feat(headers): adds re-parsing ability when getting typed headers

BREAKING CHANGE: added requirement that all HeaderFormat implementations
  must also be fmt::Debug. This likely as easy as slapping
  #[derive(Debug)] on to any custom headers.

src/header/internals/cell.rs

@@ -0,0 +1,131 @@
+use std::any::{Any, TypeId};
+use std::cell::UnsafeCell;
+use std::collections::HashMap;
+use std::fmt;
+use std::mem;
+use std::ops::Deref;
+
+pub struct OptCell<T>(UnsafeCell<Option<T>>);
+
+impl<T> OptCell<T> {
+    #[inline]
+    pub fn new(val: Option<T>) -> OptCell<T> {
+        OptCell(UnsafeCell::new(val))
+    }
+
+    #[inline]
+    pub fn set(&self, val: T) {
+        unsafe {
+            let opt = self.0.get();
+            debug_assert!((*opt).is_none());
+            *opt = Some(val)
+        }
+    }
+
+    #[inline]
+    pub unsafe fn get_mut(&mut self) -> &mut T {
+        let opt = &mut *self.0.get();
+        opt.as_mut().unwrap()
+    }
+}
+
+impl<T> Deref for OptCell<T> {
+    type Target = Option<T>;
+    #[inline]
+    fn deref<'a>(&'a self) -> &'a Option<T> {
+        unsafe { &*self.0.get() }
+    }
+}
+
+impl<T: Clone> Clone for OptCell<T> {
+    #[inline]
+    fn clone(&self) -> OptCell<T> {
+        OptCell::new((**self).clone())
+    }
+}
+
+pub struct PtrMapCell<V: ?Sized>(UnsafeCell<PtrMap<Box<V>>>);
+
+#[derive(Clone, Debug)]
+enum PtrMap<T> {
+    Empty,
+    One(TypeId, T),
+    Many(HashMap<TypeId, T>)
+}
+
+impl<V: ?Sized + fmt::Debug + Any + 'static> PtrMapCell<V> {
+    #[inline]
+    pub fn new() -> PtrMapCell<V> {
+        PtrMapCell(UnsafeCell::new(PtrMap::Empty))
+    }
+
+    #[inline]
+    pub fn get(&self, key: TypeId) -> Option<&V> {
+        let map = unsafe { &*self.0.get() };
+        match *map {
+            PtrMap::Empty => None,
+            PtrMap::One(id, ref v) => if id == key {
+                Some(v)
+            } else {
+                None
+            },
+            PtrMap::Many(ref hm) => hm.get(&key)
+        }.map(|val| &**val)
+    }
+
+    #[inline]
+    pub fn get_mut(&mut self, key: TypeId) -> Option<&mut V> {
+        let mut map = unsafe { &mut *self.0.get() };
+        match *map {
+            PtrMap::Empty => None,
+            PtrMap::One(id, ref mut v) => if id == key {
+                Some(v)
+            } else {
+                None
+            },
+            PtrMap::Many(ref mut hm) => hm.get_mut(&key)
+        }.map(|val| &mut **val)
+    }
+
+    #[inline]
+    pub unsafe fn insert(&self, key: TypeId, val: Box<V>) {
+        let mut map = &mut *self.0.get();
+        match *map {
+            PtrMap::Empty => *map = PtrMap::One(key, val),
+            PtrMap::One(..) => {
+                let one = mem::replace(map, PtrMap::Empty);
+                match one {
+                    PtrMap::One(id, one) => {
+                        debug_assert!(id != key);
+                        let mut hm = HashMap::with_capacity(2);
+                        hm.insert(id, one);
+                        hm.insert(key, val);
+                        mem::replace(map, PtrMap::Many(hm));
+                    },
+                    _ => unreachable!()
+                }
+            },
+            PtrMap::Many(ref mut hm) => { hm.insert(key, val); }
+        }
+    }
+
+    #[inline]
+    pub unsafe fn one(&self) -> &V {
+        let map = &*self.0.get();
+        match *map {
+            PtrMap::One(_, ref one) => one,
+            _ => panic!("not PtrMap::One value, {:?}", *map)
+        }
+    }
+}
+
+impl<V: ?Sized + fmt::Debug + Any + 'static> Clone for PtrMapCell<V> where Box<V>: Clone {
+    #[inline]
+    fn clone(&self) -> PtrMapCell<V> {
+        let cell = PtrMapCell::new();
+        unsafe {
+            *cell.0.get() = (&*self.0.get()).clone()
+        }
+        cell
+    }
+}