Auto merge of #48945 - clarcharr:iter_exhaust, r=Kimundi

Replace manual iterator exhaust with for_each(drop)

This originally added a dedicated method, `Iterator::exhaust`, and has since been replaced with `for_each(drop)`, which is more idiomatic.

<del>This is just shorthand for `for _ in &mut self {}` or `while let Some(_) = self.next() {}`. This states the intent a lot more clearly than the identical code: run the iterator to completion.

<del>At least personally, my eyes tend to gloss over `for _ in &mut self {}` without fully paying attention to what it does; having a `Drop` implementation akin to:

<del>`for _ in &mut self {}; unsafe { free(self.ptr); }`</del>

<del>Is not as clear as:

<del>`self.exhaust(); unsafe { free(self.ptr); }`

<del>Additionally, I've seen debate over whether `while let Some(_) = self.next() {}` or `for _ in &mut self {}` is more clear, whereas `self.exhaust()` is clearer than both.

src/liballoc/btree/map.rs

@@ -1287,8 +1287,7 @@ impl<K, V> IntoIterator for BTreeMap<K, V> {
 #[stable(feature = "btree_drop", since = "1.7.0")]
 impl<K, V> Drop for IntoIter<K, V> {
     fn drop(&mut self) {
-        for _ in &mut *self {
-        }
+        self.for_each(drop);
         unsafe {
             let leaf_node = ptr::read(&self.front).into_node();
             if let Some(first_parent) = leaf_node.deallocate_and_ascend() {

src/liballoc/linked_list.rs

@@ -1019,7 +1019,7 @@ impl<'a, T, F> Drop for DrainFilter<'a, T, F>
     where F: FnMut(&mut T) -> bool,
 {
     fn drop(&mut self) {
-        for _ in self { }
+        self.for_each(drop);
     }
 }
 

src/liballoc/vec.rs

@@ -2521,7 +2521,7 @@ impl<'a, T> DoubleEndedIterator for Drain<'a, T> {
 impl<'a, T> Drop for Drain<'a, T> {
     fn drop(&mut self) {
         // exhaust self first
-        while let Some(_) = self.next() {}
+        self.for_each(drop);
 
         if self.tail_len > 0 {
             unsafe {
@@ -2590,9 +2590,7 @@ impl<'a, I: Iterator> ExactSizeIterator for Splice<'a, I> {}
 #[stable(feature = "vec_splice", since = "1.21.0")]
 impl<'a, I: Iterator> Drop for Splice<'a, I> {
     fn drop(&mut self) {
-        // exhaust drain first
-        while let Some(_) = self.drain.next() {}
-
+        self.drain.by_ref().for_each(drop);
 
         unsafe {
             if self.drain.tail_len == 0 {
@@ -2721,8 +2719,7 @@ impl<'a, T, F> Drop for DrainFilter<'a, T, F>
     where F: FnMut(&mut T) -> bool,
 {
     fn drop(&mut self) {
-        for _ in self.by_ref() { }
-
+        self.for_each(drop);
         unsafe {
             self.vec.set_len(self.old_len - self.del);
         }

src/liballoc/vec_deque.rs

@@ -2250,7 +2250,7 @@ unsafe impl<'a, T: Send> Send for Drain<'a, T> {}
 #[stable(feature = "drain", since = "1.6.0")]
 impl<'a, T: 'a> Drop for Drain<'a, T> {
     fn drop(&mut self) {
-        for _ in self.by_ref() {}
+        self.for_each(drop);
 
         let source_deque = unsafe { self.deque.as_mut() };
 

src/librustc_data_structures/array_vec.rs

@@ -207,7 +207,7 @@ pub struct Iter<A: Array> {
 
 impl<A: Array> Drop for Iter<A> {
     fn drop(&mut self) {
-        for _ in self {}
+        self.for_each(drop);
     }
 }
 
@@ -251,7 +251,7 @@ impl<'a, A: Array> Iterator for Drain<'a, A> {
 impl<'a, A: Array> Drop for Drain<'a, A> {
     fn drop(&mut self) {
         // exhaust self first
-        while let Some(_) = self.next() {}
+        self.for_each(drop);
 
         if self.tail_len > 0 {
             unsafe {

src/libstd/collections/hash/table.rs

@@ -1124,7 +1124,7 @@ impl<'a, K, V> ExactSizeIterator for Drain<'a, K, V> {
 
 impl<'a, K: 'a, V: 'a> Drop for Drain<'a, K, V> {
     fn drop(&mut self) {
-        for _ in self {}
+        self.for_each(drop);
     }
 }