Change how the 0 flag works in format!

Now it always implies right-alignment, so that padding zeroes are placed after the sign (if any) and before the digits. In other words, it always takes precedence over explicitly specified `[[fill]align]`. This also affects the '#' flag: zeroes are placed after the prefix (0b, 0o, 0x) and before the digits.

           :05     :<05    :>05    :^05
before   |-0001| |-1000| |-0001| |-0100|
after    |-0001| |-0001| |-0001| |-0001|
          :#05x   :<#05x  :>#05x  :^#05x
before   |0x001| |0x100| |000x1| |0x010|
after    |0x001| |0x001| |0x001| |0x001|

Fixes #39997 [breaking-change]

src/libcollections/fmt.rs

@@ -366,6 +366,10 @@
 //!         like `{:08}` would yield `00000001` for the integer `1`, while the
 //!         same format would yield `-0000001` for the integer `-1`. Notice that
 //!         the negative version has one fewer zero than the positive version.
+//!         Note that padding zeroes are always placed after the sign (if any)
+//!         and before the digits. When used together with the `#` flag, a similar
+//!         rule applies: padding zeroes are inserted after the prefix but before
+//!         the digits.
 //!
 //! ## Width
 //!

src/libcore/fmt/mod.rs

@@ -1045,6 +1045,7 @@ impl<'a> Formatter<'a> {
             // is zero
             Some(min) if self.sign_aware_zero_pad() => {
                 self.fill = '0';
+                self.align = rt::v1::Alignment::Right;
                 write_prefix(self)?;
                 self.with_padding(min - width, rt::v1::Alignment::Right, |f| {
                     f.buf.write_str(buf)

src/test/run-pass/ifmt.rs

@@ -160,6 +160,22 @@ pub fn main() {
     t!(format!("{:?}", -0.0), "-0");
     t!(format!("{:?}", 0.0), "0");
 
+    // sign aware zero padding
+    t!(format!("{:<3}", 1), "1  ");
+    t!(format!("{:>3}", 1), "  1");
+    t!(format!("{:^3}", 1), " 1 ");
+    t!(format!("{:03}", 1), "001");
+    t!(format!("{:<03}", 1), "001");
+    t!(format!("{:>03}", 1), "001");
+    t!(format!("{:^03}", 1), "001");
+    t!(format!("{:+03}", 1), "+01");
+    t!(format!("{:<+03}", 1), "+01");
+    t!(format!("{:>+03}", 1), "+01");
+    t!(format!("{:^+03}", 1), "+01");
+    t!(format!("{:#05x}", 1), "0x001");
+    t!(format!("{:<#05x}", 1), "0x001");
+    t!(format!("{:>#05x}", 1), "0x001");
+    t!(format!("{:^#05x}", 1), "0x001");
 
     // Ergonomic format_args!
     t!(format!("{0:x} {0:X}", 15), "f F");