SSLR-396 Improve error message when rule is not defined

sslr-core/src/main/java/org/sonar/sslr/internal/vm/MutableGrammarCompiler.java

@@ -19,6 +19,7 @@
  */
 package org.sonar.sslr.internal.vm;
 
+import org.sonar.sslr.grammar.GrammarException;
 import org.sonar.sslr.grammar.GrammarRuleKey;
 
 import java.util.ArrayDeque;
@@ -50,6 +51,10 @@ private CompiledGrammar doCompile(CompilableGrammarRule start) {
       CompilableGrammarRule rule = compilationQueue.poll();
       GrammarRuleKey ruleKey = rule.getRuleKey();
 
+      if(rule.getExpression() == null) {
+        throw new GrammarException("The expression for rule \"" + ruleKey + "\" is undefined.");
+      }
+
       offsets.put(ruleKey, instructions.size());
       Instruction.addAll(instructions, compile(rule.getExpression()));
       instructions.add(Instruction.ret());

sslr-core/src/test/java/com/sonar/sslr/api/typed/ActionParserRuleNotDefinedTest.java

@@ -0,0 +1,108 @@
+/*
+ * SonarSource Language Recognizer
+ * Copyright (C) 2010-2023 SonarSource SA
+ * mailto:info AT sonarsource DOT com
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 3 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ */
+package com.sonar.sslr.api.typed;
+
+import com.sonar.sslr.api.AstNode;
+import com.sonar.sslr.api.GenericTokenType;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+import org.junit.Assert;
+import org.junit.Test;
+import org.sonar.sslr.grammar.GrammarException;
+import org.sonar.sslr.grammar.GrammarRuleKey;
+import org.sonar.sslr.grammar.LexerlessGrammarBuilder;
+
+import static org.fest.assertions.Assertions.assertThat;
+
+
+/**
+ * This test reproduces a common issue, when one of the rule is not defined.
+ */
+public class ActionParserRuleNotDefinedTest {
+
+  @Test
+  public void test() {
+    GrammarException exception = Assert.assertThrows(GrammarException.class, () -> parser(MyLexicalGrammar.A));
+    assertThat(exception)
+      .hasMessage("The expression for rule \"C\" is undefined.");
+  }
+
+  private ActionParser<AstNode> parser(GrammarRuleKey ruleKey) {
+    return new ActionParser<>(StandardCharsets.UTF_8, MyLexicalGrammar.builder(), MyGrammar.class, new MyTreeFactory(), new AstNodeBuilder(), ruleKey);
+  }
+
+  public enum MyLexicalGrammar implements GrammarRuleKey {
+    A, B, C, EOF;
+
+    public static LexerlessGrammarBuilder builder() {
+      LexerlessGrammarBuilder b = LexerlessGrammarBuilder.create();
+      b.rule(B).is("B").skip();
+      // The rule C is not defined. Uncommenting next line fixes the issue.
+//      b.rule(C).is("C").skip();
+      b.rule(EOF).is(b.token(GenericTokenType.EOF, b.endOfInput())).skip();
+      return b;
+    }
+  }
+
+  public static class MyGrammar {
+    private final GrammarBuilder<AstNode> b;
+    private final MyTreeFactory f;
+
+    public MyGrammar(GrammarBuilder<AstNode> b, MyTreeFactory f) {
+      this.b = b;
+      this.f = f;
+    }
+
+    public AstNode A() {
+      return b.<AstNode>nonterminal(MyLexicalGrammar.A).is(
+        f.a(
+          b.token(MyLexicalGrammar.B),
+          b.oneOrMore(b.token(MyLexicalGrammar.C))
+        )
+      );
+    }
+
+  }
+
+  public static class MyTreeFactory {
+
+    public AstNode a(AstNode token, List<AstNode> tokens) {
+      return new ANode(token, tokens);
+    }
+
+    public AstNode b(AstNode token) {
+      return new BNode(token);
+    }
+  }
+
+  public static class ANode extends AstNode {
+
+    public ANode(AstNode token, List<AstNode> tokens) {
+      super(token.getToken());
+    }
+  }
+
+  public static class BNode extends AstNode {
+
+    public BNode(AstNode tokens) {
+      super(tokens.getToken());
+    }
+  }
+}