diff --git a/src/ena/unify/backing_vec.rs b/src/ena/unify/backing_vec.rs
index 2a07444ae3..0430336c4a 100644
--- a/src/ena/unify/backing_vec.rs
+++ b/src/ena/unify/backing_vec.rs
@@ -29,7 +29,7 @@ pub trait UnificationStore:
 
     fn values_since_snapshot(&self, snapshot: &Self::Snapshot) -> Range<usize>;
 
-    fn reset_unifications(&mut self, value: impl FnMut(u32) -> VarValue<Self::Key>);
+    fn reset_unifications(&mut self, value: impl FnMut(usize) -> VarValue<Self::Key>);
 
     fn len(&self) -> usize;
 
@@ -90,8 +90,8 @@ impl<K: UnifyKey> UnificationStore for InPlace<K> {
     }
 
     #[inline]
-    fn reset_unifications(&mut self, mut value: impl FnMut(u32) -> VarValue<Self::Key>) {
-        self.values.set_all(|i| value(i as u32));
+    fn reset_unifications(&mut self, mut value: impl FnMut(usize) -> VarValue<Self::Key>) {
+        self.values.set_all(|i| value(i));
     }
 
     fn len(&self) -> usize {
@@ -182,12 +182,12 @@ impl<K: UnifyKey> UnificationStore for Persistent<K> {
     }
 
     #[inline]
-    fn reset_unifications(&mut self, mut value: impl FnMut(u32) -> VarValue<Self::Key>) {
+    fn reset_unifications(&mut self, mut value: impl FnMut(usize) -> VarValue<Self::Key>) {
         // Without extending dogged, there isn't obviously a more
         // efficient way to do this. But it's pretty dumb. Maybe
         // dogged needs a `map`. [NOTE: revisit in light of replacing dogged with im_rc!]
         for i in 0..self.values.len() {
-            self.values[i] = value(i as u32);
+            self.values[i] = value(i);
         }
     }
 
diff --git a/src/ena/unify/mod.rs b/src/ena/unify/mod.rs
index 4455e4f794..f87d56e04b 100644
--- a/src/ena/unify/mod.rs
+++ b/src/ena/unify/mod.rs
@@ -55,9 +55,9 @@ pub use self::backing_vec::Persistent;
 pub trait UnifyKey: Copy + Clone + Debug + PartialEq {
     type Value: Clone + Debug;
 
-    fn index(&self) -> u32;
+    fn index(&self) -> usize;
 
-    fn from_index(u: u32) -> Self;
+    fn from_index(u: usize) -> Self;
 
     fn tag() -> &'static str;
 
@@ -215,7 +215,7 @@ impl<S: UnificationStore> UnificationTable<S> {
     /// Creates a fresh key with the given value.
     pub fn new_key(&mut self, value: S::Value) -> S::Key {
         let len = self.values.len();
-        let key: S::Key = UnifyKey::from_index(len as u32);
+        let key: S::Key = UnifyKey::from_index(len);
         self.values.push(VarValue::new_var(key, value));
         debug!("{}: created new key: {:?}", S::tag(), key);
         key
@@ -232,7 +232,7 @@ impl<S: UnificationStore> UnificationTable<S> {
     /// the closure.
     pub fn reset_unifications(&mut self, mut value: impl FnMut(S::Key) -> S::Value) {
         self.values.reset_unifications(|i| {
-            let key = UnifyKey::from_index(i as u32);
+            let key = UnifyKey::from_index(i);
             let value = value(key);
             VarValue::new_var(key, value)
         });
@@ -251,7 +251,7 @@ impl<S: UnificationStore> UnificationTable<S> {
     /// Returns the keys of all variables created since the `snapshot`.
     pub fn vars_since_snapshot(&self, snapshot: &Snapshot<S>) -> Range<S::Key> {
         let range = self.values.values_since_snapshot(&snapshot.snapshot);
-        S::Key::from_index(range.start as u32)..S::Key::from_index(range.end as u32)
+        S::Key::from_index(range.start)..S::Key::from_index(range.end)
     }
 
     /// Obtains the current value for a particular key.
diff --git a/src/subs.rs b/src/subs.rs
index 9838395df2..88b4ca537c 100644
--- a/src/subs.rs
+++ b/src/subs.rs
@@ -9,10 +9,10 @@ pub struct Subs {
 }
 
 #[derive(Copy, PartialEq, Eq, Clone, Hash)]
-pub struct Variable(u32);
+pub struct Variable(usize);
 
 impl Variable {
-    pub fn new_for_testing_only(num: u32) -> Self {
+    pub fn new_for_testing_only(num: usize) -> Self {
         // This is a hack that should only ever be used for testing!
         Variable(num)
     }
@@ -27,11 +27,11 @@ impl fmt::Debug for Variable {
 impl UnifyKey for Variable {
     type Value = Descriptor;
 
-    fn index(&self) -> u32 {
+    fn index(&self) -> usize {
         self.0
     }
 
-    fn from_index(index: u32) -> Self {
+    fn from_index(index: usize) -> Self {
         Variable(index)
     }