Best Practices

Patterns that keep masterror-based services predictable: typed domain errors, one stable code taxonomy, transport mapping at the edge, and public messages that never leak internals.

Derive domain errors, map them once

Model each bounded context as an enum with #[derive(Error)] and declare the AppError mapping inline with #[app_error(...)]. The derive generates Display, From<...> for wrapped sources, and the conversion into AppError/AppCode — no hand-written match at every call site.

use masterror::{AppCode, AppError, AppErrorKind, Error};

#[derive(Debug, Error)]
pub enum UserError {
    #[error("user {0} not found")]
    #[app_error(kind = AppErrorKind::NotFound, code = AppCode::NotFound, message)]
    NotFound(u64),

    #[error("email already registered")]
    #[app_error(kind = AppErrorKind::Conflict, code = AppCode::Conflict, message)]
    DuplicateEmail,

    #[error("storage failure")]
    #[app_error(kind = AppErrorKind::Database, code = AppCode::Database)]
    Storage(#[from] std::io::Error)
}

let app: AppError = UserError::DuplicateEmail.into();
assert_eq!(app.kind, AppErrorKind::Conflict);

Include message only on variants whose Display output is safe to show to clients. Omit it (as on Storage) to keep the text internal — the client sees only the kind’s title. Full attribute reference: Derive Macros.

One AppCode taxonomy per service

AppCode is your public API contract; clients branch on it. Keep the set small, documented and per-service:

• Prefer the built-in codes (NOT_FOUND, CONFLICT, VALIDATION, …) — they already carry canonical HTTP/gRPC/problem-type mappings.
• Mint custom codes centrally, not inline at call sites:

use masterror::AppCode;

pub const CODE_PLAN_LIMIT: AppCode = AppCode::new("PLAN_LIMIT_EXCEEDED");

AppCode::new is const and panics at compile time on anything that is not SCREAMING_SNAKE_CASE; use AppCode::try_new for runtime strings. Renaming a code is a breaking API change — treat additions like adding an enum variant.

Map to transports at the edge only

Domain and service layers return AppResult<T> and know nothing about HTTP. The single IntoResponse/ResponseError/Status implementation in the crate does the mapping in the handler layer:

async fn get_user(id: u64, repo: &Repo) -> masterror::AppResult<User> {
    repo.find(id).await?          // sqlx::Error -> AppError::NotFound/Database
}

Never hand-construct status codes in business logic and never implement a second response conversion — the stable AppErrorKind → status table in Web Frameworks is the one source of truth.

Redact sensitive data, keep telemetry

Two independent knobs:

• Message redaction — err.redactable() (or redact(message) in #[masterror(...)]) hides detail from wire payloads while logs keep it.
• Field redaction — per-field policy applied when metadata is serialized:

use masterror::{AppError, FieldRedaction, field};

let err = AppError::bad_request("Invalid credentials")
    .with_field(field::str("email", "user@example.com").with_redaction(FieldRedaction::Hash))
    .with_field(field::str("card", "4111111111111111").with_redaction(FieldRedaction::Last4))
    .with_field(field::str("ip", "192.168.1.100").with_redaction(FieldRedaction::Redact));

Hash keeps correlational value (same input → same digest) without exposing the raw string; Last4 suits card/token suffixes; Redact removes the value entirely. Default is None. Redact anything user-identifying by default and opt out consciously, not the other way around.

Context vs derive

• Derive when the error type is part of your domain vocabulary: it has variants, appears in signatures, and its mapping is static.
• Context (via ResultExt::ctx) when wrapping an infrastructure error ad hoc at a call site and the classification depends on the operation, not the type:

#[cfg(feature = "std")] {
use masterror::{AppErrorKind, Context, ResultExt, field};

fn read_state() -> masterror::AppResult<Vec<u8>> {
    std::fs::read("/var/lib/app/state.bin").ctx(|| {
        Context::new(AppErrorKind::Internal)
            .with(field::str("path", "/var/lib/app/state.bin"))
            .track_caller()
    })
}
}

ctx is lazy — the closure runs only on the error path. Use plain .context("message") when a human-readable note is all you need. Details: Context & Metadata.

Test kinds and codes, not strings

Assert on the stable taxonomy, never on formatted messages:

use masterror::{AppCode, AppError, AppErrorKind, ProblemJson};

let err = AppError::not_found("user 42 missing");
assert_eq!(err.kind, AppErrorKind::NotFound);
assert_eq!(err.code, AppCode::NotFound);

let problem = ProblemJson::from_ref(&err);
assert_eq!(problem.status, 404);
assert_eq!(problem.code.as_str(), "NOT_FOUND");

• ProblemJson::from_ref lets integration tests assert the exact wire contract without spinning up a server.
• mapping_for_code(&code) exposes the canonical HTTP status, gRPC code and problem-type URI for table-driven tests.
• For redaction tests, assert problem.detail.is_none() on a redactable() error and check metadata().iter_with_redaction() policies.

Public message vs internal telemetry

A useful rule for every error you construct:

masterror enforces the last row (sources are never written to wire payloads), but the first two are your responsibility: if a string contains anything you would not print in a browser, put it in metadata with a redaction policy or mark the error redactable().

See also: Derive Macros · Context & Metadata · Error Kinds & Codes · Migration