refactor(header): change Header::fmt_header to take a header::Formatter

The `header::Formatter` ensures that a formatted header is written to a
line, and allows for headers that require multiple lines. The only
header to specifically require this is `Set-Cookie`.

BREAKING CHANGE: The `fmt_header` method has changed to take a different
  formatter. In most cases, if your header also implements
  `fmt::Display`, you can just call `f.fmt_line(self)`.

src/header/common/access_control_allow_credentials.rs

@@ -63,14 +63,14 @@ impl Header for AccessControlAllowCredentials {
         Err(::Error::Header)
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        f.write_str("true")
+        f.fmt_line(self)
     }
 }
 
 impl Display for AccessControlAllowCredentials {
     fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
-        self.fmt_header(f)
+        f.write_str("true")
     }
 }
 

src/header/common/access_control_allow_origin.rs

@@ -72,18 +72,18 @@ impl Header for AccessControlAllowOrigin {
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        match *self {
+        f.fmt_line(self)
-            AccessControlAllowOrigin::Any => f.write_str("*"),
-            AccessControlAllowOrigin::Null => f.write_str("null"),
-            AccessControlAllowOrigin::Value(ref url) => Display::fmt(url, f),
-        }
     }
 }
 
 impl Display for AccessControlAllowOrigin {
     fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
-        self.fmt_header(f)
+        match *self {
+            AccessControlAllowOrigin::Any => f.write_str("*"),
+            AccessControlAllowOrigin::Null => f.write_str("null"),
+            AccessControlAllowOrigin::Value(ref url) => Display::fmt(url, f),
+        }
     }
 }
 

src/header/common/authorization.rs

@@ -100,8 +100,8 @@ impl<S: Scheme + Any> Header for Authorization<S> where <S as FromStr>::Err: 'st
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/cache_control.rs

@@ -49,6 +49,7 @@ pub struct CacheControl(pub Vec<CacheDirective>);
 
 __hyper__deref!(CacheControl => Vec<CacheDirective>);
 
+//TODO: this could just be the header! macro
 impl Header for CacheControl {
     fn header_name() -> &'static str {
         static NAME: &'static str = "Cache-Control";
@@ -64,8 +65,8 @@ impl Header for CacheControl {
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/content_disposition.rs

@@ -146,8 +146,8 @@ impl Header for ContentDisposition {
     }
 
     #[inline]
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(&self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/content_length.rs

@@ -60,8 +60,8 @@ impl Header for ContentLength {
     }
 
     #[inline]
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(&self.0, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/cookie.rs

@@ -54,8 +54,8 @@ impl Header for Cookie {
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/expect.rs

@@ -54,8 +54,8 @@ impl Header for Expect {
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/host.rs

@@ -68,17 +68,17 @@ impl Header for Host {
        from_one_raw_str(raw)
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        match self.port {
+        f.fmt_line(self)
-            None | Some(80) | Some(443) => f.write_str(&self.hostname[..]),
-            Some(port) => write!(f, "{}:{}", self.hostname, port)
-        }
     }
 }
 
 impl fmt::Display for Host {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.fmt_header(f)
+        match self.port {
+            None | Some(80) | Some(443) => f.write_str(&self.hostname[..]),
+            Some(port) => write!(f, "{}:{}", self.hostname, port)
+        }
     }
 }
 

src/header/common/if_range.rs

@@ -70,17 +70,17 @@ impl Header for IfRange {
         Err(::Error::Header)
     }
 
