Improve a todo error message

compiler/solve/src/solve.rs

@@ -260,7 +260,10 @@ fn solve(
 
                     state
                 }
-                _ => todo!(),
+                other => todo!(
+                    "the case where unify for Pattern was unsuccessful with {:?}",
+                    other
+                ),
             }
         }
         Let(let_con) => {

compiler/unify/src/unify.rs

@@ -69,6 +69,7 @@ struct Context {
     second_desc: Descriptor,
 }
 
+#[derive(Debug)]
 pub enum Unified {
     Success(Pool),
     Failure(Pool, ErrorType, ErrorType),