fix: set default options in Usage (#63)

env.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 )
 
-// Options are the options for the [Load] function.
+// Options are the options for the [Load] and [Usage] functions.
 type Options struct {
 	Source   Source // The source of environment variables. The default is [OS].
 	SliceSep string // The separator used to parse slice values. The default is space.
@@ -61,21 +61,13 @@ func (e *NotSetError) Error() string {
 //   - required: marks the environment variable as required
 //   - expand: expands the value of the environment variable using [os.Expand]
 func Load(cfg any, opts *Options) error {
-	if opts == nil {
-		opts = new(Options)
-	}
-	if opts.Source == nil {
-		opts.Source = OS
-	}
-	if opts.SliceSep == "" {
-		opts.SliceSep = " "
-	}
-
 	pv := reflect.ValueOf(cfg)
 	if !structPtr(pv) {
 		panic("env: cfg must be a non-nil struct pointer")
 	}
 
+	opts = setDefaultOptions(opts)
+
 	v := pv.Elem()
 	vars := parseVars(v, opts)
 	cache[v.Type()] = vars
@@ -112,6 +104,19 @@ func Load(cfg any, opts *Options) error {
 	return nil
 }
 
+func setDefaultOptions(opts *Options) *Options {
+	if opts == nil {
+		opts = new(Options)
+	}
+	if opts.Source == nil {
+		opts.Source = OS
+	}
+	if opts.SliceSep == "" {
+		opts.SliceSep = " "
+	}
+	return opts
+}
+
 func parseVars(v reflect.Value, opts *Options) []Var {
 	var vars []Var
 

usage.go

@@ -42,6 +42,8 @@ func Usage(cfg any, w io.Writer, opts *Options) {
 		panic("env: cfg must be a non-nil struct pointer")
 	}
 
+	opts = setDefaultOptions(opts)
+
 	v := pv.Elem()
 	vars, ok := cache[v.Type()]
 	if !ok {

usage_test.go

@@ -20,6 +20,17 @@ func TestUsage(t *testing.T) {
 		assert.Equal[E](t, buf.String(), "  FOO  string  default <empty>\n")
 	})
 
+	t.Run("with Options.NameSep", func(t *testing.T) {
+		var buf bytes.Buffer
+		var cfg struct {
+			A struct {
+				Foo int `env:"FOO"`
+			} `env:"A"`
+		}
+		env.Usage(&cfg, &buf, &env.Options{NameSep: "_"})
+		assert.Equal[E](t, buf.String(), "  A_FOO  int  default 0\n")
+	})
+
 	t.Run("custom usage message", func(t *testing.T) {
 		var buf bytes.Buffer
 		var cfg Config