Treat datetime as a primitive when encoding.

This CL fixes encoding of time.Time elements, which was broken in the
following three ways:

  - datetime zone was not coerced to UTC.

  - datetime elements were not printed together with the rest of simple
    types (because isStructOrMap returned true for time.Time), and were not
    separated by a newline.

  - arrays of datetime elements resulted in a panic (because isTOMLTableType
    did not handle them as a primitive type).

encode.go

@@ -127,10 +127,12 @@ func (enc *Encoder) eElement(rv reflect.Value) error {
 		return fstr
 	}
 
-	// Extra special case. Time needs to be in ISO8601 format.
-	switch s := rv.Interface().(type) {
-	case time.Time:
-		return ws(s.UTC().Format("2006-01-02T03:04:05Z"))
+	// Special case time.Time as a primitive. Has to come before TextMarshaler
+	// below because time.Time implements encoding.TextMarshaler, but we need to
+	// always use UTC.
+	if t, ok := rv.Interface().(time.Time); ok {
+		t = t.In(time.FixedZone("UTC", 0))
+		return ws(t.Format("2006-01-02T15:04:05Z"))
 	}
 
 	// Special case. Use text marshaler if it's available for this value.
@@ -278,8 +280,11 @@ func isStructOrMap(rv reflect.Value) bool {
 	switch rv.Kind() {
 	case reflect.Interface, reflect.Ptr:
 		return isStructOrMap(rv.Elem())
-	case reflect.Map, reflect.Struct:
+	case reflect.Map:
 		return true
+	case reflect.Struct:
+		_, isDate := rv.Interface().(time.Time)
+		return !isDate
 	default:
 		return false
 	}
@@ -480,8 +485,14 @@ func tomlTypeName(rv reflect.Value) (typeName string, valueIsNil bool) {
 		return tomlTypeName(rv.Elem())
 	case reflect.String:
 		return "string", false
-	case reflect.Map, reflect.Struct:
+	case reflect.Map:
 		return "table", false
+	case reflect.Struct:
+		if _, isDate := rv.Interface().(time.Time); isDate {
+			return "datetime", false
+		} else {
+			return "table", false
+		}
 	default:
 		panic("unexpected reflect.Kind: " + k.String())
 	}
@@ -525,8 +536,11 @@ func isTOMLTableType(rt reflect.Type, rv reflect.Value) (bool, error) {
 			return isTOMLTableType(rv.Type(), rv)
 		}
 		return false, nil
-	case reflect.Map, reflect.Struct:
+	case reflect.Map:
 		return true, nil
+	case reflect.Struct:
+		_, isDate := rv.Interface().(time.Time)
+		return !isDate, nil
 	default:
 		panic("unexpected reflect.Kind: " + k.String())
 	}

encode_test.go

@@ -2,7 +2,9 @@ package toml
 
 import (
 	"bytes"
+	"fmt"
 	"testing"
+	"time"
 )
 
 // XXX(burntsushi)
@@ -13,6 +15,9 @@ func TestEncode(t *testing.T) {
 		Int int `toml:"_int"`
 	}
 
+	date := time.Date(2014, 5, 11, 20, 30, 40, 0, time.FixedZone("IST", 3600))
+	dateStr := "2014-05-11T19:30:40Z"
+
 	tests := map[string]struct {
 		input      interface{}
 		wantOutput string
@@ -64,6 +69,18 @@ func TestEncode(t *testing.T) {
 			}{"foo", 0},
 			wantOutput: `String = "foo"`,
 		},
+		"datetime field in UTC": {
+			input:      struct{ Date time.Time }{date},
+			wantOutput: fmt.Sprintf("Date = %s", dateStr),
+		},
+		"datetime field as primitive": {
+			// Using a map here to fail if isStructOrMap() returns true for time.Time.
+			input: map[string]interface{}{
+				"Date": date,
+				"Int":  1,
+			},
+			wantOutput: fmt.Sprintf("Date = %s\nInt = 1", dateStr),
+		},
 		"array fields": {
 			input: struct {
 				IntArray0 [0]int
@@ -77,6 +94,12 @@ func TestEncode(t *testing.T) {
 			},
 			wantOutput: "IntSlice0 = []\nIntSlice3 = [1, 2, 3]",
 		},
+		"datetime slices": {
+			input: struct{ DatetimeSlice []time.Time }{
+				[]time.Time{date, date},
+			},
+			wantOutput: fmt.Sprintf("DatetimeSlice = [%s, %s]", dateStr, dateStr),
+		},
 		"nested arrays and slices": {
 			input: struct {
 				SliceOfArrays         [][2]int