feat: Add support for custom field tags

field_tag.go

@@ -0,0 +1,4 @@
+package toml
+
+// tomlFieldTag is the default field tag name when encoding and decoding TOML schema.
+const tomlFieldTag = "toml"

marshaler.go

@@ -43,13 +43,15 @@ type Encoder struct {
 	indentSymbol       string
 	indentTables       bool
 	marshalJsonNumbers bool
+	fieldTag           string
 }
 
 // NewEncoder returns a new Encoder that writes to w.
 func NewEncoder(w io.Writer) *Encoder {
 	return &Encoder{
 		w:            w,
 		indentSymbol: "  ",
+		fieldTag:     tomlFieldTag,
 	}
 }
 
@@ -100,6 +102,12 @@ func (enc *Encoder) SetMarshalJsonNumbers(indent bool) *Encoder {
 	return enc
 }
 
+// SetFieldTag replaces the current field tag with the specified field tag.
+func (enc *Encoder) SetFieldTag(fieldTag string) *Encoder {
+	enc.fieldTag = fieldTag
+	return enc
+}
+
 // Encode writes a TOML representation of v to the stream.
 //
 // If v cannot be represented to TOML it returns an error.
@@ -148,7 +156,7 @@ func (enc *Encoder) SetMarshalJsonNumbers(indent bool) *Encoder {
 // # Struct tags
 //
 // The encoding of each public struct field can be customized by the format
-// string in the "toml" key of the struct field's tag. This follows
+// string in the "toml" (or custom) key of the struct field's tag. This follows
 // encoding/json's convention. The format string starts with the name of the
 // field, optionally followed by a comma-separated list of options. The name may
 // be empty in order to provide options without overriding the default name.
@@ -164,7 +172,7 @@ func (enc *Encoder) SetMarshalJsonNumbers(indent bool) *Encoder {
 // The "commented" option prefixes the value and all its children with a comment
 // symbol.
 //
-// In addition to the "toml" tag struct tag, a "comment" tag can be used to emit
+// In addition to the "toml" (or custom ) tag struct tag, a "comment" tag can be used to emit
 // a TOML comment before the value being annotated. Comments are ignored inside
 // inline tables. For array tables, the comment is only present before the first
 // element of the array.
@@ -380,8 +388,8 @@ func isNil(v reflect.Value) bool {
 	}
 }
 
-func shouldOmitEmpty(options valueOptions, v reflect.Value) bool {
-	return options.omitempty && isEmptyValue(v)
+func (enc *Encoder) shouldOmitEmpty(options valueOptions, v reflect.Value) bool {
+	return options.omitempty && enc.isEmptyValue(v)
 }
 
 func (enc *Encoder) encodeKv(b []byte, ctx encoderCtx, options valueOptions, v reflect.Value) ([]byte, error) {
@@ -418,10 +426,10 @@ func (enc *Encoder) commented(commented bool, b []byte) []byte {
 	return b
 }
 
-func isEmptyValue(v reflect.Value) bool {
+func (enc *Encoder) isEmptyValue(v reflect.Value) bool {
 	switch v.Kind() {
 	case reflect.Struct:
-		return isEmptyStruct(v)
+		return enc.isEmptyStruct(v)
 	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
 		return v.Len() == 0
 	case reflect.Bool:
@@ -438,7 +446,7 @@ func isEmptyValue(v reflect.Value) bool {
 	return false
 }
 
-func isEmptyStruct(v reflect.Value) bool {
+func (enc *Encoder) isEmptyStruct(v reflect.Value) bool {
 	// TODO: merge with walkStruct and cache.
 	typ := v.Type()
 	for i := 0; i < typ.NumField(); i++ {
@@ -449,7 +457,7 @@ func isEmptyStruct(v reflect.Value) bool {
 			continue
 		}
 
-		tag := fieldType.Tag.Get("toml")
+		tag := fieldType.Tag.Get(enc.fieldTag)
 
 		// special field name to skip field
 		if tag == "-" {
@@ -458,7 +466,7 @@ func isEmptyStruct(v reflect.Value) bool {
 
 		f := v.Field(i)
 
-		if !isEmptyValue(f) {
+		if !enc.isEmptyValue(f) {
 			return false
 		}
 	}
@@ -715,7 +723,7 @@ func (t *table) pushTable(k string, v reflect.Value, options valueOptions) {
 	t.tables = append(t.tables, entry{Key: k, Value: v, Options: options})
 }
 
