Rollup merge of #40241 - Sawyer47:fix-39997, r=alexcrichton

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.

Here's a short summary of how similar format strings work in Python and Rust:

```
              :05     :<05    :>05    :^05
Python 3.6  |-0001| |-1000| |000-1| |0-100|
Rust before |-0001| |-1000| |-0001| |-0100|
Rust after  |-0001| |-0001| |-0001| |-0001|

             :#05x   :<#05x  :>#05x  :^#05x
Python 3.6  |0x001| |0x100| |000x1| |00x10|
Rust before |0x001| |0x100| |000x1| |0x010|
Rust after  |0x001| |0x001| |0x001| |0x001|
```

Fixes #39997 [breaking-change]

src/libcollections/fmt.rs

@@ -367,6 +367,10 @@
 //!         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)
@@ -1153,8 +1154,9 @@ impl<'a> Formatter<'a> {
             // for the sign-aware zero padding, we render the sign first and
             // behave as if we had no sign from the beginning.
             let mut formatted = formatted.clone();
-            let mut align = self.align;
             let old_fill = self.fill;
+            let old_align = self.align;
+            let mut align = old_align;
             if self.sign_aware_zero_pad() {
                 // a sign always goes first
                 let sign = unsafe { str::from_utf8_unchecked(formatted.sign) };
@@ -1165,6 +1167,7 @@ impl<'a> Formatter<'a> {
                 width = if width < sign.len() { 0 } else { width - sign.len() };
                 align = rt::v1::Alignment::Right;
                 self.fill = '0';
+                self.align = rt::v1::Alignment::Right;
             }
 
             // remaining parts go through the ordinary padding process.
@@ -1177,6 +1180,7 @@ impl<'a> Formatter<'a> {
                 })
             };
             self.fill = old_fill;
+            self.align = old_align;
             ret
         } else {
             // this is the common case and we take a shortcut

src/test/run-pass/ifmt.rs

@@ -160,6 +160,34 @@ 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");
+    t!(format!("{:05}", 1.2), "001.2");
+    t!(format!("{:<05}", 1.2), "001.2");
+    t!(format!("{:>05}", 1.2), "001.2");
+    t!(format!("{:^05}", 1.2), "001.2");
+    t!(format!("{:05}", -1.2), "-01.2");
+    t!(format!("{:<05}", -1.2), "-01.2");
+    t!(format!("{:>05}", -1.2), "-01.2");
+    t!(format!("{:^05}", -1.2), "-01.2");
+    t!(format!("{:+05}", 1.2), "+01.2");
+    t!(format!("{:<+05}", 1.2), "+01.2");
+    t!(format!("{:>+05}", 1.2), "+01.2");
+    t!(format!("{:^+05}", 1.2), "+01.2");
 
     // Ergonomic format_args!
     t!(format!("{0:x} {0:X}", 15), "f F");