replace Drain::tail_len with tail_end

* add tail_len() method
upcoming splice needs this, otherwise incrementing the tail_start would require decrementing
tail_len as well.

Author: cehteh

src/drain.rs

@@ -31,8 +31,8 @@ use crate::HeaderVec;
 pub struct Drain<'a, H, T> {
     /// Index of tail to preserve
     pub(super) tail_start: usize,
-    /// Length of tail
-    pub(super) tail_len: usize,
+    /// End index of tail to preserve
+    pub(super) tail_end: usize,
     /// Current remaining range to remove
     pub(super) iter: slice::Iter<'a, T>,
     pub(super) vec: NonNull<HeaderVec<H, T>>,
@@ -115,13 +115,17 @@ impl<H, T> Drain<'_, H, T> {
                 if tail != (start + unyielded_len) {
                     let src = source_vec.as_ptr().add(tail);
                     let dst = start_ptr.add(unyielded_len);
-                    ptr::copy(src, dst, this.tail_len);
+                    ptr::copy(src, dst, this.tail_len());
                 }
             }
 
-            source_vec.set_len(start + unyielded_len + this.tail_len);
+            source_vec.set_len(start + unyielded_len + this.tail_len());
         }
     }
+
+    pub(crate) fn tail_len(&self) -> usize {
+        self.tail_end - self.tail_start
+    }
 }
 
 impl<H, T> AsRef<[T]> for Drain<'_, H, T> {
@@ -164,7 +168,7 @@ impl<H, T> Drop for Drain<'_, H, T> {
 
         impl<H, T> Drop for DropGuard<'_, '_, H, T> {
             fn drop(&mut self) {
-                if self.0.tail_len > 0 {
+                if self.0.tail_len() > 0 {
                     unsafe {
                         let source_vec = self.0.vec.as_mut();
                         // memmove back untouched tail, update to new length
@@ -173,9 +177,9 @@ impl<H, T> Drop for Drain<'_, H, T> {
                         if tail != start {
                             let src = source_vec.as_ptr().add(tail);
                             let dst = source_vec.as_mut_ptr().add(start);
-                            ptr::copy(src, dst, self.0.tail_len);
+                            ptr::copy(src, dst, self.0.tail_len());
                         }
-                        source_vec.set_len(start + self.0.tail_len);
+                        source_vec.set_len(start + self.0.tail_len());
                     }
                 }
             }
@@ -193,8 +197,8 @@ impl<H, T> Drop for Drain<'_, H, T> {
             unsafe {
                 let vec = vec.as_mut();
                 let old_len = vec.len();
-                vec.set_len(old_len + drop_len + self.tail_len);
-                vec.truncate(old_len + self.tail_len);
+                vec.set_len(old_len + drop_len + self.tail_len());
+                vec.truncate(old_len + self.tail_len());
             }
 
             return;

src/lib.rs

@@ -855,7 +855,7 @@ impl<H, T> HeaderVec<H, T> {
             let range_slice = slice::from_raw_parts(self.as_ptr().add(start), end - start);
             Drain {
                 tail_start: end,
-                tail_len: len - end,
+                tail_end: len,
                 iter: range_slice.iter(),
                 vec: NonNull::from(self),
             }