feat: add enter and leave checks for traversers

- Improved performance (can short circuit/not continue when done)
- Backwards compatible because added interfaces
- Now generating the implementation for easier updates

pkg/visitor/traverser/traverser_test.go

@@ -0,0 +1,67 @@
+package traverser_test
+
+import (
+	"testing"
+
+	"github.com/VKCOM/php-parser/pkg/ast"
+	"github.com/VKCOM/php-parser/pkg/visitor"
+	"github.com/VKCOM/php-parser/pkg/visitor/traverser"
+)
+
+// testCase is a traverser that does not go into any class statement or its children.
+// If it does, the test fails.
+type testCase struct {
+	t *testing.T
+	visitor.Null
+	traversedFunction bool
+}
+
+var _ ast.Visitor = &testCase{}
+
+func (t *testCase) EnterNode(n ast.Vertex) bool {
+	t.t.Logf("EnterNode: %T", n)
+	if _, ok := n.(*ast.StmtClass); ok {
+		return false
+	}
+
+	return true
+}
+
+func (t *testCase) LeaveNode(n ast.Vertex) {
+	t.t.Logf("LeaveNode: %T", n)
+	if _, ok := n.(*ast.Root); ok {
+		if !t.traversedFunction {
+			t.t.Error("traverser did not traverse function")
+		}
+	}
+}
+
+func (t *testCase) StmtClass(n *ast.StmtClass) {
+	t.t.Errorf("traverser got to class")
+}
+
+func (t *testCase) StmtClassMethod(n *ast.StmtClassMethod) {
+	t.t.Errorf("traverser got to method")
+}
+
+func (t *testCase) StmtFunction(n *ast.StmtFunction) {
+	t.traversedFunction = true
+}
+
+func TestEnterNodeIsRespected(t *testing.T) {
+	tc := &testCase{t: t}
+	tv := traverser.NewTraverser(tc)
+
+	root := &ast.Root{
+		Stmts: []ast.Vertex{
+			&ast.StmtFunction{},
+			&ast.StmtClass{
+				Stmts: []ast.Vertex{
+					&ast.StmtClassMethod{},
+				},
+			},
+		},
+	}
+
+	root.Accept(tv)
+}