SSLR-395 Improve error message when one method in grammar is private

sslr-core/src/main/java/com/sonar/sslr/impl/typed/GrammarBuilderInterceptor.java

@@ -87,7 +87,14 @@ public <U> NonterminalBuilder<U> nonterminal(GrammarRuleKey ruleKey) {
   @Override
   public Object is(Object method) {
     if (expressionStack.size() != 1) {
-      throw new IllegalStateException("Unexpected stack size: " + expressionStack.size());
+      StringBuilder errorMessage = new StringBuilder("Unable to find the method for rule: \"" + ruleKey + "\" in Grammar. " +
+        "Please check if the method is defined as public.\nRecognized methods: [");
+      for (Method methodKey : methodToRuleKey.keySet()) {
+        errorMessage.append("\n\t");
+        errorMessage.append(methodKey.getName());
+      }
+      errorMessage.append("\n]");
+      throw new IllegalStateException(errorMessage.toString());
     }
 
     ParsingExpression expression = pop();

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

@@ -0,0 +1,119 @@
+/*
+ * 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.lang.reflect.InvocationTargetException;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+import org.junit.Test;
+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 method in the grammar is private.
+ * It happens very often when implementing grammar top down and when IDE generates a method with private scope.
+ */
+public class ActionParserPrivateMethodTest {
+
+  @Test
+  public void test() {
+    try {
+      parser(MyLexicalGrammar.B).parse("AAA");
+    } catch (Exception e) {
+      assertThat(((InvocationTargetException)e.getCause()).getTargetException().getMessage())
+        .startsWith("Unable to find the method for rule: \"C\" in Grammar. Please check if the method is defined as public.");
+    }
+  }
+
+  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(A).is(b.regexp("A"));
+      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 B() {
+      return b.<AstNode>nonterminal(MyLexicalGrammar.B).is(
+        f.b(
+          b.token(MyLexicalGrammar.A),
+          b.oneOrMore(C())
+        )
+      );
+    }
+
+    /**
+     * This method is private on purpose, to reproduce the issue
+     */
+    private AstNode C() {
+      return b.<AstNode>nonterminal(MyLexicalGrammar.C).is(
+        f.c(
+          b.token(MyLexicalGrammar.A)
+        )
+      );
+    }
+  }
+
+  public static class MyTreeFactory {
+
+    public AstNode b(AstNode token, List<AstNode> tokens) {
+      return new BNode(token, tokens);
+    }
+
+    public AstNode c(AstNode token) {
+      return new CNode(token);
+    }
+  }
+
+  public static class BNode extends AstNode {
+
+    public BNode(AstNode token, List<AstNode> tokens) {
+      super(token.getToken());
+    }
+  }
+
+  public static class CNode extends AstNode {
+
+    public CNode(AstNode tokens) {
+      super(tokens.getToken());
+    }
+  }
+}