Only assert that rigids are introduced at given rank behind flag

Closes #2489

compiler/solve/src/solve.rs

@@ -678,6 +678,8 @@ fn solve(
                 pools.get_mut(next_rank).extend(pool_variables);
 
                 debug_assert_eq!(
+                    // Check that no variable ended up in a higher rank than the next rank.. that
+                    // would mean we generalized one level more than we need to!
                     {
                         let offenders = pools
                             .get(next_rank)
@@ -704,7 +706,7 @@ fn solve(
                 pools.get_mut(next_rank).clear();
 
                 // check that things went well
-                debug_assert!({
+                if cfg!(debug_assertions) && std::env::var("ROC_VERIFY_RIGID_RANKS").is_ok() {
                     let rigid_vars = &constraints.variables[let_con.rigid_vars.indices()];
 
                     // NOTE the `subs.redundant` check does not come from elm.
@@ -720,13 +722,9 @@ fn solve(
                         let failing: Vec<_> = it.collect();
                         println!("Rigids {:?}", &rigid_vars);
                         println!("Failing {:?}", failing);
-
+                        assert!(false)
-                        // nicer error message
+                    }
-                        failing.is_empty()
-                    } else {
-                        true
                 }
-                });
 
                 let mut new_env = env.clone();
                 for (symbol, loc_var) in local_def_vars.iter() {

compiler/solve/tests/solve_expr.rs

@@ -6215,4 +6215,23 @@ mod solve_expr {
             "a -> Task a *",
         );
     }
+
+    #[test]
+    fn export_rigid_to_lower_rank() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                app "test" provides [ foo ] to "./platform"
+
+                F a : { foo : a }
+
+                foo = \arg ->
+                    x : F b
+                    x = arg
+                    x.foo
+                "#
+            ),
+            "F b -> b",
+        )
+    }
 }