Rollup merge of #78480 - ssomers:btree-alias, r=Mark-Simulacrum

BTreeMap: fix pointer provenance rules

Fixes #78477 and includes #78476

r? `@Mark-Simulacrum`

Author: Dylan-DPC

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

@@ -72,7 +72,7 @@ where
     // is an index -- not a reference.
     let len = node.len();
     for i in 0..len {
-        let k = unsafe { node.key_at(i) };
+        let k = unsafe { node.reborrow().key_at(i) };
         match key.cmp(k.borrow()) {
             Ordering::Greater => {}
             Ordering::Equal => return (i, true),

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

@@ -78,6 +78,7 @@
 #![cfg_attr(test, feature(new_uninit))]
 #![feature(allocator_api)]
 #![feature(array_chunks)]
+#![feature(array_methods)]
 #![feature(array_value_iter)]
 #![feature(array_windows)]
 #![feature(allow_internal_unstable)]