use less intermediate formatting (-5% improvement)

src/date.rs

@@ -532,7 +532,8 @@ impl<Tz: TimeZone> Sub<Date<Tz>> for Date<Tz> {
 
 impl<Tz: TimeZone> fmt::Debug for Date<Tz> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{:?}{:?}", self.naive_local(), self.offset)
+        self.naive_local().fmt(f)?;
+        self.offset.fmt(f)
     }
 }
 
@@ -541,7 +542,8 @@ where
     Tz::Offset: fmt::Display,
 {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}{}", self.naive_local(), self.offset)
+        self.naive_local().fmt(f)?;
+        self.offset.fmt(f)
     }
 }
 

src/datetime/mod.rs

@@ -11,6 +11,7 @@ use alloc::string::{String, ToString};
 #[cfg(any(feature = "alloc", feature = "std", test))]
 use core::borrow::Borrow;
 use core::cmp::Ordering;
+use core::fmt::Write;
 use core::ops::{Add, AddAssign, Sub, SubAssign};
 use core::{fmt, hash, str};
 #[cfg(feature = "std")]
@@ -990,7 +991,8 @@ impl<Tz: TimeZone> Sub<Days> for DateTime<Tz> {
 
 impl<Tz: TimeZone> fmt::Debug for DateTime<Tz> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{:?}{:?}", self.naive_local(), self.offset)
+        self.naive_local().fmt(f)?;
+        self.offset.fmt(f)
     }
 }
 
@@ -999,7 +1001,9 @@ where
     Tz::Offset: fmt::Display,
 {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{} {}", self.naive_local(), self.offset)
+        self.naive_local().fmt(f)?;
+        f.write_char(' ')?;
+        self.offset.fmt(f)
     }
 }
 

src/format/mod.rs

@@ -737,7 +737,7 @@ fn format_inner<'a>(
                         if let (Some(d), Some(t), Some(&(_, off))) = (date, time, off) {
                             // reuse `Debug` impls which already print ISO 8601 format.
                             // this is faster in this way.
-                            write!(result, "{:?}T{:?}", d, t)?;
+                            write!(result, "{:?}", crate::NaiveDateTime::new(*d, *t))?;
                             Some(write_local_minus_utc(result, off, false, Colons::Single))
                         } else {
                             None

src/naive/datetime/mod.rs

@@ -6,6 +6,7 @@
 #[cfg(any(feature = "alloc", feature = "std", test))]
 use core::borrow::Borrow;
 use core::convert::TryFrom;
+use core::fmt::Write;
 use core::ops::{Add, AddAssign, Sub, SubAssign};
 use core::{fmt, str};
 
@@ -1629,7 +1630,9 @@ impl Sub<Days> for NaiveDateTime {
 /// ```
 impl fmt::Debug for NaiveDateTime {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{:?}T{:?}", self.date, self.time)
+        self.date.fmt(f)?;
+        f.write_char('T')?;
+        self.time.fmt(f)
     }
 }
 
@@ -1660,7 +1663,9 @@ impl fmt::Debug for NaiveDateTime {
 /// ```
 impl fmt::Display for NaiveDateTime {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{} {}", self.date, self.time)
+        self.date.fmt(f)?;
+        f.write_char(' ')?;
+        self.time.fmt(f)
     }
 }