* added check for v3 backward incompatibilities:

** labels in lexer rules

CHANGES.txt

@@ -7,6 +7,7 @@ November 18, 2012
 * added notion of one-off error
 * added check for v3 backward incompatibilities:
 ** tree grammars
+** labels in lexer rules
 
 November 17, 2012
 

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

@@ -382,6 +382,15 @@ public class BasicSemanticChecks extends GrammarTreeVisitor {
 		}
 	}
 
+	@Override
+	protected void enterLabeledLexerElement(GrammarAST tree) {
+		Token label = ((GrammarAST)tree.getChild(0)).getToken();
+		g.tool.errMgr.grammarError(ErrorType.V3_LEXER_LABEL,
+								   g.fileName,
+								   label,
+								   label.getText());
+	}
+
 	/** Check option is appropriate for grammar, rule, subrule */
 	boolean checkOptions(GrammarAST parent,
 						 Token optionID,

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

@@ -118,6 +118,9 @@ public enum ErrorType {
 
 	// Backward incompatibility errors
 	V3_TREE_GRAMMAR(200, "tree grammars are not supported in ANTLR v4", ErrorSeverity.ERROR_ONE_OFF),
+	V3_LEXER_LABEL(201, "labels in lexer rules are not supported in ANTLR v4; " +
+		"actions cannot reference elements of lexical rules but you can use " +
+		"getText() to get the entire text matched for the rule", ErrorSeverity.ERROR_ONE_OFF),
 
     // Dependency sorting errors