Auto merge of #81956 - ssomers:btree_post_75200, r=Mark-Simulacrum

BTree: remove outdated traces of coercions

The introduction of `marker::ValMut` (#75200) meant iterators no longer see mutable keys but their code still pretends it does. And settle on the majority style `Some(unsafe {…})` over `unsafe { Some(…) }`.

r? `@Mark-Simulacrum`

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

@@ -1279,7 +1279,7 @@ impl<'a, K: 'a, V: 'a> Iterator for Iter<'a, K, V> {
             None
         } else {
             self.length -= 1;
-            unsafe { Some(self.range.next_unchecked()) }
+            Some(unsafe { self.range.next_unchecked() })
         }
     }
 
@@ -1310,7 +1310,7 @@ impl<'a, K: 'a, V: 'a> DoubleEndedIterator for Iter<'a, K, V> {
             None
         } else {
             self.length -= 1;
-            unsafe { Some(self.range.next_back_unchecked()) }
+            Some(unsafe { self.range.next_back_unchecked() })
         }
     }
 }
@@ -1348,8 +1348,7 @@ impl<'a, K: 'a, V: 'a> Iterator for IterMut<'a, K, V> {
             None
         } else {
             self.length -= 1;
-            let (k, v) = unsafe { self.range.next_unchecked() };
-            Some((k, v)) // coerce k from `&mut K` to `&K`
+            Some(unsafe { self.range.next_unchecked() })
         }
     }
 
@@ -1377,8 +1376,7 @@ impl<'a, K: 'a, V: 'a> DoubleEndedIterator for IterMut<'a, K, V> {
             None
         } else {
             self.length -= 1;
-            let (k, v) = unsafe { self.range.next_back_unchecked() };
-            Some((k, v)) // coerce k from `&mut K` to `&K`
+            Some(unsafe { self.range.next_back_unchecked() })
         }
     }
 }
@@ -1700,7 +1698,7 @@ impl<'a, K, V> Iterator for Range<'a, K, V> {
     type Item = (&'a K, &'a V);
 
     fn next(&mut self) -> Option<(&'a K, &'a V)> {
-        if self.is_empty() { None } else { unsafe { Some(self.next_unchecked()) } }
+        if self.is_empty() { None } else { Some(unsafe { self.next_unchecked() }) }
     }
 
     fn last(mut self) -> Option<(&'a K, &'a V)> {
@@ -1864,12 +1862,7 @@ impl<'a, K, V> Iterator for RangeMut<'a, K, V> {
     type Item = (&'a K, &'a mut V);
 
     fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
-        if self.is_empty() {
-            None
-        } else {
-            let (k, v) = unsafe { self.next_unchecked() };
-            Some((k, v)) // coerce k from `&mut K` to `&K`
-        }
+        if self.is_empty() { None } else { Some(unsafe { self.next_unchecked() }) }
     }
 
     fn last(mut self) -> Option<(&'a K, &'a mut V)> {
@@ -1907,12 +1900,7 @@ impl<'a, K, V> RangeMut<'a, K, V> {
 #[stable(feature = "btree_range", since = "1.17.0")]
 impl<'a, K, V> DoubleEndedIterator for RangeMut<'a, K, V> {
     fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
-        if self.is_empty() {
-            None
-        } else {
-            let (k, v) = unsafe { self.next_back_unchecked() };
-            Some((k, v)) // coerce k from `&mut K` to `&K`
-        }
+        if self.is_empty() { None } else { Some(unsafe { self.next_back_unchecked() }) }
     }
 }