btreemap-alloc: fix clear impl

Author: exrook

library/alloc/src/collections/btree/map.rs

@@ -571,11 +571,12 @@ impl<K, V, A: Allocator> BTreeMap<K, V, A> {
  /// ```
  #[stable(feature = "rust1", since = "1.0.0")]
  pub fn clear(&mut self) {
- let alloc = unsafe {
- // drop all elements and retrieve allocator
- ptr::read(self).into_iter().into_alloc()
- };
- *self = BTreeMap::new_in(alloc);
+ // avoid moving the allocator
+ mem::drop(BTreeMap {
+ root: mem::replace(&mut self.root, None),
+ length: mem::replace(&mut self.length, 0),
+ alloc: ManuallyDrop::new(&*self.alloc),
+ });
  }
 
  /// Makes a new empty BTreeMap with a reasonable choice for B.
@@ -1594,11 +1595,6 @@ impl<K, V, A: Allocator> IntoIterator for BTreeMap<K, V, A> {
 #[stable(feature = "btree_drop", since = "1.7.0")]
 impl<K, V, A: Allocator> Drop for IntoIter<K, V, A> {
  fn drop(&mut self) {
- self.dealloc()
- }
-}
-impl<K, V, A: Allocator> IntoIter<K, V, A> {
- fn dealloc(&mut self) {
  struct DropGuard<'a, K, V, A: Allocator>(&'a mut IntoIter<K, V, A>);
 
  impl<'a, K, V, A: Allocator> Drop for DropGuard<'a, K, V, A> {
@@ -1649,11 +1645,6 @@ impl<K, V, A: Allocator> IntoIter<K, V, A> {
  Some(unsafe { self.range.deallocating_next_back_unchecked(&self.alloc) })
  }
  }
- fn into_alloc(mut self) -> A {
- self.dealloc(); // Deallocate, then don't drop as drop will also call dealloc
- let iter = ManuallyDrop::new(self);
- unsafe { ptr::read(&iter.alloc) }
- }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]