Add parse_from_xxx to DateTime<Utc>

As discussed in #263. Note that this breaks existing code that did not
explicitly specify the offset in calls to `DateTime::parse_from_*`, as
they are now ambiguous.

Relies on #271 for conversion back into `DateTime<Utc>` from
`DateTime<FixedOffset>` as the latter is used under the hood.

src/datetime/mod.rs

@@ -533,7 +533,7 @@ impl DateTime<FixedOffset> {
     /// ```
     /// # use chrono::{DateTime, FixedOffset, TimeZone};
     /// assert_eq!(
-    ///     DateTime::parse_from_rfc2822("Wed, 18 Feb 2015 23:16:09 GMT").unwrap(),
+    ///     DateTime::<FixedOffset>::parse_from_rfc2822("Wed, 18 Feb 2015 23:16:09 GMT").unwrap(),
     ///     FixedOffset::east(0).ymd(2015, 2, 18).and_hms(23, 16, 9)
     /// );
     /// ```
@@ -574,7 +574,7 @@ impl DateTime<FixedOffset> {
     /// ```rust
     /// use chrono::{DateTime, FixedOffset, TimeZone};
     ///
-    /// let dt = DateTime::parse_from_str(
+    /// let dt = DateTime::<FixedOffset>::parse_from_str(
     ///     "1983 Apr 13 12:09:14.274 +0000", "%Y %b %d %H:%M:%S%.3f %z");
     /// assert_eq!(dt, Ok(FixedOffset::east(0).ymd(1983, 4, 13).and_hms_milli(12, 9, 14, 274)));
     /// ```
@@ -585,6 +585,49 @@ impl DateTime<FixedOffset> {
     }
 }
 
+impl DateTime<Utc> {
+    /// Parses an RFC 2822 date and time string such as `Tue, 1 Jul 2003 10:52:37 +0200`,
+    /// then returns a new `DateTime<Utc>` instance corresponding to the UTC date/time accounting
+    /// for the difference between UTC and the parsed timezone.
+    pub fn parse_from_rfc2822(s: &str) -> ParseResult<DateTime<Utc>> {
+        DateTime::<FixedOffset>::parse_from_rfc2822(s).map(|result| result.into())
+    }
+
+    /// Parses an RFC 3339 and ISO 8601 date and time string such as `1996-12-19T16:39:57-08:00`,
+    /// then returns a new `DateTime<Utc>` instance corresponding to the UTC date/time accounting
+    /// for the difference between UTC and the parsed timezone.
+    ///
+    /// Why isn't this named `parse_from_iso8601`? That's because ISO 8601 allows some freedom
+    /// over the syntax and RFC 3339 exercises that freedom to rigidly define a fixed format.
+    pub fn parse_from_rfc3339(s: &str) -> ParseResult<DateTime<Utc>> {
+        DateTime::<FixedOffset>::parse_from_rfc3339(s).map(|result| result.into())
+    }
+
+    /// Parses a string with the specified format string and
+    /// returns a new `DateTime` with a parsed `FixedOffset`.
+    /// See the [`format::strftime` module](./format/strftime/index.html)
+    /// on the supported escape sequences.
+    ///
+    /// See also `Offset::datetime_from_str` which gives a local `DateTime` on specific time zone.
+    ///
+    /// Note that this method *requires a timezone* in the string. See
+    /// [`NaiveDateTime::parse_from_str`](./naive/struct.NaiveDateTime.html#method.parse_from_str)
+    /// for a version that does not require a timezone in the to-be-parsed str.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use chrono::{DateTime, TimeZone, Utc};
+    ///
+    /// let dt = DateTime::<Utc>::parse_from_str(
+    ///     "1983 Apr 13 12:09:14.274 +0100", "%Y %b %d %H:%M:%S%.3f %z");
+    /// assert_eq!(dt, Ok(Utc.ymd(1983, 4, 13).and_hms_milli(11, 9, 14, 274)));
+    /// ```
+    pub fn parse_from_str(s: &str, fmt: &str) -> ParseResult<DateTime<Utc>> {
+        DateTime::<FixedOffset>::parse_from_str(s, fmt).map(|result| result.into())
+    }
+}
+
 impl<Tz: TimeZone> DateTime<Tz>
 where
     Tz::Offset: fmt::Display,

