special-case rose tree field access

compiler/gen_llvm/src/llvm/build.rs

@@ -1438,6 +1438,12 @@ pub fn build_exp_expr<'a, 'ctx, 'env>(
             structure,
             wrapped: Wrapped::RecordOrSingleTagUnion,
             ..
+        }
+        | AccessAtIndex {
+            index,
+            structure,
+            wrapped: Wrapped::LikeARoseTree,
+            ..
         } => {
             // extract field from a record
             match load_symbol_and_layout(scope, structure) {

compiler/mono/src/alias_analysis.rs

@@ -880,6 +880,10 @@ fn expr_spec(
                 Wrapped::RecordOrSingleTagUnion => {
                     builder.add_get_tuple_field(block, value_id, *index as u32)
                 }
+                Wrapped::LikeARoseTree => {
+                    let result_type = layout_spec(builder, layout)?;
+                    builder.add_unknown_with(block, &[value_id], result_type)
+                }
                 Wrapped::MultiTagUnion => {
                     // Clearly this is not generally correct, but it should be for our examples
                     let hacky_is_recursive =

compiler/mono/src/decision_tree.rs

@@ -597,7 +597,7 @@ fn to_relevant_branch_help<'a>(
                                         start.extend(end);
                                     }
                                 }
-                                Wrapped::RecordOrSingleTagUnion => {
+                                Wrapped::RecordOrSingleTagUnion | Wrapped::LikeARoseTree => {
                                     let sub_positions = arguments.into_iter().enumerate().map(
                                         |(index, (pattern, _))| {
                                             let mut new_path = path.to_vec();

compiler/mono/src/ir.rs

@@ -1014,6 +1014,8 @@ pub enum Wrapped {
     EmptyRecord,
     SingleElementRecord,
     RecordOrSingleTagUnion,
+    /// Like a rose tree; recursive, but only one tag
+    LikeARoseTree,
     MultiTagUnion,
 }
 
@@ -1046,7 +1048,7 @@ impl Wrapped {
                         },
                         _ => Some(Wrapped::MultiTagUnion),
                     },
-                    NonNullableUnwrapped(_) => Some(Wrapped::RecordOrSingleTagUnion),
+                    NonNullableUnwrapped(_) => Some(Wrapped::LikeARoseTree),
 
                     NullableWrapped { .. } | NullableUnwrapped { .. } => {
                         Some(Wrapped::MultiTagUnion)

compiler/test_gen/src/gen_primitives.rs

@@ -1997,7 +1997,6 @@ fn case_or_pattern() {
 }
 
 #[test]
-#[ignore]
 fn rosetree_basic() {
     assert_non_opt_evals_to!(
         indoc!(