feat: Support variable interpolation in vars (#555)

Author: mitar

kong.go

@@ -249,8 +249,11 @@ func (k *Kong) interpolateValue(value *Value, vars Vars) (err error) {
 		vars = vars.CloneWith(varsContributor.Vars(value))
 	}
 
-	if value.Enum, err = interpolate(value.Enum, vars, nil); err != nil {
-		return fmt.Errorf("enum for %s: %s", value.Summary(), err)
+	initialVars := vars.CloneWith(nil)
+	for n, v := range initialVars {
+		if vars[n], err = interpolate(v, initialVars, nil); err != nil {
+			return fmt.Errorf("variable %s for %s: %s", n, value.Summary(), err)
+		}
 	}
 
 	if value.Default, err = interpolate(value.Default, vars, nil); err != nil {

kong_test.go

@@ -2452,6 +2452,9 @@ func TestIntEnum(t *testing.T) {
 func TestRecursiveVariableExpansion(t *testing.T) {
 	var cli struct {
 		Config string `type:"path" default:"${config_file}" help:"Default: ${default}"`
+		Embed  struct {
+			Test string `help:"Another name: ${anotherName}"`
+		} `embed:"" set:"anotherName=${config_file}"`
 	}
 	k := mustNew(t, &cli, kong.Vars{"config_file": "/etc/config"}, kong.Exit(func(int) {}))
 	w := &strings.Builder{}
@@ -2460,6 +2463,7 @@ func TestRecursiveVariableExpansion(t *testing.T) {
 	_, err := k.Parse([]string{"--help"})
 	assert.NoError(t, err)
 	assert.Contains(t, w.String(), "Default: /etc/config")
+	assert.Contains(t, w.String(), "Another name: /etc/config")
 }
 
 type afterRunCLI struct {