src/datetime/tests.rs

@@ -111,24 +111,24 @@ fn test_datetime_rfc2822_and_rfc3339() {
     );
 
     assert_eq!(
-        DateTime::parse_from_rfc2822("Wed, 18 Feb 2015 23:16:09 +0000"),
+        DateTime::<FixedOffset>::parse_from_rfc2822("Wed, 18 Feb 2015 23:16:09 +0000"),
         Ok(FixedOffset::east(0).ymd(2015, 2, 18).and_hms(23, 16, 9))
     );
     assert_eq!(
-        DateTime::parse_from_rfc2822("Wed, 18 Feb 2015 23:16:09 -0000"),
+        DateTime::<FixedOffset>::parse_from_rfc2822("Wed, 18 Feb 2015 23:16:09 -0000"),
         Ok(FixedOffset::east(0).ymd(2015, 2, 18).and_hms(23, 16, 9))
     );
     assert_eq!(
-        DateTime::parse_from_rfc3339("2015-02-18T23:16:09Z"),
+        DateTime::<FixedOffset>::parse_from_rfc3339("2015-02-18T23:16:09Z"),
         Ok(FixedOffset::east(0).ymd(2015, 2, 18).and_hms(23, 16, 9))
     );
     assert_eq!(
-        DateTime::parse_from_rfc2822("Wed, 18 Feb 2015 23:59:60 +0500"),
+        DateTime::<FixedOffset>::parse_from_rfc2822("Wed, 18 Feb 2015 23:59:60 +0500"),
         Ok(edt.ymd(2015, 2, 18).and_hms_milli(23, 59, 59, 1_000))
     );
-    assert!(DateTime::parse_from_rfc2822("31 DEC 262143 23:59 -2359").is_err());
+    assert!(DateTime::<FixedOffset>::parse_from_rfc2822("31 DEC 262143 23:59 -2359").is_err());
     assert_eq!(
-        DateTime::parse_from_rfc3339("2015-02-18T23:59:60.234567+05:00"),
+        DateTime::<FixedOffset>::parse_from_rfc3339("2015-02-18T23:59:60.234567+05:00"),
         Ok(edt.ymd(2015, 2, 18).and_hms_micro(23, 59, 59, 1_234_567))
     );
 }
@@ -209,12 +209,15 @@ fn test_datetime_from_str() {
 fn test_datetime_parse_from_str() {
     let ymdhms = |y, m, d, h, n, s, off| FixedOffset::east(off).ymd(y, m, d).and_hms(h, n, s);
     assert_eq!(
-        DateTime::parse_from_str("2014-5-7T12:34:56+09:30", "%Y-%m-%dT%H:%M:%S%z"),
+        DateTime::<FixedOffset>::parse_from_str("2014-5-7T12:34:56+09:30", "%Y-%m-%dT%H:%M:%S%z"),
         Ok(ymdhms(2014, 5, 7, 12, 34, 56, 570 * 60))
     ); // ignore offset
-    assert!(DateTime::parse_from_str("20140507000000", "%Y%m%d%H%M%S").is_err()); // no offset
-    assert!(DateTime::parse_from_str("Fri, 09 Aug 2013 23:54:35 GMT", "%a, %d %b %Y %H:%M:%S GMT")
-        .is_err());
+    assert!(DateTime::<FixedOffset>::parse_from_str("20140507000000", "%Y%m%d%H%M%S").is_err()); // no offset
+    assert!(DateTime::<FixedOffset>::parse_from_str(
+        "Fri, 09 Aug 2013 23:54:35 GMT",
+        "%a, %d %b %Y %H:%M:%S GMT"
+    )
+    .is_err());
     assert_eq!(
         Utc.datetime_from_str("Fri, 09 Aug 2013 23:54:35 GMT", "%a, %d %b %Y %H:%M:%S GMT"),
         Ok(Utc.ymd(2013, 8, 9).and_hms(23, 54, 35))

