test(engine): cover remaining directives.go branches and remove dead code

internal/engine/directives.go

@@ -203,16 +203,14 @@ func configKeyForType(mt mutator.Type) string {
 
 // collectTokenLines returns the set of source lines that contain at least
 // one non-comment AST node, used to distinguish end-of-line directives from
-// own-line ones.
+// own-line ones. Returning false on *ast.CommentGroup stops the walk before
+// it reaches the *ast.Comment children, so no Comment guard is needed.
 func collectTokenLines(set *token.FileSet, file *ast.File) map[int]bool {
 	lines := map[int]bool{}
 	ast.Inspect(file, func(n ast.Node) bool {
 		if n == nil {
 			return false
 		}
-		if _, isComment := n.(*ast.Comment); isComment {
-			return false
-		}
 		if _, isCG := n.(*ast.CommentGroup); isCG {
 			return false
 		}
@@ -264,9 +262,6 @@ func largestNodeStartingAtLine(set *token.FileSet, file *ast.File, line int) (as
 		if _, isCG := n.(*ast.CommentGroup); isCG {
 			return false
 		}
-		if _, isC := n.(*ast.Comment); isC {
-			return false
-		}
 		if set.Position(n.Pos()).Line != line {
 			return true
 		}

internal/engine/directives_internal_test.go

@@ -245,6 +245,46 @@ func F() int {
 	}
 }
 
+func TestDirectiveIndex_NilReceiverIsSafe(t *testing.T) {
+	t.Parallel()
+
+	// A nil index must answer "not suppressed" without panicking. The
+	// engine relies on this so it can call isSuppressed unconditionally
+	// even when no index has been built (currently never happens, but
+	// the guard is cheap and protects future call sites).
+	var idx *directiveIndex
+	if idx.isSuppressed(token.Position{Line: 1}, mutator.ArithmeticBase) {
+		t.Errorf("nil directiveIndex must return false from isSuppressed")
+	}
+}
+
+func TestBuildDirectiveIndex_TypedFilterWithEmptyEntries(t *testing.T) {
+	t.Parallel()
+
+	// Doubled commas / leading commas leave empty strings in the comma-
+	// split type list. Each empty entry should be skipped silently and
+	// the surrounding valid types still parsed.
+	src := `package p
+
+func F() int {
+	return 1 + 2 //nomutant:arithmetic-base,,invert-bitwise
+}
+`
+	set, file := parseSrc(t, src)
+	idx := buildDirectiveIndex(set, file)
+
+	pos := positionOf(t, set, src, "+")
+	if !idx.isSuppressed(pos, mutator.ArithmeticBase) {
+		t.Errorf("expected ArithmeticBase to be suppressed (valid type before doubled comma)")
+	}
+	if !idx.isSuppressed(pos, mutator.InvertBitwise) {
+		t.Errorf("expected InvertBitwise to be suppressed (valid type after doubled comma)")
+	}
+	if idx.isSuppressed(pos, mutator.ConditionalsBoundary) {
+		t.Errorf("did NOT expect ConditionalsBoundary to be suppressed (not in filter)")
+	}
+}
+
 func TestBuildDirectiveIndex_NestedBlocks_Additive(t *testing.T) {
 	t.Parallel()