feat: Allow ignoring fields from embedded structs (#499)

If a field in a struct is ignored with `kong:"-"`, any embedded fields
with the same name are also ignored. This allows an outer struct to
remove flags from an embedded struct by redefining it and adding a kong
ignore tag.

build.go

@@ -54,6 +54,7 @@ func flattenedFields(v reflect.Value, ptag *Tag) (out []flattenedField, err erro
 	if v.Kind() != reflect.Struct {
 		return out, nil
 	}
+	ignored := map[string]bool{}
 	for i := 0; i < v.NumField(); i++ {
 		ft := v.Type().Field(i)
 		fv := v.Field(i)
@@ -61,7 +62,8 @@ func flattenedFields(v reflect.Value, ptag *Tag) (out []flattenedField, err erro
 		if err != nil {
 			return nil, err
 		}
-		if tag.Ignored {
+		if tag.Ignored || ignored[ft.Name] {
+			ignored[ft.Name] = true
 			continue
 		}
 		// Assign group if it's not already set.
@@ -106,9 +108,27 @@ func flattenedFields(v reflect.Value, ptag *Tag) (out []flattenedField, err erro
 		}
 		out = append(out, sub...)
 	}
+	out = removeIgnored(out, ignored)
 	return out, nil
 }
 
+func removeIgnored(fields []flattenedField, ignored map[string]bool) []flattenedField {
+	j := 0
+	for i := 0; i < len(fields); i++ {
+		if ignored[fields[i].field.Name] {
+			continue
+		}
+		if i != j {
+			fields[j] = fields[i]
+		}
+		j++
+	}
+	if j != len(fields) {
+		fields = fields[:j]
+	}
+	return fields
+}
+
 // Build a Node in the Kong data model.
 //
 // "v" is the value to create the node from, "typ" is the output Node type.

kong_test.go

@@ -856,6 +856,43 @@ func TestExcludedField(t *testing.T) {
 	assert.Error(t, err)
 }
 
+func TestExcludeEmbeddedField(t *testing.T) {
+	type Embedded struct {
+		Flag     string
+		Excluded string
+	}
+	type Embedded2 struct {
+		Flag2    string
+		Excluded string
+	}
+	var cli struct {
+		Embedded
+		Excluded string `kong:"-"`
+		Embedded2
+	}
+	var cli2 struct {
+		Embedded  Embedded  `kong:"embed"`
+		Excluded  string    `kong:"-"`
+		Embedded2 Embedded2 `kong:"embed"`
+	}
+
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"--flag=foo"})
+	assert.NoError(t, err)
+	_, err = p.Parse([]string{"--flag-2=foo"})
+	assert.NoError(t, err)
+	_, err = p.Parse([]string{"--excluded=foo"})
+	assert.Error(t, err)
+
+	p = mustNew(t, &cli2)
+	_, err = p.Parse([]string{"--flag=foo"})
+	assert.NoError(t, err)
+	_, err = p.Parse([]string{"--flag-2=foo"})
+	assert.NoError(t, err)
+	_, err = p.Parse([]string{"--excluded=foo"})
+	assert.Error(t, err)
+}
+
 func TestUnnamedFieldEmbeds(t *testing.T) {
 	type Embed struct {
 		Flag string