Remove leaf bias from BTreeMap's BoxedNode and NodeRef

src/etc/gdb_rust_pretty_printing.py

@@ -333,27 +333,34 @@ def children(self):
 # Yield each key (and optionally value) from a BoxedNode.
 def children_of_node(boxed_node, height, want_values):
     node_ptr = boxed_node['ptr']['pointer']
-    if height > 0:
-        type_name = str(node_ptr.type.target()).replace('LeafNode', 'InternalNode')
-        node_type = gdb.lookup_type(type_name)
-        node_ptr = node_ptr.cast(node_type.pointer())
-        leaf = node_ptr['data']
-    else:
-        leaf = node_ptr.dereference()
-    keys = leaf['keys']
-    if want_values:
-        values = leaf['vals']
-    length = int(leaf['len'])
-    for i in xrange(0, length + 1):
+    head = node_ptr.dereference()
+    length = int(head['len'])
+    if length > 0:
         if height > 0:
-            child_ptr = node_ptr['edges'][i]['value']['value']
-            for child in children_of_node(child_ptr, height - 1, want_values):
-                yield child
-        if i < length:
-            if want_values:
-                yield (keys[i]['value']['value'], values[i]['value']['value'])
-            else:
-                yield keys[i]['value']['value']
+            type_name = str(node_ptr.type.target()).replace('NodeHeader', 'InternalNode', 1)
+            node_type = gdb.lookup_type(type_name)
+            node_ptr = node_ptr.cast(node_type.pointer())
+            leaf = node_ptr['data']
+            edges = node_ptr['edges']
+        else:
+            type_name = str(node_ptr.type.target()).replace('NodeHeader', 'LeafNode', 1)
+            node_type = gdb.lookup_type(type_name)
+            node_ptr = node_ptr.cast(node_type.pointer())
+            leaf = node_ptr.dereference()
+            edges = None
+        keys = leaf['keys']
+        if want_values:
+            values = leaf['vals']
+        for i in xrange(0, length + 1):
+            if edges:
+                child_ptr = edges[i]['value']['value']
+                for child in children_of_node(child_ptr, height - 1, want_values):
+                    yield child
+            if i < length:
+                if want_values:
+                    yield (keys[i]['value']['value'], values[i]['value']['value'])
+                else:
+                    yield keys[i]['value']['value']
 
 class RustStdBTreeSetPrinter(object):
     def __init__(self, val):

src/liballoc/collections/btree/node.rs

@@ -152,30 +152,34 @@ impl<K, V> InternalNode<K, V> {
     }
 }
 
-/// An owned pointer to a node. This basically is either `Box<LeafNode<K, V>>` or
-/// `Box<InternalNode<K, V>>`. However, it contains no information as to which of the two types
-/// of nodes is actually behind the box, and, partially due to this lack of information, has no
-/// destructor.
+/// A managed, non-null pointer to a node. This is either an owned pointer to
+/// `LeafNode<K, V>`, an owned pointer to `InternalNode<K, V>`, or a (not owned)
+/// pointer to `NodeHeader<(), ()` (more specifically, the pointer to EMPTY_ROOT_NODE).
+/// All of these types have a `NodeHeader<K, V>` prefix, meaning that they have at
+/// least the same size as `NodeHeader<K, V>` and store the same kinds of data at the same
+/// offsets; and they have a pointer alignment at least as large as `NodeHeader<K, V>`'s.
+/// So that's the pointee type we store, and `as_header()` is unconditionally safe.
+/// However, `BoxedNode` contains no information as to which of the three types
+/// of nodes it actually contains, and, partially due to this lack of information,
+/// has no destructor.
 struct BoxedNode<K, V> {
-    ptr: Unique<LeafNode<K, V>>,
+    ptr: Unique<NodeHeader<K, V>>,
 }
 
 impl<K, V> BoxedNode<K, V> {
     fn from_leaf(node: Box<LeafNode<K, V>>) -> Self {
-        BoxedNode { ptr: Box::into_unique(node) }
+        BoxedNode { ptr: Box::into_unique(node).cast() }
     }
 
     fn from_internal(node: Box<InternalNode<K, V>>) -> Self {
-        unsafe {
-            BoxedNode { ptr: Unique::new_unchecked(Box::into_raw(node) as *mut LeafNode<K, V>) }
-        }
+        BoxedNode { ptr: Box::into_unique(node).cast() }
     }
 
-    unsafe fn from_ptr(ptr: NonNull<LeafNode<K, V>>) -> Self {
+    unsafe fn from_ptr(ptr: NonNull<NodeHeader<K, V>>) -> Self {
         BoxedNode { ptr: Unique::from(ptr) }
     }
 
