avoid int formatting as much as possible (net -70%/-68% on 2822/3339 …

…respectively)

src/format/mod.rs

@@ -40,7 +40,6 @@ use alloc::string::{String, ToString};
 #[cfg(any(feature = "alloc", feature = "std", test))]
 use core::borrow::Borrow;
 use core::fmt;
-#[cfg(any(feature = "alloc", feature = "std", test))]
 use core::fmt::Write;
 use core::str::FromStr;
 #[cfg(any(feature = "std", test))]
@@ -790,21 +789,33 @@ fn write_rfc2822_inner(
     off: FixedOffset,
     locale: Locales,
 ) -> fmt::Result {
+    result.push_str(locale.short_weekdays[d.weekday().num_days_from_sunday() as usize]);
+    result.push_str(", ");
+    write_hundreds(result, d.day() as u8)?;
+    result.push(' ');
+    result.push_str(locale.short_months[d.month0() as usize]);
+    result.push(' ');
+    write_hundreds(result, (d.year() / 100) as u8)?;
+    write_hundreds(result, (d.year() % 100) as u8)?;
+    result.push(' ');
+    write_hundreds(result, t.hour() as u8)?;
+    result.push(':');
+    write_hundreds(result, t.minute() as u8)?;
+    result.push(':');
     let sec = t.second() + t.nanosecond() / 1_000_000_000;
-    write!(
-        result,
-        "{}, {:02} {} {:04} {:02}:{:02}:{:02} ",
-        locale.short_weekdays[d.weekday().num_days_from_sunday() as usize],
-        d.day(),
-        locale.short_months[d.month0() as usize],
-        d.year(),
-        t.hour(),
-        t.minute(),
-        sec
-    )?;
+    write_hundreds(result, sec as u8)?;
+    result.push(' ');
     write_local_minus_utc(result, off, false, Colons::None)
 }
 
+/// Equivalent to `{:02}` formatting for n < 100.
+pub(crate) fn write_hundreds(w: &mut impl Write, n: u8) -> fmt::Result {
+    let tens = b'0' + n / 10;
+    let ones = b'0' + n % 10;
+    w.write_char(tens as char)?;
+    w.write_char(ones as char)
+}
+
 /// Tries to format given arguments with given formatting items.
 /// Internally used by `DelayedFormat`.
 #[cfg(any(feature = "alloc", feature = "std", test))]

src/naive/date.rs

@@ -1820,14 +1820,22 @@ impl DoubleEndedIterator for NaiveDateWeeksIterator {
 /// ```
 impl fmt::Debug for NaiveDate {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use core::fmt::Write;
+
         let year = self.year();
         let mdf = self.mdf();
         if (0..=9999).contains(&year) {
-            write!(f, "{:04}-{:02}-{:02}", year, mdf.month(), mdf.day())
+            crate::format::write_hundreds(f, (year / 100) as u8)?;
+            crate::format::write_hundreds(f, (year % 100) as u8)?;
         } else {
             // ISO 8601 requires the explicit sign for out-of-range years
-            write!(f, "{:+05}-{:02}-{:02}", year, mdf.month(), mdf.day())
+            write!(f, "{:+05}", year)?;
         }
+
+        f.write_char('-')?;
+        crate::format::write_hundreds(f, mdf.month() as u8)?;
+        f.write_char('-')?;
+        crate::format::write_hundreds(f, mdf.day() as u8)
     }
 }
 

src/naive/time/mod.rs

@@ -1177,7 +1177,13 @@ impl fmt::Debug for NaiveTime {
             (sec, self.frac)
         };
 
-        write!(f, "{:02}:{:02}:{:02}", hour, min, sec)?;
+        use core::fmt::Write;
+        crate::format::write_hundreds(f, hour as u8)?;
+        f.write_char(':')?;
+        crate::format::write_hundreds(f, min as u8)?;
+        f.write_char(':')?;
+        crate::format::write_hundreds(f, sec as u8)?;
+
         if nano == 0 {
             Ok(())
         } else if nano % 1_000_000 == 0 {