diff --git a/lib/cretonne/Cargo.toml b/lib/cretonne/Cargo.toml
index cf12060989..4540a783a6 100644
--- a/lib/cretonne/Cargo.toml
+++ b/lib/cretonne/Cargo.toml
@@ -14,6 +14,7 @@ build = "build.rs"
 name = "cretonne"
 
 [dependencies]
+cretonne-entity = { path = "../entity" }
 # It is a goal of the cretonne crate to have minimal external dependencies.
 # Please don't add any unless they are essential to the task of creating binary
 # machine code. Integration tests that need external dependencies can be
diff --git a/lib/cretonne/src/lib.rs b/lib/cretonne/src/lib.rs
index 99246defe3..996bb865bd 100644
--- a/lib/cretonne/src/lib.rs
+++ b/lib/cretonne/src/lib.rs
@@ -39,9 +39,10 @@ pub use write::write_function;
 pub const VERSION: &str = env!("CARGO_PKG_VERSION");
 
 #[macro_use]
-pub mod dbg;
+pub extern crate cton_entity as entity;
+
 #[macro_use]
-pub mod entity;
+pub mod dbg;
 
 pub mod bforest;
 pub mod binemit;
@@ -52,13 +53,14 @@ pub mod flowgraph;
 pub mod ir;
 pub mod isa;
 pub mod loop_analysis;
-pub mod packed_option;
 pub mod print_errors;
 pub mod result;
 pub mod settings;
 pub mod timing;
 pub mod verifier;
 
+pub use entity::packed_option;
+
 mod abi;
 mod bitset;
 mod constant_hash;
diff --git a/lib/entity/Cargo.toml b/lib/entity/Cargo.toml
new file mode 100644
index 0000000000..c86962c809
--- /dev/null
+++ b/lib/entity/Cargo.toml
@@ -0,0 +1,21 @@
+[package]
+authors = ["The Cretonne Project Developers"]
+name = "cretonne-entity"
+version = "0.4.2"
+description = "Data structures using entity references as mapping keys"
+license = "Apache-2.0"
+documentation = "https://cretonne.readthedocs.io/"
+repository = "https://github.com/Cretonne/cretonne"
+readme = "README.md"
+keywords = ["entity", "set", "map"]
+
+[lib]
+name = "cton_entity"
+
+[features]
+default = ["std"]
+std = []
+
+[badges]
+maintenance = { status = "experimental" }
+travis-ci = { repository = "Cretonne/cretonne" }
diff --git a/lib/entity/README.md b/lib/entity/README.md
new file mode 100644
index 0000000000..abe3b6210c
--- /dev/null
+++ b/lib/entity/README.md
@@ -0,0 +1,2 @@
+This crate contains array-based data structures used by the core Cretonne code
+generator which use densely numbered entity references as mapping keys.
diff --git a/lib/cretonne/src/entity/iter.rs b/lib/entity/src/iter.rs
similarity index 99%
rename from lib/cretonne/src/entity/iter.rs
rename to lib/entity/src/iter.rs
index 156021ee86..9eeba6c7a6 100644
--- a/lib/cretonne/src/entity/iter.rs
+++ b/lib/entity/src/iter.rs
@@ -1,6 +1,6 @@
 //! A double-ended iterator over entity references and entities.
 
-use entity::EntityRef;
+use EntityRef;
 use std::marker::PhantomData;
 use std::slice;
 
diff --git a/lib/cretonne/src/entity/keys.rs b/lib/entity/src/keys.rs
similarity index 98%
rename from lib/cretonne/src/entity/keys.rs
rename to lib/entity/src/keys.rs
index 3ad5b660b6..52481097cb 100644
--- a/lib/cretonne/src/entity/keys.rs
+++ b/lib/entity/src/keys.rs
@@ -1,6 +1,6 @@
 //! A double-ended iterator over entity references.
 
