Restore Writer.write_char, see #10861.

src/libstd/io/mem.rs

@@ -419,6 +419,16 @@ mod test {
         assert_eq!(r.read_to_str(), ~"testingtesting\ntesting");
     }
 
+    #[test]
+    fn test_write_char() {
+        let mut writer = MemWriter::new();
+        writer.write_char('a');
+        writer.write_char('\n');
+        writer.write_char('ệ');
+        let mut r = BufReader::new(*writer.inner_ref());
+        assert_eq!(r.read_to_str(), ~"a\nệ");
+    }
+
     #[test]
     fn test_read_whole_string_bad() {
         let buf = [0xff];

src/libstd/io/mod.rs

@@ -284,6 +284,7 @@ Out of scope
 #[allow(missing_doc)];
 
 use cast;
+use char::Char;
 use condition::Guard;
 use container::Container;
 use int;
@@ -902,6 +903,13 @@ pub trait Writer {
         self.write(['\n' as u8]);
     }
 
+    /// Write a single char, encoded as UTF-8.
+    fn write_char(&mut self, c: char) {
+        let mut buf = [0u8, ..4];
+        let n = c.encode_utf8(buf.as_mut_slice());
+        self.write(buf.slice_to(n));
+    }
+
     /// Write the result of passing n through `int::to_str_bytes`.
     fn write_int(&mut self, n: int) {
         int::to_str_bytes(n, 10u, |bytes| self.write(bytes))