add Store constraint that does not report errors

compiler/unify/src/unify.rs

@@ -1089,7 +1089,7 @@ fn unify_rigid(subs: &mut Subs, ctx: &Context, name: &Lowercase, other: &Content
         RigidVar(_) | RecursionVar { .. } | Structure(_) | Alias(_, _, _) => {
             // Type mismatch! Rigid can only unify with flex, even if the
             // rigid names are the same.
-            mismatch!("Rigid with {:?}", &other)
+            mismatch!("Rigid {:?} with {:?}", ctx.first, &other)
         }
         Error => {
             // Error propagates.