auto merge of #10930 : DaGenix/rust/remove-unnecessary-fields, r=alexcrichton

3 minor clean-ups now that #9629 is fixed:

* Update MutChunkIter to remove the ```remainder``` that existed just to allow the size_hint() method to be implemented. This is no longer necessary since we can just access the length of the slice directly.
* Update MutSplitIterator to address the FIXME in its size_hint() method. This method was only partially implemented due to the issue. Also, implement a minor optimization in the case that its the last iteration.
* Update ByRef iterator to implement the size_hint() method.

I noticed that MutSplitIterator returns an empty slice if called on an empty slice. I don't know if this is intended or not, but I left the ```finished``` field in-place to preserve this behavior.

@TeXitoi @blake2-ppc

src/libstd/iter.rs

@@ -795,7 +795,8 @@ pub struct ByRef<'a, T> {
 impl<'a, A, T: Iterator<A>> Iterator<A> for ByRef<'a, T> {
     #[inline]
     fn next(&mut self) -> Option<A> { self.iter.next() }
-    // FIXME: #9629 we cannot implement &self methods like size_hint on ByRef
+    #[inline]
+    fn size_hint(&self) -> (uint, Option<uint>) { self.iter.size_hint() }
 }
 
 impl<'a, A, T: DoubleEndedIterator<A>> DoubleEndedIterator<A> for ByRef<'a, T> {

src/libstd/vec.rs

@@ -2127,8 +2127,7 @@ impl<'a,T> MutableVector<'a, T> for &'a mut [T] {
     #[inline]
     fn mut_chunks(self, chunk_size: uint) -> MutChunkIter<'a, T> {
         assert!(chunk_size > 0);
-        let len = self.len();
-        MutChunkIter { v: self, chunk_size: chunk_size, remaining: len }
+        MutChunkIter { v: self, chunk_size: chunk_size }
     }
 
     fn mut_shift_ref(&mut self) -> &'a mut T {
@@ -2529,13 +2528,13 @@ impl<'a, T> Iterator<&'a mut [T]> for MutSplitIterator<'a, T> {
 
     #[inline]
     fn size_hint(&self) -> (uint, Option<uint>) {
-        if self.finished { return (0, Some(0)) }
-
-        // if the predicate doesn't match anything, we yield one slice
-        // if it matches every element, we yield len+1 empty slices.
-        // FIXME #9629
-        //(1, Some(self.v.len() + 1))
-        (1, None)
+        if self.finished {
+            (0, Some(0))
+        } else {
+            // if the predicate doesn't match anything, we yield one slice
+            // if it matches every element, we yield len+1 empty slices.
+            (1, Some(self.v.len() + 1))
+        }
     }
 }
 
@@ -2548,10 +2547,7 @@ impl<'a, T> DoubleEndedIterator<&'a mut [T]> for MutSplitIterator<'a, T> {
             None => {
                 self.finished = true;
                 let tmp = util::replace(&mut self.v, &mut []);
-                let len = tmp.len();
-                let (head, tail) = tmp.mut_split_at(len);
-                self.v = tail;
-                Some(head)
+                Some(tmp)
             }
             Some(idx) => {
                 let tmp = util::replace(&mut self.v, &mut []);
@@ -2568,31 +2564,29 @@ impl<'a, T> DoubleEndedIterator<&'a mut [T]> for MutSplitIterator<'a, T> {
 /// the remainder.
 pub struct MutChunkIter<'a, T> {
     priv v: &'a mut [T],
-    priv chunk_size: uint,
-    priv remaining: uint
+    priv chunk_size: uint
 }
 
 impl<'a, T> Iterator<&'a mut [T]> for MutChunkIter<'a, T> {
     #[inline]
     fn next(&mut self) -> Option<&'a mut [T]> {
-        if self.remaining == 0 {
+        if self.v.len() == 0 {
             None
         } else {
-            let sz = cmp::min(self.remaining, self.chunk_size);
+            let sz = cmp::min(self.v.len(), self.chunk_size);
             let tmp = util::replace(&mut self.v, &mut []);
             let (head, tail) = tmp.mut_split_at(sz);
             self.v = tail;
-            self.remaining -= sz;
             Some(head)
         }
     }
 
     #[inline]
     fn size_hint(&self) -> (uint, Option<uint>) {
-        if self.remaining == 0 {
+        if self.v.len() == 0 {
             (0, Some(0))
         } else {
-            let (n, rem) = self.remaining.div_rem(&self.chunk_size);
+            let (n, rem) = self.v.len().div_rem(&self.chunk_size);
             let n = if rem > 0 { n + 1 } else { n };
             (n, Some(n))
         }
@@ -2602,15 +2596,15 @@ impl<'a, T> Iterator<&'a mut [T]> for MutChunkIter<'a, T> {
 impl<'a, T> DoubleEndedIterator<&'a mut [T]> for MutChunkIter<'a, T> {
     #[inline]
     fn next_back(&mut self) -> Option<&'a mut [T]> {
-        if self.remaining == 0 {
+        if self.v.len() == 0 {
             None
         } else {
-            let remainder = self.remaining % self.chunk_size;
+            let remainder = self.v.len() % self.chunk_size;
             let sz = if remainder != 0 { remainder } else { self.chunk_size };
             let tmp = util::replace(&mut self.v, &mut []);
-            let (head, tail) = tmp.mut_split_at(self.remaining - sz);
+            let tmp_len = tmp.len();
+            let (head, tail) = tmp.mut_split_at(tmp_len - sz);
             self.v = head;
-            self.remaining -= sz;
             Some(tail)
         }
     }