can copy field values between successive tests

this isn't going to work permanently, if there's a save field in one
test, then an intermediate test without that field, then a load field,
it won't be visible at the end test.

tea/tree.go

@@ -2,15 +2,15 @@ package tea
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 )
 
 // Run runs a tree of tests, starting from its root.
 func Run(t *testing.T, tree *Tree) {
 	t.Run(tree.name, func(t *testing.T) {
-		setup(t, tree)
-
-		clone(tree.test).Run(t)
+		test := setup(t, tree)
+		test.Run(t)
 
 		for _, child := range tree.children {
 			if t.Failed() || t.Skipped() {
@@ -22,11 +22,14 @@ func Run(t *testing.T, tree *Tree) {
 	})
 }
 
-func setup(t *testing.T, tree *Tree) {
+func setup(t *testing.T, tree *Tree) Test {
+	test := clone(tree.test)
 	if tree.parent != nil {
-		setup(t, tree.parent)
-		clone(tree.parent.test).Run(t)
+		p := setup(t, tree.parent)
+		p.Run(t)
+		test = merge(test, p)
 	}
+	return test
 }
 
 func skip(t *testing.T, tree *Tree) {
@@ -59,6 +62,8 @@ func (t *Tree) Child(test Test) *Tree {
 	return child
 }
 
+// clone clones a test value, yielding a new test value that can be executed
+// and mutated such that the original is not mutated.
 func clone(t Test) Test {
 	srcV := reflect.ValueOf(t).Elem()
 	destV := reflect.New(srcV.Type())
@@ -66,6 +71,53 @@ func clone(t Test) Test {
 	return destV.Interface().(Test)
 }
 
+// merge merges into dest the fields on the src test that are marked as worth
+// saving
+func merge(dest Test, src Test) Test {
+	destV := reflect.ValueOf(dest).Elem()
+	srcV := reflect.ValueOf(src).Elem()
+
+	for i := 0; i < srcV.NumField(); i++ {
+		sf := srcV.Type().Field(i)
+		if isSaveField(sf) {
+			df, ok := destV.Type().FieldByName(sf.Name)
+			if ok && isLoadField(df) {
+				if sf.Type == df.Type {
+					sfv := srcV.FieldByName(sf.Name)
+					dfv := destV.FieldByName(sf.Name)
+					dfv.Set(sfv)
+				}
+			}
+		}
+	}
+	return dest
+}
+
+// isSaveField takes a struct field and checks its tags for a save tag,
+// indicating that the field's value should persist between tests
+func isSaveField(f reflect.StructField) bool {
+	parts := strings.Split(f.Tag.Get("tea"), ",")
+	for _, part := range parts {
+		if part == "save" {
+			return true
+		}
+	}
+	return false
+}
+
+// isLoadField takes a struct field and checks its tags for a load tag,
+// indicating that the field's value should be populated by a saved value from
+// a prior test in the chain.
+func isLoadField(f reflect.StructField) bool {
+	parts := strings.Split(f.Tag.Get("tea"), ",")
+	for _, part := range parts {
+		if part == "load" {
+			return true
+		}
+	}
+	return false
+}
+
 func parseName(test Test) string {
 	if s, ok := test.(interface{ String() string }); ok {
 		return s.String()

tea_test.go

@@ -10,20 +10,21 @@ import (
 )
 
 type testThingSetup struct {
-	thing *Thing
+	Thing *Thing `tea:"save"`
 }
 
 func (test *testThingSetup) Run(t *testing.T) {
 	t.Logf("[%s] running testThingSetup", t.Name())
-	if test.thing != nil {
+	if test.Thing != nil {
 		t.Fatal("should be nil")
 	}
-	test.thing = new(Thing)
+	test.Thing = new(Thing)
 }
 
 func (test testThingSetup) String() string { return "thingSetup" }
 
 type setKey struct {
+	Thing *Thing `tea:"load"`
 	key   string
 	value string
 	bad   bool
@@ -35,9 +36,8 @@ func (test setKey) String() string {
 
 func (test *setKey) Run(t *testing.T) {
 	t.Logf("[%s] running setKey key: %q value: %q", t.Name(), test.key, test.value)
-	thing := new(Thing)
 
-	err := thing.Set(test.key, test.value)
+	err := test.Thing.Set(test.key, test.value)
 	if !test.bad && err != nil {
 		t.Errorf("should be able to set %q=%q but saw error %v", test.key, test.value, err)
 	}