gcc: Use __builtin_unreachable where appropriate.

There are several instances of code, mostly functions
with a switch statement that either have a default or
enumerate all cases explicitly with no default, where
NULL (or some useless value) is returned to satisfy
the function's return value. However, this is dead
code; bbs_assert(0) was frequently used to indicate this.
Instead, use __builtin_unreachable, which is more
semantic, and possibly more performant.

bbs/logger.c

@@ -273,8 +273,7 @@ static const char *loglevel2str(enum bbs_log_level level, int term)
 		case LOG_DEBUG:
 			return COLOR_LOG_OR("  DEBUG", COLOR_GREEN, term);
 	}
-	bbs_assert(0);
-	return NULL;
+	__builtin_unreachable();
 }
 
 static const char *verbose_prefix(int level)
@@ -301,7 +300,7 @@ static const char *verbose_prefix(int level)
 		default:
 			bbs_assert(0);
 	}
-	return NULL;
+	__builtin_unreachable();
 }
 
 #define log_puts(msg) fprintf(logfp, "%s", msg); fflush(logfp);

modules/mod_discord.c

@@ -231,8 +231,7 @@ static const char *status_str(enum user_status status)
 		case STATUS_NONE:
 			return "none";
 	}
-	bbs_assert(0);
-	return NULL;
+	__builtin_unreachable();
 }
 
 static void remove_user(struct discord_user *user)

modules/mod_irc_relay.c

@@ -955,7 +955,7 @@ static void command_cb(const char *clientname, enum irc_callback_msg_type type,
 			}
 			break;
 		case CMD_QUIT: /* This is academic, as QUIT is handled above */
-			bbs_assert(0);
+			__builtin_unreachable();
 #if 0
 			ourchan = MAP1_MATCH(cp, clientname, channel) ? cp->channel2 : cp->channel1;
 			snprintf(sysmsg, sizeof(sysmsg), ":%s/%s QUIT :%s", clientname, prefix, S_IF(msg));

modules/mod_mail.c

@@ -210,8 +210,7 @@ const char *mailbox_event_type_name(enum mailbox_event_type type)
 			return "UIDVALIDITYChange";
 		/* No default case */
 	}
-	bbs_assert(0);
-	return NULL;
+	__builtin_unreachable();
 }
 
 static pthread_mutex_t eventidlock = PTHREAD_MUTEX_INITIALIZER;

nets/net_smtp.c

@@ -621,7 +621,7 @@ static int handle_auth(struct smtp_session *smtp, char *s)
 		free(pass);
 		goto logindone;
 	} else {
-		bbs_assert(0);
+		__builtin_unreachable();
 	}
 	return 0;
 
@@ -1005,8 +1005,7 @@ static const char *smtp_filter_type_name(enum smtp_filter_type type)
 		case SMTP_FILTER_PREPEND: return "PREPEND";
 		/* No default */
 	}
-	bbs_assert(0);
-	return NULL;
+	__builtin_unreachable();
 }
 
 int __smtp_filter_register(struct smtp_filter_provider *provider, enum smtp_filter_type type, enum smtp_filter_scope scope, enum smtp_direction dir, int priority, void *mod)
@@ -1417,8 +1416,7 @@ static const char *delivery_action_name(enum smtp_delivery_action action)
 		case DELIVERY_EXPANDED: return "expanded";
 		/* No default */
 	}
-	bbs_assert(0);
-	return NULL;
+	__builtin_unreachable();
 }
 
 static const char *delivery_subject_name(struct smtp_delivery_outcome **f, int n)
@@ -1436,8 +1434,7 @@ static const char *delivery_subject_name(struct smtp_delivery_outcome **f, int n
 		case DELIVERY_EXPANDED: return "Delivery Status Notification (Expanded)";
 		/* No default */
 	}
-	bbs_assert(0);
-	return NULL;
+	__builtin_unreachable();
 }
 
 /* Forward declaration */

tests/test.c

@@ -71,8 +71,7 @@ static const char *loglevel2str(enum bbs_log_level level)
 		default:
 			break;
 	}
-	assert(0);
-	return NULL;
+	__builtin_unreachable();
 }
 
 /*! \brief Minimal implementation of __bbs_log, so we can use the same interface */