diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index ffa91df4e8a86..10887369b159f 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -164,6 +164,7 @@ pub mod result;
 pub mod either;
 pub mod hashmap;
 pub mod cell;
+pub mod mutable;
 pub mod trie;
 
 
diff --git a/src/libstd/mutable.rs b/src/libstd/mutable.rs
new file mode 100644
index 0000000000000..63caa52d3af1d
--- /dev/null
+++ b/src/libstd/mutable.rs
@@ -0,0 +1,298 @@
+// Copyright 2013 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.
+
+//! A mutable memory location with dynamically checked borrow rules
+
+use prelude::*;
+
+use cast;
+use util::NonCopyable;
+
+/// A mutable memory location with dynamically checked borrow rules
+#[no_freeze]
+pub struct Mut<T> {
+    priv value: T,
+    priv borrow: BorrowFlag,
+    priv nc: NonCopyable
+}
+
+// Values [1, MAX-1] represent the number of `Ref` active
+// (will not outgrow its range since `uint` is the size of the address space)
+type BorrowFlag = uint;
+static UNUSED: BorrowFlag = 0;
+static WRITING: BorrowFlag = -1;
+
+impl<T> Mut<T> {
+    /// Create a new `Mut` containing `value`
+    pub fn new(value: T) -> Mut<T> {
+        Mut {
+            value: value,
+            borrow: UNUSED,
+            nc: NonCopyable
+        }
+    }
+
+    /// Consumes the `Mut`, returning the wrapped value.
+    pub fn unwrap(self) -> T {
+        assert!(self.borrow == UNUSED);
+        self.value
+    }
+
+    unsafe fn as_mut<'a>(&'a self) -> &'a mut Mut<T> {
+        cast::transmute_mut(self)
+    }
+
+    /// Attempts to immutably borrow the wrapped value.
+    ///
+    /// The borrow lasts until the returned `Ref` exits scope. Multiple
+    /// immutable borrows can be taken out at the same time.
+    ///
+    /// Returns `None` if the value is currently mutably borrowed.
+    pub fn try_borrow<'a>(&'a self) -> Option<Ref<'a, T>> {
+        match self.borrow {
+            WRITING => None,
+            _ => {
+                unsafe { self.as_mut().borrow += 1; }
+                Some(Ref { parent: self })
+            }
+        }
+    }
+
+    /// Immutably borrows the wrapped value.
+    ///
+    /// The borrow lasts until the returned `Ref` exits scope. Multiple
+    /// immutable borrows can be taken out at the same time.
+    ///
+    /// # Failure
+    ///
+    /// Fails if the value is currently mutably borrowed.
+    pub fn borrow<'a>(&'a self) -> Ref<'a, T> {
+        match self.try_borrow() {
+            Some(ptr) => ptr,
+            None => fail!("Mut<T> already mutably borrowed")
+        }
+    }
+
+    /// Mutably borrows the wrapped value.
+    ///
+    /// The borrow lasts untile the returned `MutRef` exits scope. The value
+    /// cannot be borrowed while this borrow is active.
+    ///
+    /// Returns `None` if the value is currently borrowed.
+    pub fn try_borrow_mut<'a>(&'a self) -> Option<MutRef<'a, T>> {
+        match self.borrow {
+            UNUSED => unsafe {
+                let mut_self = self.as_mut();
+                mut_self.borrow = WRITING;
+                Some(MutRef { parent: mut_self })
+            },
+            _ => None
+        }
+    }
+
+    /// Mutably borrows the wrapped value.
+    ///
+    /// The borrow lasts untile the returned `MutRef` exits scope. The value
+    /// cannot be borrowed while this borrow is active.
+    ///
+    /// # Failure
+    ///
+    /// Fails if the value is currently borrowed.
+    pub fn borrow_mut<'a>(&'a self) -> MutRef<'a, T> {
+        match self.try_borrow_mut() {
+            Some(ptr) => ptr,
+            None => fail!("Mut<T> already borrowed")
+        }
+    }
+
+    /// Immutably borrows the wrapped value and applies `blk` to it.
+    ///
+    /// # Failure
+    ///
+    /// Fails if the value is currently mutably borrowed.
+    #[inline]
+    pub fn map<U>(&self, blk: |&T| -> U) -> U {
+        let ptr = self.borrow();
+        blk(ptr.get())
+    }
+
+    /// Mutably borrows the wrapped value and applies `blk` to it.
+    ///
+    /// # Failure
+    ///
+    /// Fails if the value is currently borrowed.
