diff --git a/src/libcore/core.rc b/src/libcore/core.rc
index f02af9c0998..4b1ace5651b 100644
--- a/src/libcore/core.rc
+++ b/src/libcore/core.rc
@@ -55,6 +55,7 @@ export hash;
 export cmp;
 export num;
 export path;
+export managed;
 
 // NDM seems to be necessary for resolve to work
 export option_iter;
@@ -261,6 +262,7 @@ mod sys;
 #[warn(non_camel_case_types)]
 mod unsafe;
 
+mod managed;
 
 // Modules supporting compiler-generated code
 // Exported but not part of the public interface
diff --git a/src/libcore/managed.rs b/src/libcore/managed.rs
new file mode 100644
index 00000000000..06fcd87282b
--- /dev/null
+++ b/src/libcore/managed.rs
@@ -0,0 +1,62 @@
+/*!
+
+Module for wrapping freezable data structures in managed boxes.
+Normally freezable data structures require an unaliased reference,
+such as `T` or `~T`, so that the compiler can track when they are
+being mutated.  The `rw<T>` type converts these static checks into
+dynamic checks: your program will fail if you attempt to perform
+mutation when the data structure should be immutable.
+
+*/
+
+#[forbid(non_camel_case_types)];
+#[forbid(deprecated_mode)];
+#[forbid(deprecated_pattern)];
+
+import util::with;
+import unsafe::transmute_immut;
+
+export Managed;
+
+enum Mode { ReadOnly, Mutable, Immutable }
+
+struct Data<T> {
+    mut value: T;
+    mut mode: Mode;
+}
+
+type Managed<T> = @Data<T>;
+
+fn Managed<T>(+t: T) -> Managed<T> {
+    @Data {value: t, mode: ReadOnly}
+}
+
+impl<T> Data<T> {
+    fn borrow_mut<R>(op: &fn(t: &mut T) -> R) -> R {
+        match self.mode {
+          Immutable => fail fmt!("%? currently immutable",
+                                 self.value),
+          ReadOnly | Mutable => {}
+        }
+
+        do with(&mut self.mode, Mutable) {
+            op(&mut self.value)
+        }
+    }
+
+    fn borrow_const<R>(op: &fn(t: &const T) -> R) -> R {
+        op(&const self.value)
+    }
+
+    fn borrow_imm<R>(op: &fn(t: &T) -> R) -> R {
+        match self.mode {
+          Mutable => fail fmt!("%? currently mutable",
+                               self.value),
+          ReadOnly | Immutable => {}
+        }
+
+        do with(&mut self.mode, Immutable) {
+            op(unsafe{transmute_immut(&mut self.value)})
+        }
+    }
+}
diff --git a/src/libcore/send_map.rs b/src/libcore/send_map.rs
index 52a63dacbfc..9b5b6e0d4f3 100644
--- a/src/libcore/send_map.rs
+++ b/src/libcore/send_map.rs
@@ -221,6 +221,13 @@ mod linear {
             self.size -= 1;
             return true;
         }
+
+        fn clear() {
+            for uint::range(0, self.buckets.len()) |idx| {
+                self.buckets[idx] = none;
+            }
+            self.size = 0;
+        }
     }
 
     priv impl<K,V> &LinearMap<K,V> {
diff --git a/src/libcore/util.rs b/src/libcore/util.rs
index c68ee317da4..8487e4ff930 100644
--- a/src/libcore/util.rs
+++ b/src/libcore/util.rs
@@ -12,6 +12,24 @@ pure fn id<T>(+x: T) -> T { x }
 /// Ignores a value.
 pure fn ignore<T>(+_x: T) { }
 
+/// Sets `*ptr` to `new_value`, invokes `op()`, and then restores the
+/// original value of `*ptr`.
+#[inline(always)]
+fn with<T: copy, R>(
+    ptr: &mut T,
+    +new_value: T,
+    op: &fn() -> R) -> R
+{
+    // NDM: if swap operator were defined somewhat differently,
+    // we wouldn't need to copy...
+
+    let old_value = *ptr;
+    *ptr = move new_value;
+    let result = op();
+    *ptr = move old_value;
+    return move result;
+}
+
 /**
  * Swap the values at two mutable locations of the same type, without
  * deinitialising or copying either one.
diff --git a/src/libstd/map.rs b/src/libstd/map.rs
index e6f44d76b4c..538a6446001 100644
--- a/src/libstd/map.rs
+++ b/src/libstd/map.rs
@@ -4,6 +4,9 @@
 
 import io::WriterUtil;
 import to_str::ToStr;
+import managed::Managed;
+import send_map::linear::LinearMap;
+
 export hashmap, hashfn, eqfn, set, map, chained, hashmap, str_hash;
 export box_str_hash;
 export bytes_hash, int_hash, uint_hash, set_add;
@@ -59,10 +62,10 @@ trait map<K: copy, V: copy> {
     fn find(+key: K) -> option<V>;
 