-func walkStruct(ctx encoderCtx, t *table, v reflect.Value) {
+func (enc *Encoder) walkStruct(ctx encoderCtx, t *table, v reflect.Value) {
 	// TODO: cache this
 	typ := v.Type()
 	for i := 0; i < typ.NumField(); i++ {
@@ -726,7 +734,7 @@ func walkStruct(ctx encoderCtx, t *table, v reflect.Value) {
 			continue
 		}
 
-		tag := fieldType.Tag.Get("toml")
+		tag := fieldType.Tag.Get(enc.fieldTag)
 
 		// special field name to skip field
 		if tag == "-" {
@@ -743,9 +751,9 @@ func walkStruct(ctx encoderCtx, t *table, v reflect.Value) {
 		if k == "" {
 			if fieldType.Anonymous {
 				if fieldType.Type.Kind() == reflect.Struct {
-					walkStruct(ctx, t, f)
+					enc.walkStruct(ctx, t, f)
 				} else if fieldType.Type.Kind() == reflect.Ptr && !f.IsNil() && f.Elem().Kind() == reflect.Struct {
-					walkStruct(ctx, t, f.Elem())
+					enc.walkStruct(ctx, t, f.Elem())
 				}
 				continue
 			} else {
@@ -775,7 +783,7 @@ func walkStruct(ctx encoderCtx, t *table, v reflect.Value) {
 func (enc *Encoder) encodeStruct(b []byte, ctx encoderCtx, v reflect.Value) ([]byte, error) {
 	var t table
 
-	walkStruct(ctx, &t, v)
+	enc.walkStruct(ctx, &t, v)
 
 	return enc.encodeTable(b, ctx, t)
 }
@@ -879,7 +887,7 @@ func (enc *Encoder) encodeTable(b []byte, ctx encoderCtx, t table) ([]byte, erro
 
 	hasNonEmptyKV := false
 	for _, kv := range t.kvs {
-		if shouldOmitEmpty(kv.Options, kv.Value) {
+		if enc.shouldOmitEmpty(kv.Options, kv.Value) {
 			continue
 		}
 		hasNonEmptyKV = true
@@ -898,7 +906,7 @@ func (enc *Encoder) encodeTable(b []byte, ctx encoderCtx, t table) ([]byte, erro
 
 	first := true
 	for _, table := range t.tables {
-		if shouldOmitEmpty(table.Options, table.Value) {
+		if enc.shouldOmitEmpty(table.Options, table.Value) {
 			continue
 		}
 		if first {
@@ -932,7 +940,7 @@ func (enc *Encoder) encodeTableInline(b []byte, ctx encoderCtx, t table) ([]byte
 
 	first := true
 	for _, kv := range t.kvs {
-		if shouldOmitEmpty(kv.Options, kv.Value) {
+		if enc.shouldOmitEmpty(kv.Options, kv.Value) {
 			continue
 		}
 

marshaler_test.go

@@ -1080,6 +1080,30 @@ Bad = ''
 	assert.Equal(t, expected, string(b))
 }
 
+func TestEncoderSetFieldTag(t *testing.T) {
+	type fieldTagTestType struct {
+		Hellllooo     string `json:"hello"`
+		Nnnummbbeerrr int    `json:"number"`
+	}
+
+	var buf bytes.Buffer
+	enc := toml.NewEncoder(&buf)
+	enc = enc.SetFieldTag("json")
+
+	input := fieldTagTestType{
+		Hellllooo:     "hi",
+		Nnnummbbeerrr: 1,
+	}
+	err := enc.Encode(input)
+	require.NoError(t, err)
+
+	expValue := `hello = 'hi'
+number = 1
+`
+
+	require.Equal(t, expValue, buf.String())
+}
+
 func TestIssue436(t *testing.T) {
 	data := []byte(`{"a": [ { "b": { "c": "d" } } ]}`)
 

unmarshaler.go

@@ -21,7 +21,7 @@ import (
 //
 // It is a shortcut for Decoder.Decode() with the default options.
 func Unmarshal(data []byte, v interface{}) error {
-	d := decoder{}
+	d := decoder{fieldTag: tomlFieldTag}
 	d.p.Reset(data)
 	return d.FromParser(v)
 }
@@ -36,11 +36,14 @@ type Decoder struct {
 
 	// toggles unmarshaler interface
 	unmarshalerInterface bool
+
+	// fieldTag is the tag name used to decode struct fields.
+	fieldTag string
 }
 
 // NewDecoder creates a new Decoder that will read from r.
 func NewDecoder(r io.Reader) *Decoder {
-	return &Decoder{r: r}
+	return &Decoder{r: r, fieldTag: tomlFieldTag}
 }
 
 // DisallowUnknownFields causes the Decoder to return an error when the
@@ -73,6 +76,11 @@ func (d *Decoder) EnableUnmarshalerInterface() *Decoder {
 	return d
 }
 
