Reset Formatter flags on exit from pad_integral

This fixes a bug where after calling pad_integral with appropriate flags, the
fill and alignment flags would be set to '0' and 'Right' and left as such even
after exiting pad_integral, which meant that future calls on the same Formatter
would get incorrect flags reported.

This is quite difficult to observe in practice, as almost all formatting
implementations in practice don't call `Display::fmt` directly, but rather use
`write!` or a similar macro, which means that they cannot observe the effects of
the wrong flags (as `write!` creates a fresh Formatter instance). However, we
include a test case.

Author: Mark-Simulacrum

src/libcore/fmt/mod.rs

@@ -1244,12 +1244,15 @@ impl<'a> Formatter<'a> {
             // The sign and prefix goes before the padding if the fill character
             // is zero
             Some(min) if self.sign_aware_zero_pad() => {
-                self.fill = '0';
-                self.align = rt::v1::Alignment::Right;
+                let old_fill = crate::mem::replace(&mut self.fill, '0');
+                let old_align = crate::mem::replace(&mut self.align, rt::v1::Alignment::Right);
                 write_prefix(self, sign, prefix)?;
                 let post_padding = self.padding(min - width, rt::v1::Alignment::Right)?;
                 self.buf.write_str(buf)?;
-                post_padding.write(self.buf)
+                post_padding.write(self.buf)?;
+                self.fill = old_fill;
+                self.align = old_align;
+                Ok(())
             }
             // Otherwise, the sign and prefix goes after the padding
             Some(min) => {

src/libcore/tests/fmt/mod.rs

@@ -28,3 +28,18 @@ fn test_estimated_capacity() {
     assert_eq!(format_args!("{}, hello!", "World").estimated_capacity(), 0);
     assert_eq!(format_args!("{}. 16-bytes piece", "World").estimated_capacity(), 32);
 }
+
+#[test]
+fn pad_integral_resets() {
+    struct Bar;
+
+    impl core::fmt::Display for Bar {
+        fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+            "1".fmt(f)?;
+            f.pad_integral(true, "", "5")?;
+            "1".fmt(f)
+        }
+    }
+
+    assert_eq!(format!("{:<03}", Bar), "1  0051  ");
+}