give better debug info for unresolved type variables

compiler/mono/src/ir.rs

@@ -4615,7 +4615,15 @@ fn reuse_function_symbol<'a>(
     original: Symbol,
 ) -> Stmt<'a> {
     match procs.partial_procs.get(&original) {
-        None => result,
+        None => {
+            // danger: a foreign symbol may not be specialized!
+            debug_assert!(
+                env.home == original.module_id() || original.module_id() == ModuleId::ATTR
+            );
+
+            result
+        }
+
         Some(partial_proc) => {
             let arg_var = arg_var.unwrap_or(partial_proc.annotation);
             // this symbol is a function, that is used by-name (e.g. as an argument to another

compiler/mono/src/layout.rs

@@ -315,7 +315,10 @@ impl<'a> Layout<'a> {
 
         match content {
             FlexVar(_) | RigidVar(_) => Err(LayoutProblem::UnresolvedTypeVar(var)),
-            RecursionVar { .. } => todo!("recursion var"),
+            RecursionVar { structure, .. } => {
+                let structure_content = env.subs.get_without_compacting(structure).content;
+                Self::new_help(env, structure, structure_content)
+            }
             Structure(flat_type) => layout_from_flat_type(env, flat_type),
 
             Alias(Symbol::NUM_INT, args, _) => {
@@ -751,7 +754,7 @@ fn layout_from_flat_type<'a>(
             // Determine the layouts of the fields, maintaining sort order
             let mut layouts = Vec::with_capacity_in(sorted_fields.len(), arena);
 
-            for (_, field) in sorted_fields {
+            for (label, field) in sorted_fields {
                 use LayoutProblem::*;
 
                 let field_var = {
@@ -776,7 +779,14 @@ fn layout_from_flat_type<'a>(
                             layouts.push(layout);
                         }
                     }
-                    Err(UnresolvedTypeVar(_)) | Err(Erroneous) => {
+                    Err(UnresolvedTypeVar(v)) => {
+                        // Invalid field!
+                        panic!(
+                            r"I hit an unresolved type var {:?} when determining the layout of {:?} of record field: {:?} : {:?}",
+                            field_var, v, label, field
+                        );
+                    }
+                    Err(Erroneous) => {
                         // Invalid field!
                         panic!("TODO gracefully handle record with invalid field.var");
                     }

examples/effect/Main.roc

@@ -1,12 +1,21 @@
 app Main provides [ main ] imports [ Effect, RBTree ]
 
-foo : RBTree.Dict Int Int
-foo = Empty # RBTree.empty
+toAndFro : Int
+toAndFro =
+    empty : RBTree.Dict Int Int
+    empty = RBTree.empty
+
+    empty
+        |> RBTree.toList
+        |> List.len
+
+
+
 
 main : Effect.Effect {} as Fx
 main =
     # if RBTree.isEmpty empty then
-    if RBTree.size foo == 0 then
+    if toAndFro == 2 then
         Effect.putLine "Yay"
             |> Effect.after (\{} -> Effect.getLine)
             |> Effect.after (\line -> Effect.putLine line)