diff --git a/crates/compiler/load_internal/src/file.rs b/crates/compiler/load_internal/src/file.rs
index 66d697d976..7d10a7a86c 100644
--- a/crates/compiler/load_internal/src/file.rs
+++ b/crates/compiler/load_internal/src/file.rs
@@ -4713,7 +4713,7 @@ fn canonicalize_and_constrain<'a>(
         &symbols_from_requires,
         &mut var_store,
     );
-    let types = Types::new();
+    let mut types = Types::new();
 
     // _after has an underscore because it's unused in --release builds
     let _after = roc_types::types::get_type_clone_count();
diff --git a/crates/compiler/solve/src/ability.rs b/crates/compiler/solve/src/ability.rs
index 41ec8267aa..e117298b4c 100644
--- a/crates/compiler/solve/src/ability.rs
+++ b/crates/compiler/solve/src/ability.rs
@@ -75,6 +75,7 @@ impl PendingDerivesTable {
                 let derive_key = RequestedDeriveKey { opaque, ability };
 
                 // Neither rank nor pools should matter here.
+                let typ = types.from_old_type(&typ);
                 let opaque_var = type_to_var(
                     subs,
                     Rank::toplevel(),
@@ -84,7 +85,7 @@ impl PendingDerivesTable {
                     &mut Pools::default(),
                     types,
                     aliases,
-                    &typ,
+                    typ,
                 );
                 let real_var = match subs.get_content_without_compacting(opaque_var) {
                     Content::Alias(_, _, real_var, AliasKind::Opaque) => real_var,
diff --git a/crates/compiler/solve/src/solve.rs b/crates/compiler/solve/src/solve.rs
index 179263aadd..39042aa3ff 100644
--- a/crates/compiler/solve/src/solve.rs
+++ b/crates/compiler/solve/src/solve.rs
@@ -1347,10 +1347,10 @@ fn solve(
                     .map(|v| Type::Variable(*v))
                     .collect();
 
-                let tag_ty = Type::TagUnion(
+                let tag_ty = can_types.from_old_type(&Type::TagUnion(
                     vec![(tag_name.clone(), payload_types)],
                     TypeExtension::Closed,
-                );
+                ));
                 let includes = type_to_var(
                     subs,
                     rank,
@@ -1360,7 +1360,7 @@ fn solve(
                     pools,
                     &mut can_types,
                     aliases,
-                    &tag_ty,
+                    tag_ty,
                 );
 
                 match unify(
@@ -2265,7 +2265,7 @@ fn type_cell_to_var(
     aliases: &mut Aliases,
     typ_cell: &Cell<Index<TypeTag>>,
 ) -> Variable {
-    let typ = typ_cell.replace(Type::EmptyTagUnion);
+    let typ = typ_cell.get();
     let var = type_to_var(
         subs,
         rank,
@@ -2275,9 +2275,12 @@ fn type_cell_to_var(
         pools,
         types,
         aliases,
-        &typ,
+        typ,
     );
-    typ_cell.replace(Type::Variable(var));
+    unsafe {
+        types.set_variable(typ, var);
+    }
+
     var
 }
 
@@ -2290,13 +2293,12 @@ pub(crate) fn type_to_var(
     pools: &mut Pools,
     types: &mut Types,
     aliases: &mut Aliases,
-    typ: &Type,
+    typ: Index<TypeTag>,
 ) -> Variable {
-    if let Type::Variable(var) = typ {
-        *var
+    if let TypeTag::Variable(var) = types[typ] {
+        var
     } else {
         let mut arena = take_scratchpad();
-        let typ = types.from_old_type(typ);
 
         let var = type_to_variable(
             subs,
diff --git a/crates/compiler/test_derive/src/util.rs b/crates/compiler/test_derive/src/util.rs
index 8c011f4a84..92fbcec7f6 100644
--- a/crates/compiler/test_derive/src/util.rs
+++ b/crates/compiler/test_derive/src/util.rs
@@ -27,6 +27,7 @@ use roc_reporting::report::{type_problem, RocDocAllocator};
 use roc_types::{
     pretty_print::{name_and_print_var, DebugPrint},
     subs::{ExposedTypesStorageSubs, Subs, Variable},
+    types::Types,
 };
 
 const DERIVED_MODULE: ModuleId = ModuleId::DERIVED_SYNTH;
@@ -343,11 +344,12 @@ fn check_derived_typechecks_and_golden(
     check_golden: impl Fn(&str),
 ) {
     // constrain the derived
+    let mut types = Types::new();
     let mut constraints = Constraints::new();
     let def_var = derived_def.expr_var;
     let mut decls = Declarations::new();
     decls.push_def(derived_def);
-    let constr = constrain_decls(&mut constraints, test_module, &decls);
+    let constr = constrain_decls(&mut types, &mut constraints, test_module, &decls);
 
     // the derived implementation on stuff from the builtin module, so
     //   - we need to add those dependencies as imported on the constraint
@@ -394,7 +396,7 @@ fn check_derived_typechecks_and_golden(
     );
     let (mut solved_subs, _, problems, _) = roc_solve::module::run_solve(
         test_module,
-        Default::default(),
+        types,
         &constraints,
         constr,
         RigidVariables::default(),
diff --git a/crates/compiler/types/src/types.rs b/crates/compiler/types/src/types.rs
index 0c52ee4a42..33ce9ff995 100644
--- a/crates/compiler/types/src/types.rs
+++ b/crates/compiler/types/src/types.rs
@@ -586,6 +586,13 @@ impl Types {
         (tags, payload_slices)
     }
 
+    /// # Safety
+    ///
+    /// May only be called if `var` is known to represent the type at `index`.
+    pub unsafe fn set_variable(&mut self, index: Index<TypeTag>, var: Variable) {
+        self.tags[index.index()] = TypeTag::Variable(var);
+    }
+
     fn reserve_type_tags(&mut self, length: usize) -> Slice<TypeTag> {
         use std::iter::repeat;
 
diff --git a/crates/reporting/tests/helpers/mod.rs b/crates/reporting/tests/helpers/mod.rs
index 195a27b6b3..b65a928648 100644
--- a/crates/reporting/tests/helpers/mod.rs
+++ b/crates/reporting/tests/helpers/mod.rs
@@ -143,11 +143,12 @@ pub fn can_expr_with<'a>(
         }
     };
 
+    let mut types = Types::new();
     let mut constraints = Constraints::new();
 
     let mut var_store = VarStore::default();
     let var = var_store.fresh();
-    let var_index = constraints.push_type(Type::Variable(var));
+    let var_index = constraints.push_variable(var);
     let expected = constraints.push_expected_type(Expected::NoExpectation(var_index));
     let mut module_ids = ModuleIds::default();
 
@@ -176,6 +177,7 @@ pub fn can_expr_with<'a>(
     );
 
     let constraint = constrain_expr(
+        &mut types,
         &mut constraints,
         &mut roc_constrain::expr::Env {
             rigids: MutMap::default(),
@@ -205,7 +207,7 @@ pub fn can_expr_with<'a>(
         var,
         constraint,
         constraints,
-        types: Default::default(),
+        types,
     })
 }