make bumpmap new functions using a trait

compiler/collections/src/all.rs

@@ -30,6 +30,26 @@ pub type SendSet<K> = im::hashset::HashSet<K, BuildHasher>;
 
 pub type BumpMap<'a, K, V> = hashbrown::HashMap<K, V, BuildHasher, hashbrown::BumpWrapper<'a>>;
 
+pub trait BumpMapDefault<'a> {
+    fn new_in(arena: &'a bumpalo::Bump) -> Self;
+
+    fn with_capacity_in(capacity: usize, arena: &'a bumpalo::Bump) -> Self;
+}
+
+impl<'a, K, V> BumpMapDefault<'a> for BumpMap<'a, K, V> {
+    fn new_in(arena: &'a bumpalo::Bump) -> Self {
+        hashbrown::HashMap::with_hasher_in(default_hasher(), hashbrown::BumpWrapper(arena))
+    }
+
+    fn with_capacity_in(capacity: usize, arena: &'a bumpalo::Bump) -> Self {
+        hashbrown::HashMap::with_capacity_and_hasher_in(
+            capacity,
+            default_hasher(),
+            hashbrown::BumpWrapper(arena),
+        )
+    }
+}
+
 pub fn arena_join<'a, I>(arena: &'a Bump, strings: &mut I, join_str: &str) -> String<'a>
 where
     I: Iterator<Item = &'a str>,

compiler/mono/src/ir.rs

@@ -6,7 +6,7 @@ use crate::layout::{
 };
 use bumpalo::collections::Vec;
 use bumpalo::Bump;
-use roc_collections::all::{default_hasher, BumpMap, MutMap, MutSet};
+use roc_collections::all::{default_hasher, BumpMap, BumpMapDefault, MutMap, MutSet};
 use roc_module::ident::{ForeignSymbol, Lowercase, TagName};
 use roc_module::low_level::LowLevel;
 use roc_module::symbol::{IdentIds, ModuleId, Symbol};
@@ -1833,8 +1833,7 @@ fn specialize_external<'a>(
     let host_exposed_layouts = if host_exposed_variables.is_empty() {
         HostExposedLayouts::NotHostExposed
     } else {
-        let mut aliases =
+        let mut aliases = BumpMap::new_in(env.arena);
-            hashbrown::HashMap::with_hasher_in(default_hasher(), hashbrown::BumpWrapper(env.arena));
 
         for (symbol, variable) in host_exposed_variables {
             let layout = layout_cache