Lift $dst outside the closure in write!

If you were writing to something along the lines of `self.foo` then with the new
closure rules it meant that you were borrowing `self` for the entirety of the
closure, meaning that you couldn't format other fields of `self` at the same
time as writing to a buffer contained in `self`.

By lifting the borrow outside of the closure the borrow checker can better
understand that you're only borrowing one of the fields at a time. This had to
use type ascription as well in order to preserve trait object coercions.

src/libstd/macros.rs

@@ -145,16 +145,18 @@ macro_rules! format(
 
 #[macro_export]
 macro_rules! write(
-    ($dst:expr, $($arg:tt)*) => (
-        format_args!(|args| { ::std::fmt::write($dst, args) }, $($arg)*)
-    )
+    ($dst:expr, $($arg:tt)*) => ({
+        let dst: &mut ::std::io::Writer = $dst;
+        format_args!(|args| { ::std::fmt::write(dst, args) }, $($arg)*)
+    })
 )
 
 #[macro_export]
 macro_rules! writeln(
-    ($dst:expr, $($arg:tt)*) => (
-        format_args!(|args| { ::std::fmt::writeln($dst, args) }, $($arg)*)
-    )
+    ($dst:expr, $($arg:tt)*) => ({
+        let dst: &mut ::std::io::Writer = $dst;
+        format_args!(|args| { ::std::fmt::writeln(dst, args) }, $($arg)*)
+    })
 )
 
 #[macro_export]