     /**
-     * Remove and return a value from the map. If the key does not exist
-     * in the map then returns none.
+     * Remove and return a value from the map. Returns true if the
+     * key was present in the map, otherwise false.
      */
-    fn remove(+key: K) -> option<V>;
+    fn remove(+key: K) -> bool;
 
     /// Clear the map, removing all key/value pairs.
     fn clear();
@@ -279,18 +282,18 @@ mod chained {
             option::unwrap(opt_v)
         }
 
-        fn remove(+k: K) -> option<V> {
+        fn remove(+k: K) -> bool {
             match self.search_tbl(&k, self.hasher(&k)) {
-              not_found => none,
+              not_found => false,
               found_first(idx, entry) => {
                 self.count -= 1u;
                 self.chains[idx] = entry.next;
-                some(entry.value)
+                true
               }
               found_after(eprev, entry) => {
                 self.count -= 1u;
                 eprev.next = entry.next;
-                some(entry.value)
+                true
               }
             }
         }
@@ -468,6 +471,93 @@ fn hash_from_uints<V: copy>(items: &[(uint, V)]) -> hashmap<uint, V> {
     hash_from_vec(uint::hash, uint::eq, items)
 }
 
+// XXX Transitionary
+impl<K: copy, V: copy> Managed<LinearMap<K, V>>: map<K, V> {
+    fn size() -> uint {
+        do self.borrow_const |p| {
+            p.len()
+        }
+    }
+
+    fn insert(+key: K, +value: V) -> bool {
+        do self.borrow_mut |p| {
+            p.insert(key, value)
+        }
+    }
+
+    fn contains_key(+key: K) -> bool {
+        do self.borrow_const |p| {
+            p.contains_key(&key)
+        }
+    }
+
+    fn contains_key_ref(key: &K) -> bool {
+        do self.borrow_const |p| {
+            p.contains_key(key)
+        }
+    }
+
+    fn get(+key: K) -> V {
+        do self.borrow_const |p| {
+            p.get(&key)
+        }
+    }
+
+    fn find(+key: K) -> option<V> {
+        do self.borrow_const |p| {
+            p.find(&key)
+        }
+    }
+
+    fn remove(+key: K) -> bool {
+        do self.borrow_mut |p| {
+            p.remove(&key)
+        }
+    }
+
+    fn clear() {
+        do self.borrow_mut |p| {
+            p.clear()
+        }
+    }
+
+    fn each(op: fn(+key: K, +value: V) -> bool) {
+        do self.borrow_imm |p| {
+            p.each(op)
+        }
+    }
+
+    fn each_key(op: fn(+key: K) -> bool) {
+        do self.borrow_imm |p| {
+            p.each_key(op)
+        }
+    }
+
+    fn each_value(op: fn(+value: V) -> bool) {
+        do self.borrow_imm |p| {
+            p.each_value(op)
+        }
+    }
+
+    fn each_ref(op: fn(key: &K, value: &V) -> bool) {
+        do self.borrow_imm |p| {
+            p.each_ref(op)
+        }
+    }
+
+    fn each_key_ref(op: fn(key: &K) -> bool) {
+        do self.borrow_imm |p| {
+            p.each_key_ref(op)
+        }
+    }
+
+    fn each_value_ref(op: fn(value: &V) -> bool) {
+        do self.borrow_imm |p| {
+            p.each_value_ref(op)
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
 