-    fn as_ptr(&self) -> NonNull<LeafNode<K, V>> {
+    fn as_ptr(&self) -> NonNull<NodeHeader<K, V>> {
         NonNull::from(self.ptr)
     }
 }
@@ -197,11 +201,7 @@ impl<K, V> Root<K, V> {
 
     pub fn shared_empty_root() -> Self {
         Root {
-            node: unsafe {
-                BoxedNode::from_ptr(NonNull::new_unchecked(
-                    &EMPTY_ROOT_NODE as *const _ as *const LeafNode<K, V> as *mut _,
-                ))
-            },
+            node: unsafe { BoxedNode::from_ptr(NonNull::from(&EMPTY_ROOT_NODE).cast()) },
             height: 0,
         }
     }
@@ -310,7 +310,7 @@ impl<K, V> Root<K, V> {
 ///   Turning this into a `NodeHeader` reference is always safe.
 pub struct NodeRef<BorrowType, K, V, Type> {
     height: usize,
-    node: NonNull<LeafNode<K, V>>,
+    node: NonNull<NodeHeader<K, V>>,
     // `root` is null unless the borrow type is `Mut`
     root: *const Root<K, V>,
     _marker: PhantomData<(BorrowType, Type)>,
@@ -372,11 +372,11 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
     /// See `NodeRef` on why the node may not be a shared root.
     unsafe fn as_leaf(&self) -> &LeafNode<K, V> {
         debug_assert!(!self.is_shared_root());
-        self.node.as_ref()
+        &*(self.node.as_ptr() as *const LeafNode<K, V>)
     }
 
     fn as_header(&self) -> &NodeHeader<K, V> {
-        unsafe { &*(self.node.as_ptr() as *const NodeHeader<K, V>) }
+        unsafe { self.node.as_ref() }
     }
 
     /// Returns whether the node is the shared, empty root.
@@ -505,7 +505,7 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
     /// This also implies you can invoke this member on the shared root, but the resulting pointer
     /// might not be properly aligned and definitely would not allow accessing keys and values.
     fn as_leaf_mut(&mut self) -> *mut LeafNode<K, V> {
-        self.node.as_ptr()
+        self.node.as_ptr() as *mut LeafNode<K, V>
     }
 
     /// The caller must ensure that the node is not the shared root.

src/test/debuginfo/pretty-std-collections.rs

@@ -20,20 +20,26 @@
 // gdb-command: print btree_map
 // gdb-check:$2 = BTreeMap<i32, i32>(len: 15) = {[0] = 0, [1] = 1, [2] = 2, [3] = 3, [4] = 4, [5] = 5, [6] = 6, [7] = 7, [8] = 8, [9] = 9, [10] = 10, [11] = 11, [12] = 12, [13] = 13, [14] = 14}
 
+// gdb-command: print empty_btree_map
+// gdb-check:$3 = BTreeMap<(), ()>(len: 0)
+
+// gdb-command: print nasty_btree_map
+// gdb-check:$4 = BTreeMap<i32, pretty_std_collections::MyNodeHeader>(len: 1) = {[1] = pretty_std_collections::MyNodeHeader (11)}
+
 // gdb-command: print vec_deque
-// gdb-check:$3 = VecDeque<i32>(len: 3, cap: 8) = {5, 3, 7}
+// gdb-check:$5 = VecDeque<i32>(len: 3, cap: 8) = {5, 3, 7}
 
 // gdb-command: print vec_deque2
-// gdb-check:$4 = VecDeque<i32>(len: 7, cap: 8) = {2, 3, 4, 5, 6, 7, 8}
+// gdb-check:$6 = VecDeque<i32>(len: 7, cap: 8) = {2, 3, 4, 5, 6, 7, 8}
 
 #![allow(unused_variables)]
-use std::collections::BTreeSet;
 use std::collections::BTreeMap;
+use std::collections::BTreeSet;
 use std::collections::VecDeque;
 
+struct MyNodeHeader(i32); // helps to ensure we don't blindly replace substring "NodeHeader"
 
 fn main() {
-
     // BTreeSet
     let mut btree_set = BTreeSet::new();
     for i in 0..15 {
@@ -45,6 +51,9 @@ fn main() {
     for i in 0..15 {
         btree_map.insert(i, i);
     }
+    let empty_btree_map: BTreeMap<(), ()> = BTreeMap::new();
+    let mut nasty_btree_map: BTreeMap<i32, MyNodeHeader> = BTreeMap::new();
+    nasty_btree_map.insert(1, MyNodeHeader(11));
 
     // VecDeque
     let mut vec_deque = VecDeque::new();
@@ -63,4 +72,6 @@ fn main() {
     zzz(); // #break
 }
 
-fn zzz() { () }
+fn zzz() {
+    ()
+}