src/format/strftime.rs

@@ -622,15 +622,15 @@ fn test_strftime_docs() {
     );
     assert_eq!(
         dt.with_timezone(&Utc),
-        DateTime::parse_from_str("2001-07-07T15:04:60.026490708Z", "%+").unwrap()
+        DateTime::<FixedOffset>::parse_from_str("2001-07-07T15:04:60.026490708Z", "%+").unwrap()
     );
     assert_eq!(
         dt.with_timezone(&Utc),
-        DateTime::parse_from_str("2001-07-07T15:04:60.026490708UTC", "%+").unwrap()
+        DateTime::<FixedOffset>::parse_from_str("2001-07-07T15:04:60.026490708UTC", "%+").unwrap()
     );
     assert_eq!(
         dt.with_timezone(&Utc),
-        DateTime::parse_from_str("2001-07-07t15:04:60.026490708utc", "%+").unwrap()
+        DateTime::<FixedOffset>::parse_from_str("2001-07-07t15:04:60.026490708utc", "%+").unwrap()
     );
 
     assert_eq!(

src/lib.rs

@@ -268,11 +268,11 @@
 //! assert_eq!("2014-11-28T21:00:09+09:00".parse::<DateTime<FixedOffset>>(), Ok(fixed_dt.clone()));
 //!
 //! // method 2
-//! assert_eq!(DateTime::parse_from_str("2014-11-28 21:00:09 +09:00", "%Y-%m-%d %H:%M:%S %z"),
+//! assert_eq!(DateTime::<FixedOffset>::parse_from_str("2014-11-28 21:00:09 +09:00", "%Y-%m-%d %H:%M:%S %z"),
 //!            Ok(fixed_dt.clone()));
-//! assert_eq!(DateTime::parse_from_rfc2822("Fri, 28 Nov 2014 21:00:09 +0900"),
+//! assert_eq!(DateTime::<FixedOffset>::parse_from_rfc2822("Fri, 28 Nov 2014 21:00:09 +0900"),
 //!            Ok(fixed_dt.clone()));
-//! assert_eq!(DateTime::parse_from_rfc3339("2014-11-28T21:00:09+09:00"), Ok(fixed_dt.clone()));
+//! assert_eq!(DateTime::<FixedOffset>::parse_from_rfc3339("2014-11-28T21:00:09+09:00"), Ok(fixed_dt.clone()));
 //!
 //! // method 3
 //! assert_eq!(Utc.datetime_from_str("2014-11-28 12:00:09", "%Y-%m-%d %H:%M:%S"), Ok(dt.clone()));
@@ -302,14 +302,14 @@
 //!
 //! ```rust
 //! // We need the trait in scope to use Utc::timestamp().
-//! use chrono::{DateTime, TimeZone, Utc};
+//! use chrono::{DateTime, FixedOffset, TimeZone, Utc};
 //!
 //! // Construct a datetime from epoch:
 //! let dt = Utc.timestamp(1_500_000_000, 0);
 //! assert_eq!(dt.to_rfc2822(), "Fri, 14 Jul 2017 02:40:00 +0000");
 //!
 //! // Get epoch value from a datetime:
-//! let dt = DateTime::parse_from_rfc2822("Fri, 14 Jul 2017 02:40:00 +0000").unwrap();
+//! let dt = DateTime::<FixedOffset>::parse_from_rfc2822("Fri, 14 Jul 2017 02:40:00 +0000").unwrap();
 //! assert_eq!(dt.timestamp(), 1_500_000_000);
 //! ```
 //!

src/naive/time/mod.rs

@@ -178,7 +178,7 @@ mod tests;
 ///
 /// let dt = Utc.ymd(2015, 6, 30).and_hms(23, 56, 5);
 /// assert_eq!(format!("{:?}", dt), "2015-06-30T23:56:05Z");
-/// assert_eq!(DateTime::parse_from_rfc3339("2015-06-30T23:56:05Z").unwrap(), dt);
+/// assert_eq!(DateTime::<Utc>::parse_from_rfc3339("2015-06-30T23:56:05Z").unwrap(), dt);
 /// ```
 ///
 /// Since Chrono alone cannot determine any existence of leap seconds,