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rust.axum.missing_error_handler

Stability High

Detects Axum handlers without proper error handling, leading to unhelpful 500 errors.

Why It Matters

Missing error handlers in Axum:

Poor UX — Generic 500 errors confuse users
No logging — Errors lost without tracing
Security risk — Internal details may leak

Example

// ❌ Before (error propagated as-is)
async fn get_user(Path(id): Path<i32>) -> impl IntoResponse {
    let user = db.find_user(id).await?; // Error becomes 500
    Json(user)
}

// ✅ After (with proper error handling)
use axum::response::IntoResponse;

#[derive(Debug)]
enum AppError {
    NotFound,
    Database(sqlx::Error),
}

impl IntoResponse for AppError {
    fn into_response(self) -> Response {
        let (status, message) = match self {
            AppError::NotFound => (StatusCode::NOT_FOUND, "Not found"),
            AppError::Database(e) => {
                tracing::error!(?e, "Database error");
                (StatusCode::INTERNAL_SERVER_ERROR, "Internal error")
            }
        };
        (status, Json(json!({ "error": message }))).into_response()
    }
}

async fn get_user(Path(id): Path<i32>) -> Result<Json<User>, AppError> {
    let user = db.find_user(id).await.map_err(AppError::Database)?;
    user.ok_or(AppError::NotFound).map(Json)
}

What Unfault Detects

Handlers returning Result without IntoResponse impl
Missing error type conversions
Anyhow errors propagated directly

Auto-Fix

Unfault can generate AppError enum with IntoResponse implementation.