Module snafu::guide::opaque

How to create opaque error types for public APIs

While creating error types on top of Rust enums allows for great flexibility inside your code, that same flexibility may not be desired in a public API. Public enums also expose their variants and the variant’s fields, allowing consumers to rely on those details.

The most conservative approach is to create an opaque error type that only implements a handful of traits. This can be done by deriving Snafu for a newtype struct that contains another SNAFU error:

#[derive(Debug, Snafu)]
pub struct Error(InnerError);

// That's all it takes! The rest is demonstration of how to use it.

pub fn login(id: i32) -> Result<(), Error> {
    validate_user(id)?;
    is_user_locked(id)?;
    Ok(())
}

#[derive(Debug, Snafu)]
enum InnerError {
    #[snafu(display("User ID {} is invalid", user_id))]
    InvalidUser { user_id: i32 },
    #[snafu(display("User ID {} is locked", user_id))]
    UserLocked { user_id: i32 },
}

fn validate_user(user_id: i32) -> Result<(), InnerError> {
    InvalidUser { user_id }.fail()
}

fn is_user_locked(user_id: i32) -> Result<(), InnerError> {
    UserLocked { user_id }.fail()
}

Delegated traits

• Error
• Display
• ErrorCompat

From

The From trait is also implemented to convert the inner type into the opaque type. This makes converting from internal errors to public errors very easy.