Merge pull request rust-num#150 from withoutboats/to_writer_trait_objects

Allow trait objects in the to_writer function.

Author: dtolnay

json/src/ser.rs

@@ -950,7 +950,7 @@ fn fmt_f64_or_null<W>(wr: &mut W, value: f64) -> Result<()>
 
 /// Encode the specified struct into a json `[u8]` writer.
 #[inline]
-pub fn to_writer<W, T>(writer: &mut W, value: &T) -> Result<()>
+pub fn to_writer<W: ?Sized, T>(writer: &mut W, value: &T) -> Result<()>
     where W: io::Write,
           T: ser::Serialize,
 {
@@ -961,7 +961,7 @@ pub fn to_writer<W, T>(writer: &mut W, value: &T) -> Result<()>
 
 /// Encode the specified struct into a json `[u8]` writer.
 #[inline]
-pub fn to_writer_pretty<W, T>(writer: &mut W, value: &T) -> Result<()>
+pub fn to_writer_pretty<W: ?Sized, T>(writer: &mut W, value: &T) -> Result<()>
     where W: io::Write,
           T: ser::Serialize,
 {