Truncate fractional seconds to picoseconds

Close https://github.com/alexcrichton/toml-rs/issues/186

src/datetime.rs

@@ -218,7 +218,10 @@ impl FromStr for Datetime {
                     return Err(DatetimeParseError { _private: () })
                 }
                 chars = whole[end..].chars();
-                match format!("0.{}", &whole[..end]).parse() {
+
+                // truncate to picoseconds precision
+                let last = if end > 12 { 12 } else { end };
+                match format!("0.{}", &whole[..last]).parse() {
                     Ok(f) => f,
                     Err(_) => return Err(DatetimeParseError { _private: () }),
                 }

tests/valid.rs

@@ -190,6 +190,9 @@ test!(example_bom,
        include_str!("valid/example-bom.toml"),
        include_str!("valid/example.json"));
 
+test!(datetime_truncate,
+      include_str!("valid/datetime-truncate.toml"),
+      include_str!("valid/datetime-truncate.json"));
 test!(table_array_nest_no_keys,
       include_str!("valid/table-array-nest-no-keys.toml"),
       include_str!("valid/table-array-nest-no-keys.json"));

tests/valid/datetime-truncate.json

@@ -0,0 +1,6 @@
+{
+    "bestdayever": {
+        "type": "datetime",
+        "value": "1987-07-05T17:45:00.123456789012Z"
+    }
+}

tests/valid/datetime-truncate.toml

@@ -0,0 +1 @@
+bestdayever = 1987-07-05T17:45:00.123456789012345Z