auto merge of #16885 : wickerwaka/rust/fmt-center, r=alexcrichton

Use '^' to specify center alignment in format strings.

```
fmt!( "[{:^5s}]", "Hi" ) -> "[ Hi  ]"
fmt!( "[{:^5s}]", "H" )  -> "[  H  ]"
fmt!( "[{:^5d}]", 1i )   -> "[  1  ]"
fmt!( "[{:^5d}]", -1i )  -> "[ -1  ]"
fmt!( "[{:^6d}]", 1i )   -> "[  1   ]"
fmt!( "[{:^6d}]", -1i )  -> "[  -1  ]"
```

If the padding is odd then the padding on the right will be one
character longer than the padding on the left.

src/libcore/fmt/mod.rs

@@ -461,19 +461,28 @@ impl<'a> Formatter<'a> {
         use char::Char;
         let align = match self.align {
             rt::AlignUnknown => default,
-            rt::AlignLeft | rt::AlignRight => self.align
+            _ => self.align
         };
-        if align == rt::AlignLeft {
-            try!(f(self));
-        }
+
+        let (pre_pad, post_pad) = match align {
+            rt::AlignLeft => (0u, padding),
+            rt::AlignRight | rt::AlignUnknown => (padding, 0u),
+            rt::AlignCenter => (padding / 2, (padding + 1) / 2),
+        };
+
         let mut fill = [0u8, ..4];
         let len = self.fill.encode_utf8(fill).unwrap_or(0);
-        for _ in range(0, padding) {
+
+        for _ in range(0, pre_pad) {
             try!(self.buf.write(fill.slice_to(len)));
         }
-        if align == rt::AlignRight {
-            try!(f(self));
+
+        try!(f(self));
+
+        for _ in range(0, post_pad) {
+            try!(self.buf.write(fill.slice_to(len)));
         }
+
         Ok(())
     }
 

src/libcore/fmt/rt.rs

@@ -43,6 +43,8 @@ pub enum Alignment {
     AlignLeft,
     /// Indication that contents should be right-aligned.
     AlignRight,
+    /// Indication that contents should be center-aligned.
+    AlignCenter,
     /// No alignment was requested.
     AlignUnknown,
 }

src/libfmt_macros/lib.rs

@@ -81,6 +81,8 @@ pub enum Alignment {
     AlignLeft,
     /// The value will be aligned to the right.
     AlignRight,
+    /// The value will be aligned in the center.
+    AlignCenter,
     /// The value will take on a default alignment.
     AlignUnknown,
 }
@@ -279,7 +281,7 @@ impl<'a> Parser<'a> {
         match self.cur.clone().next() {
             Some((_, c)) => {
                 match self.cur.clone().skip(1).next() {
-                    Some((_, '>')) | Some((_, '<')) => {
+                    Some((_, '>')) | Some((_, '<')) | Some((_, '^')) => {
                         spec.fill = Some(c);
                         self.cur.next();
                     }
@@ -293,6 +295,8 @@ impl<'a> Parser<'a> {
             spec.align = AlignLeft;
         } else if self.consume('>') {
             spec.align = AlignRight;
+        } else if self.consume('^') {
+            spec.align = AlignCenter;
         }
         // Sign flags
         if self.consume('+') {

src/libstd/fmt.rs

@@ -333,7 +333,7 @@ argument := integer | identifier
 
 format_spec := [[fill]align][sign]['#'][0][width]['.' precision][type]
 fill := character
-align := '<' | '>'
+align := '<' | '^' | '>'
 sign := '+' | '-'
 width := count
 precision := count | '*'
@@ -357,6 +357,7 @@ parameter. This indicates that if the value being formatted is smaller than
 are specified by `fill`, and the alignment can be one of two options:
 
 * `<` - the argument is left-aligned in `width` columns
+* `^` - the argument is center-aligned in `width` columns
 * `>` - the argument is right-aligned in `width` columns
 
 ### Sign/#/0

src/libsyntax/ext/format.rs

@@ -430,6 +430,9 @@ impl<'a, 'b> Context<'a, 'b> {
                     parse::AlignRight => {
                         self.ecx.path_global(sp, self.rtpath("AlignRight"))
                     }
+                    parse::AlignCenter => {
+                        self.ecx.path_global(sp, self.rtpath("AlignCenter"))
+                    }
                     parse::AlignUnknown => {
                         self.ecx.path_global(sp, self.rtpath("AlignUnknown"))
                     }

src/test/run-pass/ifmt.rs

@@ -90,11 +90,16 @@ pub fn main() {
     t!(format!("{:4s}", "a"), "a   ");
     t!(format!("{:>4s}", "a"), "   a");
     t!(format!("{:<4s}", "a"), "a   ");
+    t!(format!("{:^5s}", "a"),  "  a  ");
+    t!(format!("{:^5s}", "aa"), " aa  ");
+    t!(format!("{:^4s}", "a"),  " a  ");
+    t!(format!("{:^4s}", "aa"), " aa ");
     t!(format!("{:.4s}", "a"), "a");
     t!(format!("{:4.4s}", "a"), "a   ");
     t!(format!("{:4.4s}", "aaaaaaaaaaaaaaaaaa"), "aaaa");
     t!(format!("{:<4.4s}", "aaaaaaaaaaaaaaaaaa"), "aaaa");
     t!(format!("{:>4.4s}", "aaaaaaaaaaaaaaaaaa"), "aaaa");
+    t!(format!("{:^4.4s}", "aaaaaaaaaaaaaaaaaa"), "aaaa");
     t!(format!("{:>10.4s}", "aaaaaaaaaaaaaaaaaa"), "aaaa");
     t!(format!("{:2.4s}", "aaaaa"), "aaaa");
     t!(format!("{:2.4s}", "aaaa"), "aaaa");