wasm: Get List.map2 working

compiler/mono/src/code_gen_help/mod.rs

@@ -25,7 +25,7 @@ const ARG_2: Symbol = Symbol::ARG_2;
 pub const REFCOUNT_MAX: usize = 0;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
-enum HelperOp {
+pub enum HelperOp {
     Inc,
     Dec,
     DecRef(JoinPointId),
@@ -185,16 +185,16 @@ impl<'a> CodeGenHelp<'a> {
     /// Generate a refcount increment procedure, *without* a Call expression.
     /// *This method should be rarely used* - only when the proc is to be called from Zig.
     /// Otherwise you want to generate the Proc and the Call together, using another method.
-    /// This only supports the 'inc' operation, as it's the only real use case we have.
-    pub fn gen_refcount_inc_proc(
+    pub fn gen_refcount_proc(
         &mut self,
         ident_ids: &mut IdentIds,
         layout: Layout<'a>,
+        op: HelperOp,
     ) -> (Symbol, Vec<'a, (Symbol, ProcLayout<'a>)>) {
         let mut ctx = Context {
             new_linker_data: Vec::new_in(self.arena),
             recursive_union: None,
-            op: HelperOp::Inc,
+            op,
         };
 
         let proc_name = self.find_or_create_proc(ident_ids, &mut ctx, layout);

compiler/mono/src/code_gen_help/refcount.rs

@@ -107,7 +107,9 @@ pub fn refcount_generic<'a>(
 
     match layout {
         Layout::Builtin(Builtin::Int(_) | Builtin::Float(_) | Builtin::Bool | Builtin::Decimal) => {
-            unreachable!("Not refcounted: {:?}", layout)
+            // Generate a dummy function that immediately returns Unit
+            // Some higher-order Zig builtins *always* call an RC function on List elements.
+            rc_return_stmt(root, ident_ids, ctx)
         }
         Layout::Builtin(Builtin::Str) => refcount_str(root, ident_ids, ctx),
         Layout::Builtin(Builtin::List(elem_layout)) => {