-    fn fmt_header(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> ::std::fmt::Result {
-        match *self {
+        f.fmt_line(self)
-            IfRange::EntityTag(ref x) => Display::fmt(x, f),
-            IfRange::Date(ref x) => Display::fmt(x, f),
-        }
     }
 }
 
 impl Display for IfRange {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.fmt_header(f)
+        match *self {
+            IfRange::EntityTag(ref x) => Display::fmt(x, f),
+            IfRange::Date(ref x) => Display::fmt(x, f),
+        }
     }
 }
 

src/header/common/link.rs

@@ -423,14 +423,14 @@ impl Header for Link {
             .unwrap_or(Err(::Error::Header))
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt_delimited(f, self.values.as_slice(), ", ", ("", ""))
+        f.fmt_line(self)
     }
 }
 
 impl fmt::Display for Link {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.fmt_header(f)
+        fmt_delimited(f, self.values.as_slice(), ", ", ("", ""))
     }
 }
 

src/header/common/mod.rs

@@ -204,14 +204,13 @@ macro_rules! header {
             fn parse_header(raw: &$crate::header::Raw) -> $crate::Result<Self> {
                 $crate::header::parsing::from_comma_delimited(raw).map($id)
             }
-            fn fmt_header(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+            fn fmt_header(&self, f: &mut $crate::header::Formatter) -> ::std::fmt::Result {
-                $crate::header::parsing::fmt_comma_delimited(f, &self.0[..])
+                f.fmt_line(self)
             }
         }
         impl ::std::fmt::Display for $id {
             fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
-                use $crate::header::Header;
+                $crate::header::parsing::fmt_comma_delimited(f, &self.0[..])
-                self.fmt_header(f)
             }
         }
     };
@@ -229,14 +228,13 @@ macro_rules! header {
             fn parse_header(raw: &$crate::header::Raw) -> $crate::Result<Self> {
                 $crate::header::parsing::from_comma_delimited(raw).map($id)
             }
-            fn fmt_header(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+            fn fmt_header(&self, f: &mut $crate::header::Formatter) -> ::std::fmt::Result {
-                $crate::header::parsing::fmt_comma_delimited(f, &self.0[..])
+                f.fmt_line(self)
             }
         }
         impl ::std::fmt::Display for $id {
             fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
-                use $crate::header::Header;
+                $crate::header::parsing::fmt_comma_delimited(f, &self.0[..])
-                self.fmt_header(f)
             }
         }
     };
@@ -254,8 +252,8 @@ macro_rules! header {
             fn parse_header(raw: &$crate::header::Raw) -> $crate::Result<Self> {
                 $crate::header::parsing::from_one_raw_str(raw).map($id)
             }
-            fn fmt_header(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+            fn fmt_header(&self, f: &mut $crate::header::Formatter) -> ::std::fmt::Result {
-                ::std::fmt::Display::fmt(&**self, f)
+                f.fmt_line(self)
             }
         }
         impl ::std::fmt::Display for $id {
@@ -289,8 +287,8 @@ macro_rules! header {
             fn parse_header(raw: &$crate::header::Raw) -> $crate::Result<Self> {
                 $crate::header::parsing::from_one_raw_str::<<$value as ::std::borrow::ToOwned>::Owned>(raw).map($id::new)
             }
-            fn fmt_header(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+            fn fmt_header(&self, f: &mut $crate::header::Formatter) -> ::std::fmt::Result {
-                ::std::fmt::Display::fmt(&**self, f)
+                f.fmt_line(self)
             }
         }
         impl ::std::fmt::Display for $id {
@@ -323,7 +321,12 @@ macro_rules! header {
                 }
                 $crate::header::parsing::from_comma_delimited(raw).map($id::Items)
             }
-            fn fmt_header(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+            fn fmt_header(&self, f: &mut $crate::header::Formatter) -> ::std::fmt::Result {
+                f.fmt_line(self)
+            }
+        }
+        impl ::std::fmt::Display for $id {
+            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                 match *self {
                     $id::Any => f.write_str("*"),
                     $id::Items(ref fields) => $crate::header::parsing::fmt_comma_delimited(
@@ -331,12 +334,6 @@ macro_rules! header {
                 }
             }
         }
-        impl ::std::fmt::Display for $id {
-            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
-                use $crate::header::Header;
-                self.fmt_header(f)
-            }
-        }
     };
 
     // optional test module

src/header/common/origin.rs

@@ -104,8 +104,8 @@ impl Header for Origin {
         from_one_raw_str(raw)
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/pragma.rs

@@ -54,8 +54,8 @@ impl Header for Pragma {
         })
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/prefer.rs