-use entity::EntityRef;
+use EntityRef;
 use std::marker::PhantomData;
 
 /// Iterate over all keys in order.
diff --git a/lib/cretonne/src/entity/mod.rs b/lib/entity/src/lib.rs
similarity index 71%
rename from lib/cretonne/src/entity/mod.rs
rename to lib/entity/src/lib.rs
index 88c0977cf2..00b6412c0d 100644
--- a/lib/cretonne/src/entity/mod.rs
+++ b/lib/entity/src/lib.rs
@@ -1,6 +1,6 @@
 //! Array-based data structures using densely numbered entity references as mapping keys.
 //!
-//! This module defines a number of data structures based on arrays. The arrays are not indexed by
+//! This crate defines a number of data structures based on arrays. The arrays are not indexed by
 //! `usize` as usual, but by *entity references* which are integers wrapped in new-types. This has
 //! a couple advantages:
 //!
@@ -29,21 +29,26 @@
 //!   references allocated from an associated memory pool. It has a much smaller footprint than
 //!   `Vec`.
 
-mod iter;
-mod keys;
-mod list;
-mod map;
-mod primary;
-mod set;
-mod sparse;
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
+#![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
+#![cfg_attr(feature = "cargo-clippy",
+            allow(new_without_default, new_without_default_derive, redundant_field_names))]
 
-pub use self::iter::{Iter, IterMut};
-pub use self::keys::Keys;
-pub use self::list::{EntityList, ListPool};
-pub use self::map::EntityMap;
-pub use self::primary::PrimaryMap;
-pub use self::set::EntitySet;
-pub use self::sparse::{SparseMap, SparseMapValue, SparseSet};
+// Turns on no_std and alloc features if std is not available.
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(feature = "std"), feature(alloc))]
+
+/// This replaces `std` in builds with `core`.
+#[cfg(not(feature = "std"))]
+mod std {
+    extern crate alloc;
+    pub use self::alloc::{boxed, string, vec};
+    pub use core::*;
+}
+
+// Re-export core so that the macros works with both std and no_std crates
+#[doc(hidden)]
+pub extern crate core as __core;
 
 /// A type wrapping a small integer index should implement `EntityRef` so it can be used as the key
 /// of an `EntityMap` or `SparseMap`.
@@ -61,9 +66,9 @@ pub trait EntityRef: Copy + Eq {
 macro_rules! entity_impl {
     // Basic traits.
     ($entity:ident) => {
-        impl $crate::entity::EntityRef for $entity {
+        impl $crate::EntityRef for $entity {
             fn new(index: usize) -> Self {
-                debug_assert!(index < (::std::u32::MAX as usize));
+                debug_assert!(index < ($crate::__core::u32::MAX as usize));
                 $entity(index as u32)
             }
 
@@ -74,7 +79,7 @@ macro_rules! entity_impl {
 
         impl $crate::packed_option::ReservedValue for $entity {
             fn reserved_value() -> $entity {
-                $entity(::std::u32::MAX)
+                $entity($crate::__core::u32::MAX)
             }
         }
     };
@@ -84,16 +89,34 @@ macro_rules! entity_impl {
     ($entity:ident, $display_prefix:expr) => {
         entity_impl!($entity);
 
-        impl ::std::fmt::Display for $entity {
-            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+        impl $crate::__core::fmt::Display for $entity {
+            fn fmt(&self, f: &mut $crate::__core::fmt::Formatter) -> $crate::__core::fmt::Result {
                 write!(f, "{}{}", $display_prefix, self.0)
             }
         }
 
-        impl ::std::fmt::Debug for $entity {
-            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
-                (self as &::std::fmt::Display).fmt(f)
+        impl $crate::__core::fmt::Debug for $entity {
+            fn fmt(&self, f: &mut $crate::__core::fmt::Formatter) -> $crate::__core::fmt::Result {
+                (self as &$crate::__core::fmt::Display).fmt(f)
             }
         }
     };
 }
+
+pub mod packed_option;
+
+mod iter;
+mod keys;
+mod list;
+mod map;
+mod primary;
+mod set;
+mod sparse;
+
+pub use self::iter::{Iter, IterMut};
+pub use self::keys::Keys;
+pub use self::list::{EntityList, ListPool};
+pub use self::map::EntityMap;
+pub use self::primary::PrimaryMap;
+pub use self::set::EntitySet;
+pub use self::sparse::{SparseMap, SparseMapValue, SparseSet};
diff --git a/lib/cretonne/src/entity/list.rs b/lib/entity/src/list.rs
similarity index 99%
rename from lib/cretonne/src/entity/list.rs
rename to lib/entity/src/list.rs
index 545b1e146b..803504bc76 100644
--- a/lib/cretonne/src/entity/list.rs
+++ b/lib/entity/src/list.rs
@@ -1,5 +1,5 @@
 //! Small lists of entity references.
-use entity::EntityRef;
+use EntityRef;
 use std::hash::{Hash, Hasher};
 use std::marker::PhantomData;
 use std::mem;
@@ -481,8 +481,12 @@ impl<T: EntityRef> EntityList<T> {
 mod tests {
     use super::*;
     use super::{sclass_for_length, sclass_size};
-    use entity::EntityRef;
-    use ir::Inst;
+    use EntityRef;
+
+    /// An opaque reference to an instruction in a function.
+    #[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+    pub struct Inst(u32);
+    entity_impl!(Inst, "inst");
 
