Error-Handling Skill

thiserror / anyhow design and refactors - an Agent Skill for Rust 1.97.0 services and libraries.

What This Skill Does

Refactors string errors and unwrap chains into thiserror enums in libraries, anyhow::Result in binaries, context via .context(), and Axum IntoResponse mappings. Adds From impls and #[non_exhaustive] where APIs evolve.

When to Invoke

• PR introduces unwrap in production path
• Library exposes Box<dyn Error>
• HTTP 500 returns raw internal messages
• Migrating failure or error-chain legacy code

Inputs

Outputs

• error.rs module with typed enums
• main returning anyhow::Result
• Handler Result<T, AppError> with IntoResponse
• Test cases for error display redaction

Guardrails

1. Libraries never use anyhow in public signatures.
2. No expect in lib - return Result.
3. Context at operation boundaries - "loading config", not every line.
4. Redact secrets in Display - manual impl when needed.
5. Single error enum per crate when practical - avoid fragmentation.

Recipe

rg 'unwrap\(|expect\(' src/ --type rust
cargo clippy -- -D clippy::unwrap_used
cargo test error_

Working Example

use axum::{http::StatusCode, response::IntoResponse};
use thiserror::Error;
 
#[derive(Debug, Error)]
pub enum AppError {
    #[error("not found")]
    NotFound,
    #[error(transparent)]
    Internal(#[from] anyhow::Error),
}
 
impl IntoResponse for AppError {
    fn into_response(self) -> axum::response::Response {
        let status = match self {
            AppError::NotFound => StatusCode::NOT_FOUND,
            AppError::Internal(_) => StatusCode::INTERNAL_SERVER_ERROR,
        };
        (status, self.to_string()).into_response()
    }
}

FAQs

Related

• anyhow & thiserror
• Custom error types
• Error handling rules
• Axum Service Skill

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.