Is it good practice to directly return a `String` as an error in Rust?

No, it's generally not good practice. While technically possible, returning a `String` directly as an error lacks structure and makes it difficult to handle errors consistently. It doesn't allow for easy error categorization, chaining, or providing context. It's much better to define a custom error type (using `enum` and `Error` trait or a crate like `thiserror`) that encapsulates the error message and potentially other relevant information.