Add an "embed" tag.

README.md

@@ -414,6 +414,7 @@ Tag                    | Description
 `group:"X"`            | Logical group for a flag or command.
 `prefix:"X"`           | Prefix for all sub-flags.
 `set:"K=V"`            | Set a variable for expansion by child elements. Multiples can occur.
+`embed`                | If present, this field's children will be embedded in the parent. Useful for composition.
 
 <a id="markdown-variable-interpolation" name="variable-interpolation"></a>
 ## Variable interpolation

build.go

@@ -52,7 +52,7 @@ func flattenedFields(v reflect.Value) (out []flattenedField) {
 		if tag.Ignored {
 			continue
 		}
-		if ft.Anonymous {
+		if ft.Anonymous || tag.Embed {
 			if fv.Kind() == reflect.Interface {
 				fv = fv.Elem()
 			}

kong_test.go

@@ -626,3 +626,19 @@ func TestExcludedField(t *testing.T) {
 	_, err = p.Parse([]string{"--excluded=foo"})
 	require.Error(t, err)
 }
+
+func TestUnnamedFieldEmbeds(t *testing.T) {
+	type Embed struct {
+		Flag string
+	}
+	var cli struct {
+		One Embed `prefix:"one-" embed:""`
+		Two Embed `prefix:"two-" embed:""`
+	}
+	buf := &strings.Builder{}
+	p := mustNew(t, &cli, kong.Writers(buf, buf), kong.Exit(func(int) {}))
+	_, err := p.Parse([]string{"--help"})
+	require.NoError(t, err)
+	require.Contains(t, buf.String(), `--one-flag=STRING`)
+	require.Contains(t, buf.String(), `--two-flag=STRING`)
+}

tag.go

@@ -29,6 +29,7 @@ type Tag struct {
 	Group       string
 	Vars        Vars
 	Prefix      string // Optional prefix on anonymous structs. All sub-flags will have this prefix.
+	Embed       bool
 
 	// Storage for all tag keys for arbitrary lookups.
 	items map[string][]string
@@ -151,6 +152,7 @@ func parseTag(fv reflect.Value, ft reflect.StructField) *Tag {
 	t.Sep, _ = t.GetRune("sep")
 	t.Group = t.Get("group")
 	t.Prefix = t.Get("prefix")
+	t.Embed = t.Has("embed")
 	if t.Sep == 0 {
 		if t.Get("sep") == "none" {
 			t.Sep = -1

tag_test.go

@@ -138,3 +138,14 @@ func TestTagSetOnCommand(t *testing.T) {
 	require.NoError(t, err)
 	require.Contains(t, buf.String(), `A key from somewhere.`)
 }
+
+func TestTagSetOnFlag(t *testing.T) {
+	var cli struct {
+		Flag string `set:"where=somewhere" help:"A key from ${where}."`
+	}
+	buf := &strings.Builder{}
+	p := mustNew(t, &cli, kong.Writers(buf, buf), kong.Exit(func(int) {}))
+	_, err := p.Parse([]string{"--help"})
+	require.NoError(t, err)
+	require.Contains(t, buf.String(), `A key from somewhere.`)
+}