expose rigids introduced by builtins

they aren't used yet, because their names can 'leak'. Not sure what the best way forward is here

compiler/can/src/def.rs

@@ -199,7 +199,7 @@ pub fn canonicalize_defs<'a>(
                             region: loc_lowercase.region,
                         });
                     } else {
-                        panic!("TODO handle phantom type variables, they are not allowed!");
+                        panic!("TODO handle phantom type variables, they are not allowed!\nThe {:?} variable in the definition of {:?} gives trouble", loc_lowercase, symbol);
                     }
                 }
 

compiler/constrain/src/module.rs

@@ -43,7 +43,7 @@ pub fn constrain_imported_values(
     imports: Vec<Import<'_>>,
     body_con: Constraint,
     var_store: &VarStore,
-) -> Constraint {
+) -> (Vec<Variable>, Constraint) {
     use Constraint::*;
     let mut def_types = SendMap::default();
     let mut rigid_vars = Vec::new();
@@ -81,14 +81,17 @@ pub fn constrain_imported_values(
         }
     }
 
-    Let(Box::new(LetConstraint {
-        rigid_vars,
-        flex_vars: Vec::new(),
-        def_types,
-        def_aliases: SendMap::default(),
-        defs_constraint: True,
-        ret_constraint: body_con,
-    }))
+    (
+        rigid_vars.clone(),
+        Let(Box::new(LetConstraint {
+            rigid_vars,
+            flex_vars: Vec::new(),
+            def_types,
+            def_aliases: SendMap::default(),
+            defs_constraint: True,
+            ret_constraint: body_con,
+        })),
+    )
 }
 
 pub fn load_builtin_aliases(
@@ -110,10 +113,14 @@ pub fn load_builtin_aliases(
         let mut vars = Vec::with_capacity(builtin_alias.vars.len());
 
         for (loc_lowercase, index) in builtin_alias.vars.iter().zip(1..) {
-            let var = free_vars
-                .unnamed_vars
-                .get(&VarId::from_u32(index))
-                .expect("var_id was not instantiated (is it phantom?)");
+            let var = if let Some(result) = free_vars.unnamed_vars.get(&VarId::from_u32(index)) {
+                result
+            } else {
+                panic!(
+                    "var_id {:?} was not instantiated in the body of {:?} : {:?} (is it phantom?)",
+                    index, symbol, &builtin_alias
+                )
+            };
 
             vars.push(Located::at(
                 loc_lowercase.region,

compiler/src/load/mod.rs

@@ -844,7 +844,10 @@ fn solve_module(
     }
 
     // Wrap the existing module constraint in these imported constraints.
-    let constraint = constrain_imported_values(imported_symbols, constraint, &var_store);
+
+    // TODO what to do with the introduced rigids?
+    let (_introduced_rigids, constraint) =
+        constrain_imported_values(imported_symbols, constraint, &var_store);
     let constraint = constrain_imported_aliases(imported_aliases, constraint, &var_store);
     let mut constraint = load_builtin_aliases(&stdlib.aliases, constraint, &var_store);
 

compiler/tests/helpers/mod.rs

@@ -152,8 +152,10 @@ pub fn uniq_expr_with(
         .collect();
 
     // load builtin values
-    let constraint =
-        roc_constrain::module::constrain_imported_values(imports, constraint, &var_store);
+
+    // TODO what to do with those rigids?
+    let (_introduced_rigids, constraint) =
+        roc::constrain::module::constrain_imported_values(imports, constraint, &var_store);
 
     // load builtin types
     let mut constraint =
@@ -205,7 +207,7 @@ pub fn can_expr_with(arena: &Bump, home: ModuleId, expr_str: &str) -> CanExprOut
     let mut scope = Scope::new(home);
     let dep_idents = IdentIds::exposed_builtins(0);
     let mut env = Env::new(home, dep_idents, &module_ids, IdentIds::default());
-    let (loc_expr, output) = canonicalize_expr(
+    let (loc_expr, mut output) = canonicalize_expr(
         &mut env,
         &var_store,
         &mut scope,
@@ -234,8 +236,13 @@ pub fn can_expr_with(arena: &Bump, home: ModuleId, expr_str: &str) -> CanExprOut
         .collect();
 
     //load builtin values
-    let constraint =
-        roc_constrain::module::constrain_imported_values(imports, constraint, &var_store);
+    let (_introduced_rigids, constraint) =
+        roc::constrain::module::constrain_imported_values(imports, constraint, &var_store);
+
+    // TODO determine what to do with those rigids
+    //    for var in introduced_rigids {
+    //        output.ftv.insert(var, format!("internal_{:?}", var).into());
+    //    }
 
     //load builtin types
     let mut constraint = roc_constrain::module::load_builtin_aliases(

compiler/tests/test_uniqueness_infer.rs

@@ -2097,4 +2097,22 @@ mod test_infer_uniq {
             "Attr * Int",
         );
     }
+
+    #[test]
+    fn update_cost() {
+        infer_eq(
+            indoc!(
+                r#"
+                f = \r ->
+                    g = r.q
+                    h = r.p
+
+                    42
+
+                f
+                "#
+            ),
+            "Attr * (Attr (* | a | b) { p : (Attr b *), q : (Attr a *) }* -> Attr * Int)",
+        );
+    }
 }