diff --git a/compiler/can/src/annotation.rs b/compiler/can/src/annotation.rs
index 98ba3636f2..69a3a3d564 100644
--- a/compiler/can/src/annotation.rs
+++ b/compiler/can/src/annotation.rs
@@ -1,6 +1,6 @@
 use crate::env::Env;
 use crate::scope::Scope;
-use roc_collections::all::{ImMap, MutSet, SendMap};
+use roc_collections::all::{ImMap, MutMap, MutSet, SendMap};
 use roc_module::ident::{Ident, Lowercase, TagName};
 use roc_module::symbol::Symbol;
 use roc_parse::ast::{AssignedField, Tag, TypeAnnotation};
@@ -58,6 +58,12 @@ impl IntroducedVariables {
     }
 }
 
+#[derive(Debug, Default)]
+pub struct VariablySizedTypes {
+    rigids: MutMap<Lowercase, ()>,
+    aliases: MutMap<Symbol, ()>,
+}
+
 pub fn canonicalize_annotation(
     env: &mut Env,
     scope: &mut Scope,
diff --git a/compiler/load/src/file.rs b/compiler/load/src/file.rs
index 68ed8b7eaa..283127a09a 100644
--- a/compiler/load/src/file.rs
+++ b/compiler/load/src/file.rs
@@ -5,6 +5,7 @@ use crossbeam::deque::{Injector, Stealer, Worker};
 use crossbeam::thread;
 use parking_lot::Mutex;
 use roc_builtins::std::{Mode, StdLib};
+use roc_can::annotation::VariablySizedTypes;
 use roc_can::constraint::Constraint;
 use roc_can::def::Declaration;
 use roc_can::module::{canonicalize_module_defs, Module};
@@ -210,6 +211,7 @@ struct ModuleCache<'a> {
     found_specializations: MutMap<ModuleId, FoundSpecializationsModule<'a>>,
     external_specializations_requested: MutMap<ModuleId, ExternalSpecializations>,
     documentation: MutMap<ModuleId, ModuleDocumentation>,
+    host_exposed_variably_sized_types: MutMap<Symbol, VariablySizedTypes>,
 }
 
 fn start_phase<'a>(module_id: ModuleId, phase: Phase, state: &mut State<'a>) -> BuildTask<'a> {