@@ -65,8 +65,8 @@ impl Header for Prefer {
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/preference_applied.rs

@@ -63,8 +63,8 @@ impl Header for PreferenceApplied {
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/range.rs

@@ -190,8 +190,8 @@ impl Header for Range {
         from_one_raw_str(raw)
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        Display::fmt(self, f)
+        f.fmt_line(self)
     }
 
 }

src/header/common/referrer_policy.rs

@@ -79,8 +79,8 @@ impl Header for ReferrerPolicy {
         Err(::Error::Header)
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/retry_after.rs

@@ -121,7 +121,13 @@ impl Header for RetryAfter {
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> ::std::fmt::Result {
+        f.fmt_line(self)
+    }
+}
+
+impl fmt::Display for RetryAfter {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
             RetryAfter::Delay(ref duration) => {
                 write!(f, "{}", duration.num_seconds())

src/header/common/set_cookie.rs

@@ -91,16 +91,7 @@ impl Header for SetCookie {
         }
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        if self.0.len() == 1 {
-            write!(f, "{}", &self.0[0])
-        } else {
-            panic!("SetCookie with multiple cookies cannot be used with fmt_header, must use fmt_multi_header");
-        }
-    }
-
-
-    fn fmt_multi_header(&self, f: &mut ::header::MultilineFormatter) -> fmt::Result {
         for cookie in &self.0 {
             try!(f.fmt_line(cookie));
         }

src/header/common/strict_transport_security.rs

@@ -129,8 +129,8 @@ impl Header for StrictTransportSecurity {
         parsing::from_one_raw_str(raw)
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
-        fmt::Display::fmt(self, f)
+        f.fmt_line(self)
     }
 }
 

src/header/common/warning.rs

@@ -96,7 +96,13 @@ impl Header for Warning {
         from_one_raw_str(raw)
     }
 
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut ::header::Formatter) -> fmt::Result {
+        f.fmt_line(self)
+    }
+}
+
+impl fmt::Display for Warning {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self.date {
             Some(date) => write!(f, "{:03} {} \"{}\" \"{}\"", self.code, self.agent, self.text, date),
             None => write!(f, "{:03} {} \"{}\"", self.code, self.agent, self.text)

src/header/internals/item.rs

@@ -4,7 +4,7 @@ use std::fmt;
 use std::str::from_utf8;
 
 use super::cell::{OptCell, PtrMapCell};
-use header::{Header, MultilineFormatter, Multi, raw, Raw};
+use header::{Header, Formatter, Multi, raw, Raw};
 
 
 #[derive(Clone)]
@@ -47,7 +47,7 @@ impl Item {
         }
 
         let mut raw = raw::new();
-        self.write_h1(&mut MultilineFormatter(Multi::Raw(&mut raw))).expect("fmt failed");
+        self.write_h1(&mut Formatter(Multi::Raw(&mut raw))).expect("fmt failed");
         self.raw.set(raw);
 
         self.raw.as_ref().unwrap()
@@ -93,7 +93,7 @@ impl Item {
         }.map(|typed| unsafe { typed.downcast_unchecked() })
     }
 
-    pub fn write_h1(&self, f: &mut MultilineFormatter) -> fmt::Result {
+    pub fn write_h1(&self, f: &mut Formatter) -> fmt::Result {
         match *self.raw {
             Some(ref raw) => {
                 for part in raw.iter() {
@@ -111,7 +111,7 @@ impl Item {
             },
             None => {
                 let typed = unsafe { self.typed.one() };
-                typed.fmt_multi_header(f)
+                typed.fmt_header(f)
             }
         }
     }

