diff --git a/compiler/load/src/file.rs b/compiler/load/src/file.rs
index ad541252f7..543c75c037 100644
--- a/compiler/load/src/file.rs
+++ b/compiler/load/src/file.rs
@@ -8,7 +8,7 @@ use roc_builtins::std::StdLib;
 use roc_can::constraint::Constraint;
 use roc_can::def::{Declaration, Def};
 use roc_can::module::{canonicalize_module_defs, Module};
-use roc_collections::all::{default_hasher, BumpMap, BumpMapDefault, BumpSet, MutMap, MutSet};
+use roc_collections::all::{default_hasher, BumpMap, MutMap, MutSet};
 use roc_constrain::module::{
     constrain_imports, pre_constrain_imports, ConstrainableImports, Import,
 };
@@ -553,7 +553,7 @@ fn start_phase<'a>(
                     ident_ids,
                 } = typechecked;
 
-                let mut imported_module_thunks = BumpSet::new_in(arena);
+                let mut imported_module_thunks = bumpalo::collections::Vec::new_in(arena);
 
                 if let Some(imports) = state.module_cache.imports.get(&module_id) {
                     for imported in imports.iter() {
@@ -570,7 +570,7 @@ fn start_phase<'a>(
                     module_id,
                     module_timing,
                     solved_subs,
-                    imported_module_thunks,
+                    imported_module_thunks: imported_module_thunks.into_bump_slice(),
                     decls,
                     ident_ids,
                     exposed_to_host: state.exposed_to_host.clone(),
@@ -1035,7 +1035,7 @@ enum BuildTask<'a> {
         module_timing: ModuleTiming,
         layout_cache: LayoutCache<'a>,
         solved_subs: Solved<Subs>,
-        imported_module_thunks: BumpSet<Symbol>,
+        imported_module_thunks: &'a [Symbol],
         module_id: ModuleId,
         ident_ids: IdentIds,
         decls: Vec<Declaration>,
@@ -3985,7 +3985,7 @@ fn make_specializations<'a>(
 fn build_pending_specializations<'a>(
     arena: &'a Bump,
     solved_subs: Solved<Subs>,
-    imported_module_thunks: BumpSet<Symbol>,
+    imported_module_thunks: &'a [Symbol],
     home: ModuleId,
     mut ident_ids: IdentIds,
     decls: Vec<Declaration>,
diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index 61a5869d46..f64f338c21 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -350,7 +350,7 @@ impl<'a> ExternalSpecializations<'a> {
 #[derive(Clone, Debug)]
 pub struct Procs<'a> {
     pub partial_procs: BumpMap<Symbol, PartialProc<'a>>,
-    pub imported_module_thunks: BumpSet<Symbol>,
+    pub imported_module_thunks: &'a [Symbol],
     pub module_thunks: BumpSet<Symbol>,
     pub pending_specializations:
         Option<BumpMap<Symbol, MutMap<ProcLayout<'a>, PendingSpecialization<'a>>>>,
@@ -364,7 +364,7 @@ impl<'a> Procs<'a> {
     pub fn new_in(arena: &'a Bump) -> Self {
         Self {
             partial_procs: BumpMap::new_in(arena),
-            imported_module_thunks: BumpSet::new_in(arena),
+            imported_module_thunks: &[],
             module_thunks: BumpSet::new_in(arena),
             pending_specializations: Some(BumpMap::new_in(arena)),
             specialized: BumpMap::new_in(arena),
@@ -382,6 +382,10 @@ pub enum InProgressProc<'a> {
 }
 
 impl<'a> Procs<'a> {
+    fn is_imported_module_thunk(&self, symbol: Symbol) -> bool {
+        self.imported_module_thunks.iter().any(|x| *x == symbol)
+    }
+
     pub fn get_specialized_procs_without_rc(
         self,
         env: &mut Env<'a, '_>,
@@ -6091,7 +6095,7 @@ fn reuse_function_symbol<'a>(
                         .raw_from_var(env.arena, arg_var, env.subs)
                         .expect("creating layout does not fail");
 
-                    if procs.imported_module_thunks.contains(&original) {
+                    if procs.is_imported_module_thunk(original) {
                         let layout = match raw {
                             RawFunctionLayout::ZeroArgumentThunk(layout) => layout,
                             RawFunctionLayout::Function(_, lambda_set, _) => {
@@ -6562,7 +6566,7 @@ fn call_by_name_help<'a>(
         add_needed_external(procs, env, original_fn_var, proc_name);
 
         debug_assert_ne!(proc_name.module_id(), ModuleId::ATTR);
-        if procs.imported_module_thunks.contains(&proc_name) {
+        if procs.is_imported_module_thunk(proc_name) {
             force_thunk(
                 env,
                 proc_name,