auto merge of #14258 : alexcrichton/rust/dox-format-writer, r=cmr

This commit fills in the documentation holes for the FormatWriter trait which
were previously accidentally left blank. Additionally, this adds the `write_fmt`
method to the trait to allow usage of the `write!` macro with implementors of
the `FormatWriter` trait. This is not useful for consumers of the standard
library who should generally avoid the `FormatWriter` trait, but it is useful
for consumers of the core library who are not using the standard library.

src/test/run-pass/colorful-write-macros.rs

@@ -14,12 +14,22 @@
 #![feature(macro_rules)]
 
 use std::io::MemWriter;
+use std::fmt;
+use std::fmt::FormatWriter;
 
 struct Foo<'a> {
     writer: &'a mut Writer,
     other: &'a str,
 }
 
+struct Bar;
+
+impl fmt::FormatWriter for Bar {
+    fn write(&mut self, _: &[u8]) -> fmt::Result {
+        Ok(())
+    }
+}
+
 fn borrowing_writer_from_struct_and_formatting_struct_field(foo: Foo) {
     write!(foo.writer, "{}", foo.other);
 }
@@ -29,4 +39,7 @@ fn main() {
     write!(&mut w as &mut Writer, "");
     write!(&mut w, ""); // should coerce
     println!("ok");
+
+    let mut s = Bar;
+    write!(&mut s, "test");
 }