Decode: improve errors on integers and strings (#891)

unmarshaler.go

@@ -149,12 +149,16 @@ type errorContext struct {
 }
 
 func (d *decoder) typeMismatchError(toml string, target reflect.Type) error {
+	return fmt.Errorf("toml: %s", d.typeMismatchString(toml, target))
+}
+
+func (d *decoder) typeMismatchString(toml string, target reflect.Type) string {
 	if d.errorContext != nil && d.errorContext.Struct != nil {
 		ctx := d.errorContext
 		f := ctx.Struct.FieldByIndex(ctx.Field)
-		return fmt.Errorf("toml: cannot decode TOML %s into struct field %s.%s of type %s", toml, ctx.Struct, f.Name, f.Type)
+		return fmt.Sprintf("cannot decode TOML %s into struct field %s.%s of type %s", toml, ctx.Struct, f.Name, f.Type)
 	}
-	return fmt.Errorf("toml: cannot decode TOML %s into a Go value of type %s", toml, target)
+	return fmt.Sprintf("cannot decode TOML %s into a Go value of type %s", toml, target)
 }
 
 func (d *decoder) expr() *unstable.Node {
@@ -963,7 +967,7 @@ func (d *decoder) unmarshalInteger(value *unstable.Node, v reflect.Value) error
 	case reflect.Interface:
 		r = reflect.ValueOf(i)
 	default:
-		return d.typeMismatchError("integer", v.Type())
+		return unstable.NewParserError(d.p.Raw(value.Raw), d.typeMismatchString("integer", v.Type()))
 	}
 
 	if !r.Type().AssignableTo(v.Type()) {
@@ -982,7 +986,7 @@ func (d *decoder) unmarshalString(value *unstable.Node, v reflect.Value) error {
 	case reflect.Interface:
 		v.Set(reflect.ValueOf(string(value.Data)))
 	default:
-		return unstable.NewParserError(d.p.Raw(value.Raw), "cannot store TOML string into a Go %s", v.Kind())
+		return unstable.NewParserError(d.p.Raw(value.Raw), d.typeMismatchString("string", v.Type()))
 	}
 
 	return nil

unmarshaler_test.go

@@ -2048,6 +2048,68 @@ func TestUnmarshalErrors(t *testing.T) {
 	require.Equal(t, "toml: cannot decode TOML integer into struct field toml_test.mystruct.Bar of type string", err.Error())
 }
 
+func TestUnmarshalStringInvalidStructField(t *testing.T) {
+	type Server struct {
+		Path string
+		Port int
+	}
+
+	type Cfg struct {
+		Server Server
+	}
+
+	var cfg Cfg
+
+	data := `[server]
+path = "/my/path"
+port = "bad"
+`
+
+	file := strings.NewReader(data)
+	err := toml.NewDecoder(file).Decode(&cfg)
+	require.Error(t, err)
+
+	x := err.(*toml.DecodeError)
+	require.Equal(t, "toml: cannot decode TOML string into struct field toml_test.Server.Port of type int", x.Error())
+	expected := `1| [server]
+2| path = "/my/path"
+3| port = "bad"
+ |        ~~~~~ cannot decode TOML string into struct field toml_test.Server.Port of type int`
+
+	require.Equal(t, expected, x.String())
+}
+
+func TestUnmarshalIntegerInvalidStructField(t *testing.T) {
+	type Server struct {
+		Path string
+		Port int
+	}
+
+	type Cfg struct {
+		Server Server
+	}
+
+	var cfg Cfg
+
+	data := `[server]
+path = 100
+port = 50
+`
+
+	file := strings.NewReader(data)
+	err := toml.NewDecoder(file).Decode(&cfg)
+	require.Error(t, err)
+
+	x := err.(*toml.DecodeError)
+	require.Equal(t, "toml: cannot decode TOML integer into struct field toml_test.Server.Path of type string", x.Error())
+	expected := `1| [server]
+2| path = 100
+ |        ~~~ cannot decode TOML integer into struct field toml_test.Server.Path of type string
+3| port = 50`
+
+	require.Equal(t, expected, x.String())
+}
+
 func TestUnmarshalInvalidTarget(t *testing.T) {
 	x := "foo"
 	err := toml.Unmarshal([]byte{}, x)