Correctly handle and report unterminated multiline comments in Lexer

core/src/main/java/it/cavallium/warppi/math/rules/dsl/frontend/Lexer.java

@@ -139,8 +139,11 @@ public class Lexer {
 		matchWhile(c -> c != '\n');
 	}
 
-	private void multiLineComment() {
+	private void multiLineComment() throws SyntaxException {
 		while (!(matchChar('*') && matchChar('/'))) {
+			if (atEnd()) {
+				throw new SyntaxException(new UnterminatedComment(startOfLexeme));
+			}
 			popChar();
 		}
 	}

core/src/main/java/it/cavallium/warppi/math/rules/dsl/frontend/UnterminatedComment.java

@@ -0,0 +1,40 @@
+package it.cavallium.warppi.math.rules.dsl.frontend;
+
+import it.cavallium.warppi.math.rules.dsl.DslError;
+
+import java.util.Objects;
+
+/**
+ * Occurs when DSL source code contains a multiline comment which is never terminated (closed).
+ */
+public class UnterminatedComment implements DslError {
+	private final int position;
+
+	public UnterminatedComment(final int position) {
+		this.position = position;
+	}
+
+	@Override
+	public int getPosition() {
+		return position;
+	}
+
+	@Override
+	public int getLength() {
+		return 2; // Length of comment start marker: "/*"
+	}
+
+	@Override
+	public boolean equals(final Object o) {
+		if (!(o instanceof UnterminatedComment)) {
+			return false;
+		}
+		final UnterminatedComment other = (UnterminatedComment) o;
+		return this.position == other.position;
+	}
+
+	@Override
+	public int hashCode() {
+		return Objects.hash(position);
+	}
+}

core/src/test/java/it/cavallium/warppi/math/rules/dsl/frontend/LexerTest.java

@@ -127,6 +127,22 @@ public class LexerTest {
 		assertEquals(expectedErrors, errors);
 	}
 
+	@Test
+	public void unterminatedComment() {
+		final Lexer lexer = new Lexer("reduction /* test:\n x -> x", errors::add);
+
+		final List<Token> expectedTokens = Arrays.asList(
+				new Token(REDUCTION, "reduction", 0),
+				new Token(EOF, "", 26)
+		);
+		assertEquals(expectedTokens, lexer.lex());
+
+		final List<DslError> expectedErrors = Collections.singletonList(
+				new UnterminatedComment(10)
+		);
+		assertEquals(expectedErrors, errors);
+	}
+
 	@Test
 	public void errorOrder() {
 		final Lexer lexer = new Lexer(".2. @", errors::add);