Pick a feature name for write_char

src/libcore/fmt/mod.rs

@@ -93,7 +93,7 @@ pub trait Write {
     /// # Errors
     ///
     /// This function will return an instance of `FormatError` on error.
-    #[stable(feature = "rust1", since = "1.1.0")]
+    #[stable(feature = "fmt_write_char", since = "1.1.0")]
     fn write_char(&mut self, c: char) -> Result {
         let mut utf_8 = [0u8; 4];
         let bytes_written = c.encode_utf8(&mut utf_8).unwrap_or(0);