     #[test]
     fn size_classes() {
diff --git a/lib/cretonne/src/entity/map.rs b/lib/entity/src/map.rs
similarity index 98%
rename from lib/cretonne/src/entity/map.rs
rename to lib/entity/src/map.rs
index ff402b104f..8256ba9c90 100644
--- a/lib/cretonne/src/entity/map.rs
+++ b/lib/entity/src/map.rs
@@ -1,6 +1,6 @@
 //! Densely numbered entity references as mapping keys.
 
-use entity::{EntityRef, Iter, IterMut, Keys};
+use {EntityRef, Iter, IterMut, Keys};
 use std::marker::PhantomData;
 use std::ops::{Index, IndexMut};
 use std::slice;
diff --git a/lib/cretonne/src/packed_option.rs b/lib/entity/src/packed_option.rs
similarity index 100%
rename from lib/cretonne/src/packed_option.rs
rename to lib/entity/src/packed_option.rs
diff --git a/lib/cretonne/src/entity/primary.rs b/lib/entity/src/primary.rs
similarity index 99%
rename from lib/cretonne/src/entity/primary.rs
rename to lib/entity/src/primary.rs
index 5d690a7553..805ccc6866 100644
--- a/lib/cretonne/src/entity/primary.rs
+++ b/lib/entity/src/primary.rs
@@ -1,5 +1,5 @@
 //! Densely numbered entity references as mapping keys.
-use entity::{EntityRef, Iter, IterMut, Keys};
+use {EntityRef, Iter, IterMut, Keys};
 use std::marker::PhantomData;
 use std::ops::{Index, IndexMut};
 use std::slice;
diff --git a/lib/cretonne/src/entity/set.rs b/lib/entity/src/set.rs
similarity index 99%
rename from lib/cretonne/src/entity/set.rs
rename to lib/entity/src/set.rs
index e4acf47723..392813401c 100644
--- a/lib/cretonne/src/entity/set.rs
+++ b/lib/entity/src/set.rs
@@ -1,6 +1,6 @@
 //! Densely numbered entity references as set keys.
 
-use entity::{EntityRef, Keys};
+use {EntityRef, Keys};
 use std::marker::PhantomData;
 use std::vec::Vec;
 
diff --git a/lib/cretonne/src/entity/sparse.rs b/lib/entity/src/sparse.rs
similarity index 98%
rename from lib/cretonne/src/entity/sparse.rs
rename to lib/entity/src/sparse.rs
index 6894c89498..95d0bba41b 100644
--- a/lib/cretonne/src/entity/sparse.rs
+++ b/lib/entity/src/sparse.rs
@@ -7,7 +7,7 @@
 //! > Briggs, Torczon, *An efficient representation for sparse sets*,
 //!   ACM Letters on Programming Languages and Systems, Volume 2, Issue 1-4, March-Dec. 1993.
 
-use entity::{EntityMap, EntityRef};
+use {EntityMap, EntityRef};
 use std::mem;
 use std::slice;
 use std::u32;
@@ -229,8 +229,12 @@ pub type SparseSet<T> = SparseMap<T, T>;
 #[cfg(test)]
 mod tests {
     use super::*;
-    use entity::EntityRef;
-    use ir::Inst;
+    use EntityRef;
+
+    /// An opaque reference to an instruction in a function.
+    #[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+    pub struct Inst(u32);
+    entity_impl!(Inst, "inst");
 
     // Mock key-value object for testing.
     #[derive(PartialEq, Eq, Debug)]