Add compiler warning 158: FRAGMENT_ACTION_IGNORED (fixes #472)

tool/src/org/antlr/v4/semantics/BasicSemanticChecks.java

@@ -116,6 +116,13 @@ public class BasicSemanticChecks extends GrammarTreeVisitor {
 	 */
 	protected int nonFragmentRuleCount;
 
+	/**
+	 * This is {@code true} from the time {@link #discoverLexerRule} is called
+	 * for a lexer rule with the {@code fragment} modifier until
+	 * {@link #exitLexerRule} is called.
+	 */
+	private boolean inFragmentRule;
+
 	public BasicSemanticChecks(Grammar g, RuleCollector ruleCollector) {
 		this.g = g;
 		this.ruleCollector = ruleCollector;
@@ -200,20 +207,24 @@ public class BasicSemanticChecks extends GrammarTreeVisitor {
 	{
 		checkInvalidRuleDef(ID.token);
 
-		boolean fragmentRule = false;
 		if (modifiers != null) {
 			for (GrammarAST tree : modifiers) {
 				if (tree.getType() == ANTLRParser.FRAGMENT) {
-					fragmentRule = true;
+					inFragmentRule = true;
 				}
 			}
 		}
 
-		if (!fragmentRule) {
+		if (!inFragmentRule) {
 			nonFragmentRuleCount++;
 		}
 	}
 
+	@Override
+	protected void exitLexerRule(GrammarAST tree) {
+		inFragmentRule = false;
+	}
+
 	@Override
 	public void ruleRef(GrammarAST ref, ActionAST arg) {
 		checkInvalidRuleRef(ref.token);
@@ -381,6 +392,21 @@ public class BasicSemanticChecks extends GrammarTreeVisitor {
 	@Override
 	protected void enterLexerCommand(GrammarAST tree) {
 		checkElementIsOuterMostInSingleAlt(tree);
+
+		if (inFragmentRule) {
+			String fileName = tree.token.getInputStream().getSourceName();
+			String ruleName = currentRuleName;
+			g.tool.errMgr.grammarError(ErrorType.FRAGMENT_ACTION_IGNORED, fileName, tree.token, ruleName);
+		}
+	}
+
+	@Override
+	public void actionInAlt(ActionAST action) {
+		if (inFragmentRule) {
+			String fileName = action.token.getInputStream().getSourceName();
+			String ruleName = currentRuleName;
+			g.tool.errMgr.grammarError(ErrorType.FRAGMENT_ACTION_IGNORED, fileName, action.token, ruleName);
+		}
 	}
 
 	/**

tool/src/org/antlr/v4/tool/ErrorType.java

@@ -852,6 +852,37 @@ public enum ErrorType {
 	 * @since 4.2.1
 	 */
 	UNRECOGNIZED_ASSOC_OPTION(157, "rule '<arg>' contains an 'assoc' terminal option in an unrecognized location", ErrorSeverity.WARNING),
+	/**
+	 * Compiler Warning 158.
+	 *
+	 * <p>fragment rule '<em>rule</em>' contains an action or command which can
+	 * never be executed</p>
+	 *
+	 * <p>A lexer rule which is marked with the {@code fragment} modifier
+	 * contains an embedded action or lexer command. ANTLR lexers only execute
+	 * commands and embedded actions located in the top-level matched rule.
+	 * Since fragment rules can never be the top-level rule matched by a lexer,
+	 * actions or commands placed in these rules can never be executed during
+	 * the lexing process.</p>
+	 *
+	 * <p>The following rule produces this warning.</p>
+	 *
+	 * <pre>
+	 * X1 : 'x' -> more    // ok
+	 *    ;
+	 * Y1 : 'x' {more();}  // ok
+	 *    ;
+	 * fragment
+	 * X2 : 'x' -> more    // warning 158
+	 *    ;
+	 * fragment
+	 * Y2 : 'x' {more();}  // warning 158
+	 *    ;
+	 * </pre>
+	 *
+	 * @since 4.2.1
+	 */
+	FRAGMENT_ACTION_IGNORED(158, "fragment rule '<arg>' contains an action or command which can never be executed", ErrorSeverity.WARNING),
 
 	/*
 	 * Backward incompatibility errors

tool/test/org/antlr/v4/test/TestToolSyntaxErrors.java

@@ -473,4 +473,33 @@ public class TestToolSyntaxErrors extends BaseTest {
 
 		super.testErrors(pair, true);
 	}
+
+	/**
+	 * This test ensures the {@link ErrorType#FRAGMENT_ACTION_IGNORED} warning
+	 * is produced as described in the documentation.
+	 */
+	@Test public void testFragmentActionIgnored() {
+		String grammar =
+			"lexer grammar A;\n" +
+			"X1 : 'x' -> more    // ok\n" +
+			"   ;\n" +
+			"Y1 : 'x' {more();}  // ok\n" +
+			"   ;\n" +
+			"fragment\n" +
+			"X2 : 'x' -> more    // warning 158\n" +
+			"   ;\n" +
+			"fragment\n" +
+			"Y2 : 'x' {more();}  // warning 158\n" +
+			"   ;\n";
+		String expected =
+			"warning(" + ErrorType.FRAGMENT_ACTION_IGNORED.code + "): A.g4:7:12: fragment rule 'X2' contains an action or command which can never be executed\n" +
+			"warning(" + ErrorType.FRAGMENT_ACTION_IGNORED.code + "): A.g4:10:9: fragment rule 'Y2' contains an action or command which can never be executed\n";
+
+		String[] pair = new String[] {
+			grammar,
+			expected
+		};
+
+		super.testErrors(pair, true);
+	}
 }