add index

compiler/mono/src/borrow.rs

@@ -565,6 +565,11 @@ impl<'a> BorrowInfState<'a> {
             EmptyArray => {
                 self.own_var(z);
             }
+
+            Call(call) => self.collect_call(z, call),
+
+            Literal(_) | RuntimeErrorFunction(_) => {}
+
             AccessAtIndex { structure: x, .. } => {
                 // if the structure (record/tag/array) is owned, the extracted value is
                 if self.is_owned(*x) {
@@ -577,12 +582,16 @@ impl<'a> BorrowInfState<'a> {
                 }
             }
 
-            Call(call) => self.collect_call(z, call),
+            CoerceToTagId { structure: x, .. } => {
+                // if the structure (record/tag/array) is owned, the extracted value is
+                if self.is_owned(*x) {
+                    self.own_var(z);
+                }
 
-            Literal(_) | RuntimeErrorFunction(_) => {}
+                // if the extracted value is owned, the structure must be too
-
+                if self.is_owned(z) {
-            CoerceToTagId { .. } => {
+                    self.own_var(*x);
-                // do nothing for now; when we extract the fields here, that must change
+                }
             }
 
             GetTagId { structure: x, .. } => {

compiler/mono/src/inc_dec.rs

@@ -743,17 +743,6 @@ impl<'a> Context<'a> {
                 self.arena.alloc(Stmt::Let(z, v, l, b)),
                 &b_live_vars,
             ),
-            AccessAtIndex { structure: x, .. } => {
-                let b = self.add_dec_if_needed(x, b, b_live_vars);
-                let info_x = self.get_var_info(x);
-                let b = if info_x.consume {
-                    self.add_inc(z, 1, b)
-                } else {
-                    b
-                };
-
-                self.arena.alloc(Stmt::Let(z, v, l, b))
-            }
 
             Call(crate::ir::Call {
                 call_type,
@@ -766,6 +755,18 @@ impl<'a> Context<'a> {
                 self.arena.alloc(Stmt::Let(z, v, l, b))
             }
 
+            AccessAtIndex { structure: x, .. } => {
+                let b = self.add_dec_if_needed(x, b, b_live_vars);
+                let info_x = self.get_var_info(x);
+                let b = if info_x.consume {
+                    self.add_inc(z, 1, b)
+                } else {
+                    b
+                };
+
+                self.arena.alloc(Stmt::Let(z, v, l, b))
+            }
+
             GetTagId { structure: x, .. } => {
                 let b = self.add_dec_if_needed(x, b, b_live_vars);
                 let info_x = self.get_var_info(x);

compiler/mono/src/ir.rs

@@ -1200,6 +1200,8 @@ pub enum Expr<'a> {
     CoerceToTagId {
         structure: Symbol,
         tag_id: u8,
+        union_layout: UnionLayout<'a>,
+        index: u64,
     },
 
     Array {