diff --git a/src/libstd/smallintmap.rs b/src/libstd/smallintmap.rs
index a7b40bbb0f9..04b04f505a4 100644
--- a/src/libstd/smallintmap.rs
+++ b/src/libstd/smallintmap.rs
@@ -80,13 +80,13 @@ impl<V: copy> smallintmap<V>: map::map<uint, V> {
         insert(self, key, value);
         return !exists;
     }
-    fn remove(+key: uint) -> option<V> {
+    fn remove(+key: uint) -> bool {
         if key >= self.v.len() {
-            return none;
+            return false;
         }
         let old = self.v.get_elt(key);
         self.v.set_elt(key, none);
-        old
+        old.is_some()
     }
     fn clear() {
         self.v.set(~[mut]);
diff --git a/src/test/bench/core-map.rs b/src/test/bench/core-map.rs
new file mode 100644
index 00000000000..9a267a5a25d
--- /dev/null
+++ b/src/test/bench/core-map.rs
@@ -0,0 +1,178 @@
+/*
+
+
+
+*/
+
+use std;
+import rand;
+import std::map;
+import managed::Managed;
+import send_map::linear::*;
+import io::WriterUtil;
+
+struct Results {
+    sequential_ints: float;
+    random_ints: float;
+    delete_ints: float;
+
+    sequential_strings: float;
+    random_strings: float;
+    delete_strings: float;
+}
+
+fn timed(result: &mut float,
+         op: fn()) {
+    let start = std::time::precise_time_s();
+    op();
+    let end = std::time::precise_time_s();
+    *result = (end - start);
+}
+
+fn int_benchmarks<M: map::map<uint, uint>>(make_map: fn() -> M,
+                                           rng: @rand::Rng,
+                                           num_keys: uint,
+                                           results: &mut Results) {
+
+    {
+        let map = make_map();
+        do timed(&mut results.sequential_ints) {
+            for uint::range(0, num_keys) |i| {
+                map.insert(i, i+1);
+            }
+
+            for uint::range(0, num_keys) |i| {
+                assert map.get(i) == i+1;
+            }
+        }
+    }
+
+    {
+        let map = make_map();
+        do timed(&mut results.random_ints) {
+            for uint::range(0, num_keys) |i| {
+                map.insert(rng.next() as uint, i);
+            }
+        }
+    }
+
+    {
+        let map = make_map();
+        for uint::range(0, num_keys) |i| {
+            map.insert(i, i);;
+        }
+
+        do timed(&mut results.delete_ints) {
+            for uint::range(0, num_keys) |i| {
+                assert map.remove(i);
+            }
+        }
+    }
+}
+
+fn str_benchmarks<M: map::map<~str, uint>>(make_map: fn() -> M,
+                                           rng: @rand::Rng,
+                                           num_keys: uint,
+                                           results: &mut Results) {
+    {
+        let map = make_map();
+        do timed(&mut results.sequential_strings) {
+            for uint::range(0, num_keys) |i| {
+                let s = uint::to_str(i, 10);
+                map.insert(s, i);
+            }
+
+            for uint::range(0, num_keys) |i| {
+                let s = uint::to_str(i, 10);
+                assert map.get(s) == i;
+            }
+        }
+    }
+
+    {
+        let map = make_map();
+        do timed(&mut results.random_strings) {
+            for uint::range(0, num_keys) |i| {
+                let s = uint::to_str(rng.next() as uint, 10);
+                map.insert(s, i);
+            }
+        }
+    }
+
+    {
+        let map = make_map();
+        for uint::range(0, num_keys) |i| {
+            map.insert(uint::to_str(i, 10), i);
+        }
+        do timed(&mut results.delete_strings) {
+            for uint::range(0, num_keys) |i| {
+                assert map.remove(uint::to_str(i, 10));
+            }
+        }
+    }
+}
+
+fn write_header(header: &str) {
+    io::stdout().write_str(header);
+    io::stdout().write_str("\n");
+}
+
+fn write_row(label: &str, value: float) {
+    io::stdout().write_str(fmt!("%30s %f s\n", label, value));
+}
+
+fn write_results(label: &str, results: &Results) {
+    write_header(label);
+    write_row("sequential_ints", results.sequential_ints);
+    write_row("random_ints", results.random_ints);
+    write_row("delete_ints", results.delete_ints);
+    write_row("sequential_strings", results.sequential_strings);
+    write_row("random_strings", results.random_strings);
+    write_row("delete_strings", results.delete_strings);
+}
+
+fn empty_results() -> Results {
+    Results {
+        sequential_ints: 0f,
+        random_ints: 0f,
+        delete_ints: 0f,
+
+        sequential_strings: 0f,
+        random_strings: 0f,
+        delete_strings: 0f,
+    }
+}
+
+fn main(args: ~[~str]) {
+    let num_keys = {
+        if args.len() == 2 {
+            uint::from_str(args[1]).get()
+        } else {
+            100 // woefully inadequate for any real measurement
+        }
+    };
+
+    let seed = ~[1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+
+    {
+        let rng = rand::seeded_rng(copy seed);
+        let mut results = empty_results();
+        int_benchmarks::<map::hashmap<uint, uint>>(
+            map::uint_hash, rng, num_keys, &mut results);
+        str_benchmarks::<map::hashmap<~str, uint>>(
+            map::str_hash, rng, num_keys, &mut results);
+        write_results("libstd::map::hashmap", &results);
+    }
+
+    {
+        let rng = rand::seeded_rng(copy seed);
+        let mut results = empty_results();
+        int_benchmarks::<Managed<LinearMap<uint, uint>>>(
+            || Managed(linear_map(uint::hash, uint::eq)),
+            rng, num_keys, &mut results);
+        str_benchmarks::<Managed<LinearMap<~str, uint>>>(
+            || Managed(linear_map(str::hash, str::eq)),
+            rng, num_keys, &mut results);
+        write_results("libstd::map::hashmap", &results);
+    }
+}
\ No newline at end of file