src/header/mod.rs

@@ -41,7 +41,7 @@
 //!
 //! ```
 //! use std::fmt;
-//! use hyper::header::{Header, Raw};
+//! use hyper::header::{self, Header, Raw};
 //!
 //! #[derive(Debug, Clone, Copy)]
 //! struct Dnt(bool);
@@ -66,16 +66,16 @@
 //!         Err(hyper::Error::Header)
 //!     }
 //!
-//!     fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+//!     fn fmt_header(&self, f: &mut header::Formatter) -> fmt::Result {
-//!         if self.0 {
+//!         let value = if self.0 {
-//!             f.write_str("1")
+//!             "1"
 //!         } else {
-//!             f.write_str("0")
+//!             "0"
-//!         }
+//!         };
+//!         f.fmt_line(&value)
 //!     }
 //! }
 //! ```
-use std::any::{Any, TypeId};
 use std::borrow::{Cow, ToOwned};
 use std::iter::{FromIterator, IntoIterator};
 use std::{mem, fmt};
@@ -83,6 +83,7 @@ use std::{mem, fmt};
 use unicase::UniCase;
 
 use self::internals::{Item, VecMap, Entry};
+use self::sealed::{GetType, HeaderClone};
 
 pub use self::shared::*;
 pub use self::common::*;
@@ -100,7 +101,7 @@ pub mod parsing;
 ///
 /// This trait represents the construction and identification of headers,
 /// and contains trait-object unsafe methods.
-pub trait Header: HeaderClone + Any + GetType + Send + Sync {
+pub trait Header: HeaderClone + GetType + Send + Sync {
     /// Returns the name of the header field this belongs to.
     ///
     /// This will become an associated constant once available.
@@ -113,26 +114,27 @@ pub trait Header: HeaderClone + Any + GetType + Send + Sync {
     /// than one field value. If that's the case, you **should** return `None`
     /// if `raw.len() > 1`.
     fn parse_header(raw: &Raw) -> ::Result<Self> where Self: Sized;
-    /// Format a header to be output into a TcpStream.
+    /// Format a header to outgoing stream.
     ///
-    /// This method is not allowed to introduce an Err not produced
+    /// Most headers should be formatted on one line, and so a common pattern
-    /// by the passed-in Formatter.
+    /// would be to implement `std::fmt::Display` for this type as well, and
-    fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result;
+    /// then just call `f.fmt_line(self)`.
-    /// Formats a header over multiple lines.
+    ///
+    /// ## Note
+    ///
+    /// This has the ability to format a header over multiple lines.
     ///
     /// The main example here is `Set-Cookie`, which requires that every
-    /// cookie being set be specified in a separate line.
+    /// cookie being set be specified in a separate line. Almost every other
-    ///
+    /// case should only format as 1 single line.
-    /// The API here is still being explored, so this is hidden by default.
-    /// The passed in formatter doesn't have any public methods, so it would
-    /// be quite difficult to depend on this externally.
-    #[doc(hidden)]
     #[inline]
-    fn fmt_multi_header(&self, f: &mut MultilineFormatter) -> fmt::Result {
+    fn fmt_header(&self, f: &mut Formatter) -> fmt::Result;
-        f.fmt_line(&FmtHeader(self))
-    }
 }
 
+mod sealed {
+    use std::any::{Any, TypeId};
+    use super::Header;
+
     #[doc(hidden)]
     pub trait GetType: Any {
         #[inline(always)]
@@ -143,6 +145,18 @@ pub trait GetType: Any {
 
     impl<T: Any> GetType for T {}
 
+    #[doc(hidden)]
+    pub trait HeaderClone {
+        fn clone_box(&self) -> Box<Header + Send + Sync>;
+    }
+
+    impl<T: Header + Clone> HeaderClone for T {
+        #[inline]
+        fn clone_box(&self) -> Box<Header + Send + Sync> {
+            Box::new(self.clone())
+        }
+    }
+
     #[test]
     fn test_get_type() {
         use ::header::{ContentLength, UserAgent};
@@ -159,10 +173,12 @@ fn test_get_type() {
         assert_eq!(TypeId::of::<ContentLength>(), (*len).get_type());
         assert_eq!(TypeId::of::<UserAgent>(), (*agent).get_type());
     }
+}
 
