Disallow syntax rules that match the empty string (for now)

src/compiler/build_tables/build_parse_table.cc

@@ -177,7 +177,7 @@ class ParseTableBuilder {
             parse_table.add_terminal_action(state_id, lookahead, action);
           } else {
             ParseAction &existing_action = entry.actions[0];
-            if (allow_any_conflict) {
+            if (existing_action.type == ParseActionTypeAccept || allow_any_conflict) {
               entry.actions.push_back(action);
             } else {
               int existing_precedence = existing_action.precedence();

src/compiler/prepare_grammar/flatten_grammar.cc

@@ -13,6 +13,7 @@ namespace tree_sitter {
 namespace prepare_grammar {
 
 using std::find;
+using std::pair;
 using std::vector;
 
 class FlattenRule : public rules::RuleFn<void> {
@@ -87,16 +88,35 @@ SyntaxVariable flatten_rule(const Variable &variable) {
   return SyntaxVariable(variable.name, variable.type, productions);
 }
 
-SyntaxGrammar flatten_grammar(const InitialSyntaxGrammar &grammar) {
+pair<SyntaxGrammar, CompileError> flatten_grammar(const InitialSyntaxGrammar &grammar) {
   SyntaxGrammar result;
   result.expected_conflicts = grammar.expected_conflicts;
   result.extra_tokens = grammar.extra_tokens;
 
+  bool is_start = true;
   for (const Variable &variable : grammar.variables) {
-    result.variables.push_back(flatten_rule(variable));
+    SyntaxVariable syntax_variable = flatten_rule(variable);
+
+    if (!is_start) {
+      for (const Production &production : syntax_variable.productions) {
+        if (production.empty()) {
+          return {
+            result,
+            CompileError(
+              TSCompileErrorTypeEpsilonRule,
+              "The rule `" + variable.name + "` matches the empty string.\n" +
+              "Tree-sitter currently does not support syntactic rules that match the empty string.\n"
+            )
+          };
+        }
+      }
+    }
+
+    result.variables.push_back(syntax_variable);
+    is_start = false;
   }
 
-  return result;
+  return {result, CompileError::none()};
 }
 
 }  // namespace prepare_grammar

src/compiler/prepare_grammar/flatten_grammar.h

@@ -3,6 +3,7 @@
 
 #include <string>
 #include "tree_sitter/compiler.h"
+#include "compiler/compile_error.h"
 #include "compiler/syntax_grammar.h"
 
 namespace tree_sitter {
@@ -11,7 +12,7 @@ namespace prepare_grammar {
 struct InitialSyntaxGrammar;
 
 SyntaxVariable flatten_rule(const Variable &variable);
-SyntaxGrammar flatten_grammar(const InitialSyntaxGrammar &);
+std::pair<SyntaxGrammar, CompileError> flatten_grammar(const InitialSyntaxGrammar &);
 
 }  // namespace prepare_grammar
 }  // namespace tree_sitter

src/compiler/prepare_grammar/prepare_grammar.cc

@@ -51,7 +51,11 @@ tuple<SyntaxGrammar, LexicalGrammar, CompileError> prepare_grammar(
   /*
    * Flatten syntax rules into lists of productions.
    */
-  SyntaxGrammar syntax_grammar = flatten_grammar(syntax_grammar1);
+  auto flatten_result = flatten_grammar(syntax_grammar1);
+  SyntaxGrammar syntax_grammar = flatten_result.first;
+  error = flatten_result.second;
+  if (error.type)
+    return make_tuple(SyntaxGrammar(), LexicalGrammar(), error);
 
   /*
    * Ensure all lexical rules are in a consistent format.