error: invalid key type `Struct`
   ╭▸ test/ui/invalid_key_type.aldrin:29:20
   │
29 │ newtype Set4 = set<Struct>;
   │                    ━━━━━━ type used here
   │
   ├ help: allowed key types are `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`,
   │       `string`, `uuid` and newtypes resolving to one of those
   ╰ note: `Struct` is a struct