Merge #247

247: Remove Clone for Guard r=jeehoonkang a=stjepang

While `Guard::clone()` is a nice convenience, it's not really a method we *need*.

By removing the clone impl, we enable some kinds of API. For example, a data structure might want to return a `Guard` to the user and then retrieve it back knowing it has never been cloned.

@Amanieu needs to forbid `Guard::clone()` in a certain use case. I don't remember the exact details, so perhaps he could comment what it is about.

Co-authored-by: Stjepan Glavina <stjepang@gmail.com>

crossbeam-epoch/src/guard.rs

@@ -1,6 +1,5 @@
 use core::fmt;
 use core::mem;
-use core::ptr;
 
 use atomic::Shared;
 use collector::Collector;
@@ -65,15 +64,6 @@ use internal::Local;
 /// assert!(!epoch::is_pinned());
 /// ```
 ///
-/// The same can be achieved by cloning guards:
-///
-/// ```
-/// use crossbeam_epoch as epoch;
-///
-/// let guard1 = epoch::pin();
-/// let guard2 = guard1.clone();
-/// ```
-///
 /// [`pin`]: fn.pin.html
 pub struct Guard {
     pub(crate) local: *const Local,
@@ -433,16 +423,6 @@ impl Drop for Guard {
     }
 }
 
-impl Clone for Guard {
-    #[inline]
-    fn clone(&self) -> Guard {
-        match unsafe { self.local.as_ref() } {
-            None => Guard { local: ptr::null() },
-            Some(local) => local.pin(),
-        }
-    }
-}
-
 impl fmt::Debug for Guard {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.pad("Guard { .. }")