Decoder: time allows extra precision (#710)

As discussed[1], this change allows times to provide precision beyond the
nanosecond (nine digits fractional part). Extra precision is truncated according
to the TOML specificiation.

[1]: https://github.com/pelletier/go-toml/discussions/707

decode.go

@@ -232,6 +232,7 @@ func parseLocalTime(b []byte) (LocalTime, []byte, error) {
 
 	if len(b) >= 1 && b[0] == '.' {
 		frac := 0
+		precision := 0
 		digits := 0
 
 		for i, c := range b[1:] {
@@ -241,23 +242,31 @@ func parseLocalTime(b []byte) (LocalTime, []byte, error) {
 				}
 				break
 			}
+			digits++
 
 			const maxFracPrecision = 9
 			if i >= maxFracPrecision {
-				return t, nil, newDecodeError(b[i-1:i], "maximum precision for date time is nanosecond")
+				// go-toml allows decoding fractional seconds
+				// beyond the supported precision of 9
+				// digits. It truncates the fractional component
+				// to the supported precision and ignores the
+				// remaining digits.
+				//
+				// https://github.com/pelletier/go-toml/discussions/707
+				continue
 			}
 
 			frac *= 10
 			frac += int(c - '0')
-			digits++
+			precision++
 		}
 
-		if digits == 0 {
+		if precision == 0 {
 			return t, nil, newDecodeError(b[:1], "nanoseconds need at least one digit")
 		}
 
-		t.Nanosecond = frac * nspow[digits]
-		t.Precision = digits
+		t.Nanosecond = frac * nspow[precision]
+		t.Precision = precision
 
 		return t, b[1+digits:], nil
 	}

unmarshaler.go

@@ -60,7 +60,8 @@ func (d *Decoder) SetStrict(strict bool) *Decoder {
 //
 // When a TOML local date, time, or date-time is decoded into a time.Time, its
 // value is represented in time.Local timezone. Otherwise the approriate Local*
-// structure is used.
+// structure is used. For time values, precision up to the nanosecond is
+// supported by truncating extra digits.
 //
 // Empty tables decoded in an interface{} create an empty initialized
 // map[string]interface{}.

unmarshaler_test.go

@@ -2311,6 +2311,21 @@ func TestIssue708(t *testing.T) {
 	require.Equal(t, map[string]string{"0": ""}, v)
 }
 
+func TestIssue710(t *testing.T) {
+	v := map[string]toml.LocalTime{}
+	err := toml.Unmarshal([]byte(`0=00:00:00.0000000000`), &v)
+	require.NoError(t, err)
+	require.Equal(t, map[string]toml.LocalTime{"0": {Precision: 9}}, v)
+	v1 := map[string]toml.LocalTime{}
+	err = toml.Unmarshal([]byte(`0=00:00:00.0000000001`), &v1)
+	require.NoError(t, err)
+	require.Equal(t, map[string]toml.LocalTime{"0": {Precision: 9}}, v1)
+	v2 := map[string]toml.LocalTime{}
+	err = toml.Unmarshal([]byte(`0=00:00:00.1111111119`), &v2)
+	require.NoError(t, err)
+	require.Equal(t, map[string]toml.LocalTime{"0": {Nanosecond: 111111111, Precision: 9}}, v2)
+}
+
 func TestUnmarshalDecodeErrors(t *testing.T) {
 	examples := []struct {
 		desc string
@@ -2325,18 +2340,10 @@ func TestUnmarshalDecodeErrors(t *testing.T) {
 			desc: "local time with fractional",
 			data: `a = 11:22:33.x`,
 		},
-		{
-			desc: "local time frac precision too large",
-			data: `a = 2021-05-09T11:22:33.99999999999`,
-		},
 		{
 			desc: "wrong time offset separator",
 			data: `a = 1979-05-27T00:32:00.-07:00`,
 		},
-		{
-			desc: "missing fractional with tz",
-			data: `a = 2021-05-09T11:22:33.99999999999`,
-		},
 		{
 			desc: "wrong time offset separator",
 			data: `a = 1979-05-27T00:32:00Z07:00`,