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.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libsyntax/ext/format.rs b/src/libsyntax/ext/format.rs
index 0994abaadc7..aa2c51c0225 100644
--- a/src/libsyntax/ext/format.rs
+++ b/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"))
                     }