Fix variance of BTreeMap and its companion structs.

src/libcollections/btree/map.rs

@@ -12,6 +12,7 @@ use core::cmp::Ordering;
 use core::fmt::Debug;
 use core::hash::{Hash, Hasher};
 use core::iter::FromIterator;
+use core::marker::PhantomData;
 use core::ops::Index;
 use core::{fmt, intrinsics, mem, ptr};
 
@@ -158,7 +159,8 @@ impl<K, Q: ?Sized> super::Recover<Q> for BTreeMap<K, ()>
             Found(handle) => {
                 Some(OccupiedEntry {
                     handle: handle,
-                    length: &mut self.length
+                    length: &mut self.length,
+                    _marker: PhantomData,
                 }.remove_kv().0)
             },
             GoDown(_) => None
@@ -172,7 +174,8 @@ impl<K, Q: ?Sized> super::Recover<Q> for BTreeMap<K, ()>
                 VacantEntry {
                     key: key,
                     handle: handle,
-                    length: &mut self.length
+                    length: &mut self.length,
+                    _marker: PhantomData,
                 }.insert(());
                 None
             }
@@ -223,7 +226,10 @@ pub struct Range<'a, K: 'a, V: 'a> {
 /// A mutable iterator over a sub-range of BTreeMap's entries.
 pub struct RangeMut<'a, K: 'a, V: 'a> {
     front: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
-    back: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>
+    back: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
+
+    // Be invariant in `K` and `V`
+    _marker: PhantomData<&'a mut (K, V)>,
 }
 
 /// A view into a single entry in a map, which may either be vacant or occupied.
@@ -247,7 +253,10 @@ pub enum Entry<'a, K: 'a, V: 'a> {
 pub struct VacantEntry<'a, K: 'a, V: 'a> {
     key: K,
     handle: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
-    length: &'a mut usize
+    length: &'a mut usize,
+
+    // Be invariant in `K` and `V`
+    _marker: PhantomData<&'a mut (K, V)>,
 }
 
 /// An occupied Entry.
@@ -259,7 +268,10 @@ pub struct OccupiedEntry<'a, K: 'a, V: 'a> {
         marker::LeafOrInternal
     >, marker::KV>,
 
-    length: &'a mut usize
+    length: &'a mut usize,
+
+    // Be invariant in `K` and `V`
+    _marker: PhantomData<&'a mut (K, V)>,
 }
 
 impl<K: Ord, V> BTreeMap<K, V> {
@@ -415,7 +427,8 @@ impl<K: Ord, V> BTreeMap<K, V> {
             Found(handle) => {
                 Some(OccupiedEntry {
                     handle: handle,
-                    length: &mut self.length
+                    length: &mut self.length,
+                    _marker: PhantomData,
                 }.remove())
             },
             GoDown(_) => None
@@ -568,7 +581,8 @@ impl<K: Ord, V> BTreeMap<K, V> {
 
         RangeMut {
             front: front,
-            back: back
+            back: back,
+            _marker: PhantomData
         }
     }
 
@@ -593,12 +607,14 @@ impl<K: Ord, V> BTreeMap<K, V> {
         match search::search_tree(self.root.as_mut(), &key) {
             Found(handle) => Occupied(OccupiedEntry {
                 handle: handle,
-                length: &mut self.length
+                length: &mut self.length,
+                _marker: PhantomData,
             }),
             GoDown(handle) => Vacant(VacantEntry {
                 key: key,
                 handle: handle,
-                length: &mut self.length
+                length: &mut self.length,
+                _marker: PhantomData,
             })
         }
     }
@@ -1235,6 +1251,7 @@ impl<K, V> BTreeMap<K, V> {
             range: RangeMut {
                 front: first_leaf_edge(root1),
                 back: last_leaf_edge(root2),
+                _marker: PhantomData,
             },
             length: self.length
         }

src/libcollections/btree/node.rs

@@ -97,13 +97,13 @@ impl<K, V> BoxedNode<K, V> {
         }
     }
 
-    unsafe fn from_ptr(ptr: NonZero<*mut LeafNode<K, V>>) -> Self {
-        BoxedNode { ptr: Unique::new(*ptr) }
+    unsafe fn from_ptr(ptr: NonZero<*const LeafNode<K, V>>) -> Self {
+        BoxedNode { ptr: Unique::new(*ptr as *mut LeafNode<K, V>) }
     }
 
-    fn as_ptr(&self) -> NonZero<*mut LeafNode<K, V>> {
+    fn as_ptr(&self) -> NonZero<*const LeafNode<K, V>> {
         unsafe {
-            NonZero::new(*self.ptr)
+            NonZero::new(*self.ptr as *const LeafNode<K, V>)
         }
     }
 }
@@ -209,6 +209,11 @@ impl<K, V> Root<K, V> {
     }
 }
 
+// N.B. `NodeRef` is always covariant in `K` and `V`, even when the `BorrowType`
+// is `Mut`. This is technically wrong, but cannot result in any unsafety due to
+// internal use of `NodeRef` because we stay completely generic over `K` and `V`.
+// However, whenever a public type wraps `NodeRef`, make sure that it has the
+// correct variance.
 /// A reference to a node.
 ///
 /// This type has a number of paramaters that controls how it acts:
@@ -223,8 +228,8 @@ impl<K, V> Root<K, V> {
 ///   `NodeRef` could be pointing to either type of node.
 pub struct NodeRef<BorrowType, K, V, Type> {
     height: usize,
-    node: NonZero<*mut LeafNode<K, V>>,
-    root: *mut Root<K, V>,
+    node: NonZero<*const LeafNode<K, V>>,
+    root: *const Root<K, V>,
     _marker: PhantomData<(BorrowType, Type)>
 }
 
