Rollup merge of #70979 - ssomers:btreemap_the_alice_merton_variations, r=Amanieu

Follow up on BTreeMap comments

r? @Amanieu (for the first commit)

Author: Centril

src/liballoc/collections/btree/map.rs

@@ -2771,8 +2771,6 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInter
                             pos.next_unchecked();
                         }
                     }
-
-                    // This internal node might be underfull, but only if it's the root.
                     break;
                 }
             }

src/liballoc/collections/btree/node.rs

@@ -161,7 +161,7 @@ impl<K, V> Root<K, V> {
         NodeRef {
             height: self.height,
             node: self.node.as_ptr(),
-            root: self as *const _ as *mut _,
+            root: ptr::null(),
             _marker: PhantomData,
         }
     }
@@ -179,7 +179,7 @@ impl<K, V> Root<K, V> {
         NodeRef {
             height: self.height,
             node: self.node.as_ptr(),
-            root: ptr::null_mut(), // FIXME: Is there anything better to do here?
+            root: ptr::null(),
             _marker: PhantomData,
         }
     }