Rollup merge of #67725 - ssomers:into_key_slice_mut, r=RalfJung

Simplify into_key_slice_mut

Remove a rare and tiny but superfluous run-time check from into_key_slice_mut.

In #67459, I wrote that "`get_mut` [...] does visit `into_key_slice_mut`" and that was wrong. No function that operates on a map that (still) has a shared root ever dives into `into_key_slice_mut`.  So it's more clear to remove the (previously existing, and always incomplete) code it has for dealing with shared roots, as well as a petty performance improvement for those using exotically aligned key types.

~~Also, some testing of the `range` function initially added to #67686 but hardly related.~~

r? @RalfJung

1 files changed, 10 insertions, 13 deletions

diff --git a/src/liballoc/collections/btree/node.rs b/src/liballoc/collections/btree/node.rs
index 260e51d635d..f40e0b0c304 100644
--- a/src/liballoc/collections/btree/node.rs
+++ b/src/liballoc/collections/btree/node.rs
@@ -397,6 +397,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
 
     /// Borrows a view into the values stored in the node.
     /// The caller must ensure that the node is not the shared root.
+    /// This function is not public, so doesn't have to support shared roots like `keys` does.
     fn vals(&self) -> &[V] {
         self.reborrow().into_val_slice()
     }
@@ -514,6 +515,7 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
     }
 
     /// The caller must ensure that the node is not the shared root.
+    /// This function is not public, so doesn't have to support shared roots like `keys` does.
     fn keys_mut(&mut self) -> &mut [K] {
         unsafe { self.reborrow_mut().into_key_slice_mut() }
     }
@@ -589,20 +591,15 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
         unsafe { &mut *(self.root as *mut Root<K, V>) }
     }
 
+    /// The caller must ensure that the node is not the shared root.
     fn into_key_slice_mut(mut self) -> &'a mut [K] {
-        // Same as for `into_key_slice` above, we try to avoid a run-time check.
-        if (mem::align_of::<NodeHeader<K, V, K>>() > mem::align_of::<NodeHeader<K, V>>()
-            || mem::size_of::<NodeHeader<K, V, K>>() != mem::size_of::<NodeHeader<K, V>>())
-            && self.is_shared_root()
-        {
-            &mut []
-        } else {
-            unsafe {
-                slice::from_raw_parts_mut(
-                    MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).keys),
-                    self.len(),
-                )
-            }
+        debug_assert!(!self.is_shared_root());
+        // We cannot be the shared root, so `as_leaf_mut` is okay.
+        unsafe {
+            slice::from_raw_parts_mut(
+                MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).keys),
+                self.len(),
+            )
         }
     }