Do not use Offset's Debug impl when serializing DateTime

src/datetime/serde.rs

@@ -1,13 +1,14 @@
 #![cfg_attr(docsrs, doc(cfg(feature = "serde")))]
 
 use core::fmt;
+use core::fmt::{Debug, Write};
 use serde::{de, ser};
 
 use super::DateTime;
 use crate::naive::datetime::serde::serde_from;
 #[cfg(feature = "clock")]
 use crate::offset::Local;
-use crate::offset::{FixedOffset, TimeZone, Utc};
+use crate::offset::{FixedOffset, Offset, TimeZone, Utc};
 
 #[doc(hidden)]
 #[derive(Debug)]
@@ -25,7 +26,9 @@ pub struct MicroSecondsTimestampVisitor;
 #[derive(Debug)]
 pub struct MilliSecondsTimestampVisitor;
 
-/// Serialize into a rfc3339 time string
+/// Serialize into a rfc3339 time string.
+/// In addition to rfc3339 the offset may contain seconds when present, instead of only hours and
+/// minutes.
 ///
 /// See [the `serde` module](./serde/index.html) for alternate
 /// serializations.
@@ -34,18 +37,25 @@ impl<Tz: TimeZone> ser::Serialize for DateTime<Tz> {
     where
         S: ser::Serializer,
     {
-        struct FormatWrapped<'a, D: 'a> {
-            inner: &'a D,
+        struct DisplayModifiedRfc3339<'a, Tz: TimeZone> {
+            inner: &'a DateTime<Tz>,
         }
 
-        impl<'a, D: fmt::Debug> fmt::Display for FormatWrapped<'a, D> {
+        impl<'a, Tz: TimeZone> fmt::Display for DisplayModifiedRfc3339<'a, Tz> {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-                self.inner.fmt(f)
+                // Reuse `Debug` impls, which is faster.
+                Debug::fmt(&self.inner.naive_local(), f)?;
+                let offset = (*self.inner.offset()).fix();
+                // Choose a more compact representation (`Z`) for UTC datetimes.
+                if Some(offset) == FixedOffset::east_opt(0) {
+                    f.write_char('Z')
+                } else {
+                    Debug::fmt(&offset, f)
+                }
             }
         }
 
-        // Debug formatting is correct RFC3339, and it allows Zulu.
-        serializer.collect_str(&FormatWrapped { inner: &self })
+        serializer.collect_str(&DisplayModifiedRfc3339 { inner: &self })
     }
 }