+func (d *Decoder) SetFieldTag(fieldTag string) *Decoder {
+	d.fieldTag = fieldTag
+	return d
+}
+
 // Decode the whole content of r into v.
 //
 // By default, values in the document that don't exist in the target Go value
@@ -125,6 +133,7 @@ func (d *Decoder) Decode(v interface{}) error {
 			Enabled: d.strict,
 		},
 		unmarshalerInterface: d.unmarshalerInterface,
+		fieldTag:             d.fieldTag,
 	}
 	dec.p.Reset(b)
 
@@ -166,6 +175,9 @@ type decoder struct {
 
 	// Current context for the error.
 	errorContext *errorContext
+
+	// fieldTag is the tag name used to decode struct fields.
+	fieldTag string
 }
 
 type errorContext struct {
@@ -512,7 +524,7 @@ func (d *decoder) handleKeyPart(key unstable.Iterator, v reflect.Value, nextFn h
 			v.SetMapIndex(mk, mv)
 		}
 	case reflect.Struct:
-		path, found := structFieldPath(v, string(key.Node().Data))
+		path, found := d.structFieldPath(v, string(key.Node().Data))
 		if !found {
 			d.skipUntilTable = true
 			return reflect.Value{}, nil
@@ -1152,7 +1164,7 @@ func (d *decoder) handleKeyValuePart(key unstable.Iterator, value *unstable.Node
 			v.SetMapIndex(mk, mv)
 		}
 	case reflect.Struct:
-		path, found := structFieldPath(v, string(key.Node().Data))
+		path, found := d.structFieldPath(v, string(key.Node().Data))
 		if !found {
 			d.skipUntilTable = true
 			break
@@ -1261,7 +1273,7 @@ type fieldPathsMap = map[string][]int
 
 var globalFieldPathsCache atomic.Value // map[danger.TypeID]fieldPathsMap
 
-func structFieldPath(v reflect.Value, name string) ([]int, bool) {
+func (d *decoder) structFieldPath(v reflect.Value, name string) ([]int, bool) {
 	t := v.Type()
 
 	cache, _ := globalFieldPathsCache.Load().(map[danger.TypeID]fieldPathsMap)
@@ -1270,7 +1282,7 @@ func structFieldPath(v reflect.Value, name string) ([]int, bool) {
 	if !ok {
 		fieldPaths = map[string][]int{}
 
-		forEachField(t, nil, func(name string, path []int) {
+		d.forEachField(t, nil, func(name string, path []int) {
 			fieldPaths[name] = path
 			// extra copy for the case-insensitive match
 			fieldPaths[strings.ToLower(name)] = path
@@ -1291,7 +1303,7 @@ func structFieldPath(v reflect.Value, name string) ([]int, bool) {
 	return path, ok
 }
 
-func forEachField(t reflect.Type, path []int, do func(name string, path []int)) {
+func (d *decoder) forEachField(t reflect.Type, path []int, do func(name string, path []int)) {
 	n := t.NumField()
 	for i := 0; i < n; i++ {
 		f := t.Field(i)
@@ -1304,7 +1316,7 @@ func forEachField(t reflect.Type, path []int, do func(name string, path []int))
 		fieldPath := append(path, i)
 		fieldPath = fieldPath[:len(fieldPath):len(fieldPath)]
 
-		name := f.Tag.Get("toml")
+		name := f.Tag.Get(d.fieldTag)
 		if name == "-" {
 			continue
 		}
@@ -1320,7 +1332,7 @@ func forEachField(t reflect.Type, path []int, do func(name string, path []int))
 			}
 
 			if t2.Kind() == reflect.Struct {
-				forEachField(t2, fieldPath, do)
+				d.forEachField(t2, fieldPath, do)
 			}
 			continue
 		}

unmarshaler_test.go

@@ -112,6 +112,21 @@ func TestDecodeReaderError(t *testing.T) {
 	require.Error(t, err)
 }
 
+func TestDecoderSetFieldTag(t *testing.T) {
+	type fieldTagTestType struct {
+		A string `json:"aaa"`
+		B string `json:"bbb"`
+	}
+
+	d := toml.NewDecoder(strings.NewReader("aaa = \"foo\"\nbbb = \"bar\""))
+	d.SetFieldTag("json")
+	var output fieldTagTestType
+	err := d.Decode(&output)
+	require.NoError(t, err)
+	assert.Equal(t, "foo", output.A)
+	assert.Equal(t, "bar", output.B)
+}
+
 // nolint:funlen
 func TestUnmarshal_Integers(t *testing.T) {
 	examples := []struct {