monst(mdef, rnd(10), -1)) {
-            if (vis) {
+            if (vis && canspotmon(mdef)) {
                 Strcpy(buf, Monnam(mdef));
                 pline("%s is put to sleep by %s.", buf, mon_nam(magr));
             }
@@ -1092,7 +1099,7 @@ register struct attack *mattk;
         break;
     case AD_PLYS:
         if (!cancelled && mdef->mcanmove) {
-            if (vis) {
+            if (vis && canspotmon(mdef)) {
                 Strcpy(buf, Monnam(mdef));
                 pline("%s is frozen by %s.", buf, mon_nam(magr));
             }
@@ -1105,7 +1112,7 @@ register struct attack *mattk;
 
             mon_adjust_speed(mdef, -1, (struct obj *) 0);
             mdef->mstrategy &= ~STRAT_WAITFORU;
-            if (mdef->mspeed != oldspeed && vis)
+            if (mdef->mspeed != oldspeed && vis && canspotmon(mdef))
                 pline("%s slows down.", Monnam(mdef));
         }
         break;
@@ -1115,7 +1122,7 @@ register struct attack *mattk;
          * we still should check for it).
          */
         if (!magr->mcan && !mdef->mconf && !magr->mspec_used) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline("%s looks confused.", Monnam(mdef));
             mdef->mconf = 1;
             mdef->mstrategy &= ~STRAT_WAITFORU;
@@ -1125,7 +1132,7 @@ register struct attack *mattk;
         if (can_blnd(magr, mdef, mattk->aatyp, (struct obj *) 0)) {
             register unsigned rnd_tmp;
 
-            if (vis && mdef->mcansee)
+            if (vis && mdef->mcansee && canspotmon(mdef))
                 pline("%s is blinded.", Monnam(mdef));
             rnd_tmp = d((int) mattk->damn, (int) mattk->damd);
             if ((rnd_tmp += mdef->mblinded) > 127)
@@ -1138,7 +1145,7 @@ register struct attack *mattk;
         break;
     case AD_HALU:
         if (!magr->mcan && haseyes(pd) && mdef->mcansee) {
-            if (vis)
+            if (vis && canseemon(mdef))
                 pline("%s looks %sconfused.", Monnam(mdef),
                       mdef->mconf ? "more " : "");
             mdef->mconf = 1;
@@ -1155,7 +1162,7 @@ register struct attack *mattk;
             if (is_were(pd) && pd->mlet != S_HUMAN)
                 were_change(mdef);
             if (pd == &mons[PM_CLAY_GOLEM]) {
-                if (vis) {
+                if (vis && canseemon(mdef)) {
                     pline("Some writing vanishes from %s head!",
                           s_suffix(mon_nam(mdef)));
                     pline("%s is destroyed!", Monnam(mdef));
@@ -1171,7 +1178,7 @@ register struct attack *mattk;
             if (!Deaf) {
                 if (!vis)
                     You_hear("laughter.");
-                else
+                else if (canseemon(magr))
                     pline("%s chuckles.", Monnam(magr));
             }
         }
@@ -1192,20 +1199,21 @@ register struct attack *mattk;
             add_to_minv(magr, gold);
         }
         mdef->mstrategy &= ~STRAT_WAITFORU;
-        if (vis) {
+        if (vis && canseemon(mdef)) {
             Strcpy(buf, Monnam(magr));
             pline("%s steals some gold from %s.", buf, mon_nam(mdef));
         }
         if (!tele_restrict(magr)) {
+            boolean couldspot = canspotmon(magr);
             (void) rloc(magr, TRUE);
-            if (vis && !canspotmon(magr))
+            if (vis && couldspot && !canspotmon(magr))
                 pline("%s suddenly disappears!", buf);
         }
         break;
     case AD_DRLI:
         if (!cancelled && !rn2(3) && !resists_drli(mdef)) {
             tmp = d(2, 6);
-            if (vis)
+            if (vis && canspotmon(mdef))
                 pline("%s suddenly seems weaker!", Monnam(mdef));
             mdef->mhpmax -= tmp;
             if (mdef->m_lev == 0)
@@ -1249,7 +1257,7 @@ register struct attack *mattk;
             if (vis)
                 Strcpy(onambuf, doname(otmp));
             (void) add_to_minv(magr, otmp);
-            if (vis) {
+            if (vis && canseemon(mdef)) {
                 Strcpy(buf, Monnam(magr));
                 pline("%s steals %s from %s!", buf, onambuf, mdefnambuf);
             }
@@ -1260,8 +1268,9 @@ register struct attack *mattk;
                 return (MM_DEF_DIED
                         | (grow_up(magr, mdef) ? 0 : MM_AGR_DIED));
             if (pa->mlet == S_NYMPH && !tele_restrict(magr)) {
+                boolean couldspot = canspotmon(magr);
                 (void) rloc(magr, TRUE);
-                if (vis && !canspotmon(magr))
+                if (vis && couldspot && !canspotmon(magr))
                     pline("%s suddenly disappears!", buf);
             }
         }
@@ -1269,25 +1278,25 @@ register struct attack *mattk;
         break;
     case AD_DREN:
         if (!cancelled && !rn2(4))
-            xdrainenergym(mdef, vis && mattk->aatyp != AT_ENGL);
+            xdrainenergym(mdef, vis && canspotmon(mdef) && mattk->aatyp != AT_ENGL);
         tmp = 0;
         break;
     case AD_DRST:
     case AD_DRDX:
     case AD_DRCO:
         if (!cancelled && !rn2(8)) {
-            if (vis)
+            if (vis && canspotmon(magr))
                 pline("%s %s was poisoned!", s_suffix(Monnam(magr)),
                       mpoisons_subj(magr, mattk));
             if (resists_poison(mdef)) {
-                if (vis)
+                if (vis && canspotmon(mdef) && canspotmon(magr))
                     pline_The("poison doesn't seem to affect %s.",
                               mon_nam(mdef));
             } else {
                 if (rn2(10))
                     tmp += rn1(10, 6);
                 else {
-                    if (vis)
+                    if (vis && canspotmon(mdef))
                         pline_The("poison was deadly...");
                     tmp = mdef->mhp;
                 }
@@ -1296,14 +1305,14 @@ register struct attack *mattk;
         break;
     case AD_DRIN:
         if (notonhead || !has_head(pd)) {
-            if (vis)
+            if (vis && canspotmon(mdef))
                 pline("%s doesn't seem harmed.", Monnam(mdef));
             /* Not clear what to do for green slimes */
             tmp = 0;
             break;
         }
         if ((mdef->misc_worn_check & W_ARMH) && rn2(8)) {
-            if (vis) {
+            if (vis && canspotmon(magr) && canseemon(mdef)) {
                 Strcpy(buf, s_suffix(Monnam(mdef)));
                 pline("%s helmet blocks %s attack to %s head.", buf,
                       s_suffix(mon_nam(magr)), mhis(mdef));
@@ -1317,7 +1326,7 @@ register struct attack *mattk;
             break; /* physical damage only */
         if (!rn2(4) && !slimeproof(pd)) {
             if (!munslime(mdef, FALSE) && mdef->mhp > 0) {
-                if (newcham(mdef, &mons[PM_GREEN_SLIME], FALSE, vis))
+                if (newcham(mdef, &mons[PM_GREEN_SLIME], FALSE, vis && canseemon(mdef)))
                     pd = mdef->data;
                 mdef->mstrategy &= ~STRAT_WAITFORU;
                 res = MM_HIT;