-#[doc(hidden)]
+
+/// A formatter used to serialize headers to an output stream.
 #[allow(missing_debug_implementations)]
-pub struct MultilineFormatter<'a, 'b: 'a>(Multi<'a, 'b>);
+pub struct Formatter<'a, 'b: 'a>(Multi<'a, 'b>);
 
 enum Multi<'a, 'b: 'a> {
     Line(&'a str, &'a mut fmt::Formatter<'b>),
@@ -170,8 +186,20 @@ enum Multi<'a, 'b: 'a> {
     Raw(&'a mut Raw),
 }
 
-impl<'a, 'b> MultilineFormatter<'a, 'b> {
+impl<'a, 'b> Formatter<'a, 'b> {
-    fn fmt_line(&mut self, line: &fmt::Display) -> fmt::Result {
+
+    /// Format one 'line' of a header.
+    ///
+    /// This writes the header name plus the `Display` value as a single line.
+    ///
+    /// ## Note
+    ///
+    /// This has the ability to format a header over multiple lines.
+    ///
+    /// The main example here is `Set-Cookie`, which requires that every
+    /// cookie being set be specified in a separate line. Almost every other
+    /// case should only format as 1 single line.
+    pub fn fmt_line(&mut self, line: &fmt::Display) -> fmt::Result {
         use std::fmt::Write;
         match self.0 {
             Multi::Line(ref name, ref mut f) => {
@@ -198,28 +226,19 @@ impl<'a, 'b> MultilineFormatter<'a, 'b> {
     }
 }
 
-// Internal helper to wrap fmt_header into a fmt::Display
-struct FmtHeader<'a, H: ?Sized + 'a>(&'a H);
-
-impl<'a, H: Header + ?Sized + 'a> fmt::Display for FmtHeader<'a, H> {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.0.fmt_header(f)
-    }
-}
-
 struct ValueString<'a>(&'a Item);
 
 impl<'a> fmt::Debug for ValueString<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         try!(f.write_str("\""));
-        try!(self.0.write_h1(&mut MultilineFormatter(Multi::Join(true, f))));
+        try!(self.0.write_h1(&mut Formatter(Multi::Join(true, f))));
         f.write_str("\"")
     }
 }
 
 impl<'a> fmt::Display for ValueString<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.0.write_h1(&mut MultilineFormatter(Multi::Join(true, f)))
+        self.0.write_h1(&mut Formatter(Multi::Join(true, f)))
     }
 }
 
@@ -229,7 +248,7 @@ impl<'a, H: Header> fmt::Debug for HeaderValueString<'a, H> {
     #[inline]
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         try!(f.write_str("\""));
-        try!(self.0.fmt_multi_header(&mut MultilineFormatter(Multi::Join(true, f))));
+        try!(self.0.fmt_header(&mut Formatter(Multi::Join(true, f))));
         f.write_str("\"")
     }
 }
@@ -254,17 +273,6 @@ impl<'a, F: fmt::Write + 'a> fmt::Write for NewlineReplacer<'a, F> {
     }
 }
 
-#[doc(hidden)]
-pub trait HeaderClone {
-    fn clone_box(&self) -> Box<Header + Send + Sync>;
-}
-
-impl<T: Header + Clone> HeaderClone for T {
-    #[inline]
-    fn clone_box(&self) -> Box<Header + Send + Sync> {
-        Box::new(self.clone())
-    }
-}
 
 impl Header + Send + Sync {
     // A trait object looks like this:
@@ -606,7 +614,7 @@ impl<'a> HeaderView<'a> {
 
 impl<'a> fmt::Display for HeaderView<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.1.write_h1(&mut MultilineFormatter(Multi::Line(self.0.as_ref(), f)))
+        self.1.write_h1(&mut Formatter(Multi::Line(self.0.as_ref(), f)))
     }
 }
 
@@ -746,7 +754,13 @@ mod tests {
             }
         }
 
-        fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fn fmt_header(&self, f: &mut super::Formatter) -> fmt::Result {
+            f.fmt_line(self)
+        }
+    }
+
+    impl fmt::Display for CrazyLength {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
             let CrazyLength(ref opt, ref value) = *self;
             write!(f, "{:?}, {:?}", opt, value)
         }