Update for review comments

Thank you Gankro & Alex Crichton

Author: Ulrik Sverdrup

src/libcollections/string.rs

@@ -17,7 +17,6 @@ use core::prelude::*;
 use core::fmt;
 use core::hash;
 use core::iter::FromIterator;
-use core::marker::PhantomData;
 use core::mem;
 use core::ops::{self, Deref, Add, Index};
 use core::ptr;
@@ -745,10 +744,9 @@ impl String {
 
         Drain {
             start: start,
-            tail_start: end,
+            end: end,
             iter: chars_iter,
             string: self_ptr,
-            _marker: PhantomData,
         }
     }
 }
@@ -1132,14 +1130,14 @@ impl fmt::Write for String {
 /// A draining iterator for `String`.
 #[unstable(feature = "collections_drain", reason = "recently added")]
 pub struct Drain<'a> {
+    /// Will be used as &'a mut String in the destructor
     string: *mut String,
     /// Start of part to remove
     start: usize,
-    /// Index of tail to preserve
-    tail_start: usize,
+    /// End of part to remove
+    end: usize,
     /// Current remaining range to remove
     iter: Chars<'a>,
-    _marker: PhantomData<&'a mut String>,
 }
 
 unsafe impl<'a> Sync for Drain<'a> {}
@@ -1149,15 +1147,12 @@ unsafe impl<'a> Send for Drain<'a> {}
 impl<'a> Drop for Drain<'a> {
     fn drop(&mut self) {
         unsafe {
-            // memmove back untouched tail, then truncate & reset length
+            // Use Vec::drain. "Reaffirm" the bounds checks to avoid
+            // panic code being inserted again.
             let self_vec = (*self.string).as_mut_vec();
-            let tail_len = self_vec.len() - self.tail_start;
-            if tail_len > 0 {
-                let src = self_vec.as_ptr().offset(self.tail_start as isize);
-                let dst = self_vec.as_mut_ptr().offset(self.start as isize);
-                ptr::copy(src, dst, tail_len);
+            if self.start <= self.end && self.end <= self_vec.len() {
+                self_vec.drain(self.start..self.end);
             }
-            self_vec.set_len(self.start + tail_len);
         }
     }
 }
@@ -1183,6 +1178,3 @@ impl<'a> DoubleEndedIterator for Drain<'a> {
         self.iter.next_back()
     }
 }
-
-#[unstable(feature = "collections_drain", reason = "recently added")]
-impl<'a> ExactSizeIterator for Drain<'a> { }