Don't fail when zero condition <> has no end-of-inpt rule $.

Previously re2c failed with an error in the case when end-of-input rule `$` was
enabled and the special zero condition `<>` was used. Now it no longer fails and
generates code withot end-of-input check for the zero condition (which is
handled in a special way by the code, bypassing some of the transformations that
rely on the presence of end-of-input rule).

src/adfa/adfa.cc

@@ -320,7 +320,7 @@ void Adfa::prepare(const opt_t* opts) {
         }
 
         // last state, add end-of-input rule $ if needed (see note [end-of-input rule])
-        if (!s->next && opts->fill_eof != NOEOF) {
+        if (!s->next && opts->fill_eof != NOEOF && cond != "0") {
             eof_state = new State;
             eof_state->action.set_rule(eof_rule);
             finstates[eof_rule] = eof_state;

src/main.cc

@@ -55,7 +55,7 @@ LOCAL_NODISCARD(Ret ast_to_dfa(
     const loc_t& loc = block.loc;
     Msg& msg = output.msg;
     const std::vector<AstRule>& ast = gram.rules;
-    const std::string&cond = gram.name;
+    const std::string& cond = gram.name;
     const std::string name = make_name(output, cond, loc);
     const std::string& setup = gram.setup.empty() ? "" : gram.setup[0]->text;
 
@@ -87,7 +87,9 @@ LOCAL_NODISCARD(Ret ast_to_dfa(
         output.skeletons.insert(name);
     }
 
-    cutoff_dead_rules(dfa, opts, cond, msg);
+    if (cond != "0") {
+        cutoff_dead_rules(dfa, opts, cond, msg);
+    }
 
     insert_fallback_tags(dfa);
 

src/parse/ast.cc

@@ -391,12 +391,13 @@ Ret check_and_merge_special_rules(AstGrams& grams, const opt_t* opts, Msg& msg,
 
     // Check that `re2c:eof` configuration and the $ rule are used together. This must be done after
     // merging rules inherited from other blocks and <*> condition (because they might add $ rule).
+    // Skip "0" condition, as it's a special one that has no rules and always matches empty string.
     for (const AstGram& g : grams) {
         if (!g.eofs.empty() && opts->fill_eof == NOEOF) {
             RET_FAIL(msg.error(g.eofs[0]->loc,
                                "%s$ rule found, but `re2c:eof` configuration is not set",
                                incond(g.name).c_str()));
-        } else if (g.eofs.empty() && opts->fill_eof != NOEOF) {
+        } else if (g.eofs.empty() && opts->fill_eof != NOEOF && g.name != "0") {
             RET_FAIL(error("%s`re2c:eof` configuration is set, but no $ rule found",
                            incond(g.name).c_str()));
         }

test/conditions/condition_16.c

@@ -1 +1,57 @@
-re2c: error: in condition '0' `re2c:eof` configuration is set, but no $ rule found
+/* Generated by re2c */
+// re2c $INPUT -o $OUTPUT -ci
+
+{
+	YYCTYPE yych;
+	switch (YYGETCONDITION()) {
+		case yyc0: goto yyc_0;
+		case yycx: goto yyc_x;
+		case yycy: goto yyc_y;
+	}
+/* *********************************** */
+yyc_0:
+	{ return NULL; }
+/* *********************************** */
+yyc_x:
+yyFillLabel0:
+	yych = *YYCURSOR;
+	switch (yych) {
+		case 'a': goto yy3;
+		default:
+			if (YYLIMIT <= YYCURSOR) {
+				if (YYFILL() == 0) goto yyFillLabel0;
+				goto yy4;
+			}
+			goto yy2;
+	}
+yy2:
+	++YYCURSOR;
+	{ return "*"; }
+yy3:
+	++YYCURSOR;
+	{ return "a"; }
+yy4:
+	{ return "$"; }
+/* *********************************** */
+yyc_y:
+yyFillLabel1:
+	yych = *YYCURSOR;
+	switch (yych) {
+		case 'a': goto yy7;
+		default:
+			if (YYLIMIT <= YYCURSOR) {
+				if (YYFILL() == 0) goto yyFillLabel1;
+				goto yy8;
+			}
+			goto yy6;
+	}
+yy6:
+	++YYCURSOR;
+	{ return "*"; }
+yy7:
+	++YYCURSOR;
+	{ return "a"; }
+yy8:
+	{ return "$y"; }
+}
+