+    #[inline]
+    pub fn map_mut<U>(&self, blk: |&mut T| -> U) -> U {
+        let mut ptr = self.borrow_mut();
+        blk(ptr.get())
+    }
+}
+
+impl<T: Clone> Clone for Mut<T> {
+    fn clone(&self) -> Mut<T> {
+        let x = self.borrow();
+        Mut::new(x.get().clone())
+    }
+}
+
+impl<T: DeepClone> DeepClone for Mut<T> {
+    fn deep_clone(&self) -> Mut<T> {
+        let x = self.borrow();
+        Mut::new(x.get().deep_clone())
+    }
+}
+
+impl<T: Eq> Eq for Mut<T> {
+    fn eq(&self, other: &Mut<T>) -> bool {
+        let a = self.borrow();
+        let b = other.borrow();
+        a.get() == b.get()
+    }
+}
+
+/// Wraps a borrowed reference to a value in a `Mut` box.
+pub struct Ref<'box, T> {
+    priv parent: &'box Mut<T>
+}
+
+#[unsafe_destructor]
+impl<'box, T> Drop for Ref<'box, T> {
+    fn drop(&mut self) {
+        assert!(self.parent.borrow != WRITING && self.parent.borrow != UNUSED);
+        unsafe { self.parent.as_mut().borrow -= 1; }
+    }
+}
+
+impl<'box, T> Ref<'box, T> {
+    /// Retrieve an immutable reference to the stored value.
+    #[inline]
+    pub fn get<'a>(&'a self) -> &'a T {
+        &self.parent.value
+    }
+}
+
+/// Wraps a mutable borrowed reference to a value in a `Mut` box.
+pub struct MutRef<'box, T> {
+    priv parent: &'box mut Mut<T>
+}
+
+#[unsafe_destructor]
+impl<'box, T> Drop for MutRef<'box, T> {
+    fn drop(&mut self) {
+        assert!(self.parent.borrow == WRITING);
+        unsafe { self.parent.as_mut().borrow = UNUSED; }
+    }
+}
+
+impl<'box, T> MutRef<'box, T> {
+    /// Retrieve a mutable reference to the stored value.
+    #[inline]
+    pub fn get<'a>(&'a mut self) -> &'a mut T {
+        &mut self.parent.value
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn double_imm_borrow() {
+        let x = Mut::new(0);
+        let _b1 = x.borrow();
+        x.borrow();
+    }
+
+    #[test]
+    fn no_mut_then_imm_borrow() {
+        let x = Mut::new(0);
+        let _b1 = x.borrow_mut();
+        assert!(x.try_borrow().is_none());
+    }
+
+    #[test]
+    fn no_imm_then_borrow_mut() {
+        let x = Mut::new(0);
+        let _b1 = x.borrow();
+        assert!(x.try_borrow_mut().is_none());
+    }
+
+    #[test]
+    fn no_double_borrow_mut() {
+        let x = Mut::new(0);
+        let _b1 = x.borrow_mut();
+        assert!(x.try_borrow_mut().is_none());
+    }
+
+    #[test]
+    fn imm_release_borrow_mut() {
+        let x = Mut::new(0);
+        {
+            let _b1 = x.borrow();
+        }
+        x.borrow_mut();
+    }
+
+    #[test]
+    fn mut_release_borrow_mut() {
+        let x = Mut::new(0);
+        {
+            let _b1 = x.borrow_mut();
+        }
+        x.borrow();
+    }
+
+    #[test]
+    fn double_borrow_single_release_no_borrow_mut() {
+        let x = Mut::new(0);
+        let _b1 = x.borrow();
+        {
+            let _b2 = x.borrow();
+        }
+        assert!(x.try_borrow_mut().is_none());
+    }
+
+    #[test]
+    fn map_ok() {
+        let x = Mut::new(0);
+        assert_eq!(1, x.map(|x| *x+1));
+    }
+
+    #[test]
+    #[should_fail]
+    fn mut_borrow_map() {
+        let x = Mut::new(0);
+        let _b1 = x.borrow_mut();
+        x.map(|x| *x+1);
+    }
+
+    #[test]
+    fn borrow_map() {
+        let x = Mut::new(0);
+        let _b1 = x.borrow();
+        assert_eq!(1, x.map(|x| *x+1));
+    }
+
+    #[test]
+    fn map_mut_ok() {
+        let x = Mut::new(0);
+        x.map_mut(|x| *x += 1);
+        let b = x.borrow();
+        assert_eq!(1, *b.get());
+    }
+
+    #[test]
+    #[should_fail]
+    fn borrow_map_mut() {
+        let x = Mut::new(0);
+        let _b = x.borrow();
+        x.map_mut(|x| *x += 1);
+    }
+}
diff --git a/src/test/compile-fail/mut-not-freeze.rs b/src/test/compile-fail/mut-not-freeze.rs
new file mode 100644
index 0000000000000..9a9669cbb3bbd
--- /dev/null
+++ b/src/test/compile-fail/mut-not-freeze.rs
@@ -0,0 +1,18 @@
+// Copyright 2013 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.
+
+use std::mutable::Mut;
+
+fn f<T: Freeze>(_: T) {}
+
+fn main() {
+    let x = Mut::new(0);
+    f(x); //~ ERROR: which does not fulfill `Freeze`
+}