@@ -401,7 +406,7 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
 
     fn as_leaf_mut(&mut self) -> &mut LeafNode<K, V> {
         unsafe {
-            &mut **self.node
+            &mut *(*self.node as *mut LeafNode<K, V>)
         }
     }
 
@@ -434,7 +439,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
 impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
     pub fn into_root_mut(self) -> &'a mut Root<K, V> {
         unsafe {
-            &mut *self.root
+            &mut *(self.root as *mut Root<K, V>)
         }
     }
 

src/libcollectionstest/btree/map.rs

@@ -378,6 +378,22 @@ fn test_clone() {
     }
 }
 
+#[test]
+fn test_variance() {
+    use std::collections::btree_map::{Iter, IntoIter, Range, Keys, Values};
+
+    fn map_key<'new>(v: BTreeMap<&'static str, ()>) -> BTreeMap<&'new str, ()> { v }
+    fn map_val<'new>(v: BTreeMap<(), &'static str>) -> BTreeMap<(), &'new str> { v }
+    fn iter_key<'a, 'new>(v: Iter<'a, &'static str, ()>) -> Iter<'a, &'new str, ()> { v }
+    fn iter_val<'a, 'new>(v: Iter<'a, (), &'static str>) -> Iter<'a, (), &'new str> { v }
+    fn into_iter_key<'new>(v: IntoIter<&'static str, ()>) -> IntoIter<&'new str, ()> { v }
+    fn into_iter_val<'new>(v: IntoIter<(), &'static str>) -> IntoIter<(), &'new str> { v }
+    fn range_key<'a, 'new>(v: Range<'a, &'static str, ()>) -> Range<'a, &'new str, ()> { v }
+    fn range_val<'a, 'new>(v: Range<'a, (), &'static str>) -> Range<'a, (), &'new str> { v }
+    fn keys<'a, 'new>(v: Keys<'a, &'static str, ()>) -> Keys<'a, &'new str, ()> { v }
+    fn vals<'a, 'new>(v: Values<'a, (), &'static str>) -> Values<'a, (), &'new str> { v }
+}
+
 mod bench {
     use std::collections::BTreeMap;
     use std::__rand::{Rng, thread_rng};

src/test/compile-fail/variance-btree-invariant-types.rs

@@ -0,0 +1,63 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(rustc_attrs)]
+
+use std::collections::btree_map::{IterMut, OccupiedEntry, VacantEntry};
+
+fn iter_cov_key<'a, 'new>(v: IterMut<'a, &'static (), ()>) -> IterMut<'a, &'new (), ()> {
+    v //~ ERROR mismatched types
+}
+fn iter_cov_val<'a, 'new>(v: IterMut<'a, (), &'static ()>) -> IterMut<'a, (), &'new ()> {
+    v //~ ERROR mismatched types
+}
+fn iter_contra_key<'a, 'new>(v: IterMut<'a, &'new (), ()>) -> IterMut<'a, &'static (), ()> {
+    v //~ ERROR mismatched types
+}
+fn iter_contra_val<'a, 'new>(v: IterMut<'a, (), &'new ()>) -> IterMut<'a, (), &'static ()> {
+    v //~ ERROR mismatched types
+}
+
+fn occ_cov_key<'a, 'new>(v: OccupiedEntry<'a, &'static (), ()>)
+                         -> OccupiedEntry<'a, &'new (), ()> {
+    v //~ ERROR mismatched types
+}
+fn occ_cov_val<'a, 'new>(v: OccupiedEntry<'a, (), &'static ()>)
+                         -> OccupiedEntry<'a, (), &'new ()> {
+    v //~ ERROR mismatched types
+}
+fn occ_contra_key<'a, 'new>(v: OccupiedEntry<'a, &'new (), ()>)
+                            -> OccupiedEntry<'a, &'static (), ()> {
+    v //~ ERROR mismatched types
+}
+fn occ_contra_val<'a, 'new>(v: OccupiedEntry<'a, (), &'new ()>)
+                            -> OccupiedEntry<'a, (), &'static ()> {
+    v //~ ERROR mismatched types
+}
+
+fn vac_cov_key<'a, 'new>(v: VacantEntry<'a, &'static (), ()>)
+                         -> VacantEntry<'a, &'new (), ()> {
+    v //~ ERROR mismatched types
+}
+fn vac_cov_val<'a, 'new>(v: VacantEntry<'a, (), &'static ()>)
+                         -> VacantEntry<'a, (), &'new ()> {
+    v //~ ERROR mismatched types
+}
+fn vac_contra_key<'a, 'new>(v: VacantEntry<'a, &'new (), ()>)
+                            -> VacantEntry<'a, &'static (), ()> {
+    v //~ ERROR mismatched types
+}
+fn vac_contra_val<'a, 'new>(v: VacantEntry<'a, (), &'new ()>)
+                            -> VacantEntry<'a, (), &'static ()> {
+    v //~ ERROR mismatched types
+}
+
+#[rustc_error]
+fn main() { }