Attribute Macro opaque_enum 

#[opaque_enum]

Hides enum variants behind an opaque struct wrapper.

This macro lets a public type keep an enum-like authoring experience while exposing an opaque wrapper instead of public enum variants. This prevents breaking changes when you add, remove, or modify variants.

Examples

use std::fmt::{self, Display, Formatter};

#[opaque_enum]
#[derive(Debug)]
pub enum DatabaseError {
    ConnectionFailed(String),
    QueryFailed { query: String, reason: String },
    PermissionDenied,
}

#[opaque_enum]
impl Display for DatabaseError {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        match self {
            Self::ConnectionFailed(err) => write!(f, "connection failed: {err}"),
            Self::QueryFailed { query, reason } => {
                write!(f, "query `{query}` failed: {reason}")
            }
            Self::PermissionDenied => write!(f, "permission denied"),
        }
    }
}

You can also opt-in to boxing the representation by specifying wrapper = Box:

#[opaque_enum(wrapper = Box)]
#[derive(Debug)]
pub enum LargeError {
    Variant1([u8; 1024]),
    Variant2,
}