优化链表drop方法

src/too-many-lists/advanced-lists/double-singly.md

@@ -71,9 +71,8 @@ impl<T> Stack<T> {
 
 impl<T> Drop for Stack<T> {
     fn drop(&mut self) {
-        let mut cur_link = self.head.take();
-        while let Some(mut boxed_node) = cur_link {
-            cur_link = boxed_node.next.take();
+        while let Some(box_node) = self.head.take() {
+            self.head = box_node.next;
         }
     }
 }

src/too-many-lists/bad-stack/final-code.md

@@ -137,9 +137,8 @@ impl Drop for Node {
 ```rust
 impl Drop for List {
     fn drop(&mut self) {
-        let mut cur_link = mem::replace(&mut self.head, Link::Empty);
-        while let Link::More(mut boxed_node) = cur_link {
-            cur_link = mem::replace(&mut boxed_node.next, Link::Empty);
+        while let Link::More(box_node) = mem::replace(&mut self.head, Link::Empty) {
+            self.head = box_node.next;
             // boxed_node 在这里超出作用域并被 drop,
             // 由于它的 `next` 字段拥有的 `Node` 被设置为 Link::Empty,
             // 因此这里并不会有无边界的递归发生
@@ -222,10 +221,8 @@ impl List {
 
 impl Drop for List {
     fn drop(&mut self) {
-        let mut cur_link = mem::replace(&mut self.head, Link::Empty);
-
-        while let Link::More(mut boxed_node) = cur_link {
-            cur_link = mem::replace(&mut boxed_node.next, Link::Empty);
+        while let Link::More(box_node) = mem::replace(&mut self.head, Link::Empty) {
+            self.head = box_node.next;
         }
     }
 }

src/too-many-lists/ok-stack/itermut.md

@@ -193,9 +193,8 @@ impl<T> List<T> {
 
 impl<T> Drop for List<T> {
     fn drop(&mut self) {
-        let mut cur_link = self.head.take();
-        while let Some(mut boxed_node) = cur_link {
-            cur_link = boxed_node.next.take();
+        while let Some(box_node) = self.head.take() {
+            self.head = box_node.next;
         }
     }
 }

src/too-many-lists/ok-stack/type-optimizing.md

@@ -53,9 +53,8 @@ impl List {
 
 impl Drop for List {
     fn drop(&mut self) {
-        let mut cur_link = mem::replace(&mut self.head, None);
-        while let Some(mut boxed_node) = cur_link {
-            cur_link = mem::replace(&mut boxed_node.next, None);
+        while let Some(box_node) = mem::replace(&mut self.head, None) {
+            self.head = box_node.next;
         }
     }
 }
@@ -103,9 +102,8 @@ impl List {
 
 impl Drop for List {
     fn drop(&mut self) {
-        let mut cur_link = self.head.take();
-        while let Some(mut boxed_node) = cur_link {
-            cur_link = boxed_node.next.take();
+        while let Some(box_node) = self.head.take() {
+            self.head = box_node.next;
         }
     }
 }
@@ -177,9 +175,8 @@ impl<T> List<T> {
 
 impl<T> Drop for List<T> {
     fn drop(&mut self) {
-        let mut cur_link = self.head.take();
-        while let Some(mut boxed_node) = cur_link {
-            cur_link = boxed_node.next.take();
+        while let Some(box_node) = self.head.take() {
+            self.head = box_node.next;
         }
     }
 }

src/too-many-lists/persistent-stack/drop-arc.md

@@ -5,9 +5,8 @@
 ```rust
 impl<T> Drop for List<T> {
     fn drop(&mut self) {
-        let mut cur_link = self.head.take();
-        while let Some(mut boxed_node) = cur_link {
-            cur_link = boxed_node.next.take();
+        while let Some(box_node) = self.head.take() {
+            self.head = box_node.next;
         }
     }
 }