Feature Flags

masterror keeps the default build lean: only std is enabled out of the box. Everything else — web transports, telemetry, library integrations — is opt-in. This page is the complete reference for every flag declared in Cargo.toml.

[dependencies]
masterror = { version = "0.28", default-features = false }
# or with features:
# masterror = { version = "0.28", features = [
#   "std", "axum", "actix", "openapi",
#   "serde_json", "tracing", "metrics", "backtrace",
#   "colored", "sqlx", "sqlx-migrate", "reqwest",
#   "redis", "validator", "config", "tokio",
#   "multipart", "teloxide", "init-data", "tonic",
#   "frontend", "turnkey", "benchmarks"
# ] }

Core

Flag	What it enables	Extra deps
`std` (default)	Standard library support; required by all runtime integrations. Disable for `no_std` (see No-Std)	—

Web transports

Flag	What it enables	Extra deps
`axum`	`IntoResponse` for `AppError` and `ProblemJson` with RFC 7807 JSON bodies; `AppErrorKind::status_code()`	`axum` (json, multipart), `serde_json`
`actix`	Actix Web `ResponseError` for `AppError` and `Responder` for `ProblemJson`	`actix-web`
`multipart`	Maps `axum::extract::multipart::MultipartError` → `BadRequest` (implies `axum`)	via `axum`
`openapi`	`utoipa::ToSchema` for `ErrorResponse` and `AppCode` so error payloads appear in OpenAPI specs	`utoipa`
`serde_json`	Structured JSON `details` on `AppError`/`ErrorResponse`/`ProblemJson`; `FieldValue::Json` and `field::json`	`serde_json`
`tonic`	Conversion of errors into `tonic::Status` with sanitized metadata; exports `StatusConversionError`	`tonic`

Telemetry and observability

Flag	What it enables	Extra deps
`tracing`	Structured `tracing` events emitted when errors are constructed	`tracing`, `log`, `log-mdc`
`metrics`	Increments an `error_total{code,category}` counter for each `AppError`	`metrics`
`backtrace`	Lazy `std::backtrace::Backtrace` capture (honours `RUST_BACKTRACE`), `with_backtrace()` builder	—
`colored`	Colored multi-line terminal output with automatic TTY detection; richer `Display` for `AppError`	`owo-colors`

Async and IO integrations

Each integration flag adds a From<...> for AppError conversion that classifies the library error into the taxonomy:

Flag	Conversion	Extra deps
`sqlx`	`sqlx_core::Error` → `NotFound` / `Database` (lean `sqlx-core`, no drivers or TLS)	`sqlx-core`
`sqlx-migrate`	`sqlx::migrate::MigrateError` → `Database` (full `sqlx` with `migrate` feature only)	`sqlx`
`redis`	`redis::RedisError` → `Cache`	`redis`
`reqwest`	`reqwest::Error` → `Timeout` / `Network` / `ExternalApi`	`reqwest`
`tokio`	`tokio::time::error::Elapsed` → `Timeout`	`tokio` (time)
`validator`	`validator::ValidationErrors` → `Validation`	`validator`
`config`	`config::ConfigError` → `Config`	`config`

sqlx and sqlx-migrate are split deliberately: error classification only needs sqlx-core, while migration error mapping pulls the full sqlx crate.

Messaging and bots

Flag	Conversion	Extra deps
`teloxide`	`teloxide_core::RequestError` → `RateLimited` / `Network` / `ExternalApi` / `Deserialization` / `Internal`	`teloxide-core`
`init-data`	`init_data_rs::InitDataError` → `TelegramAuth` (Telegram Mini Apps init-data validation)	`init-data-rs`

Front-end and domain

Flag	What it enables	Extra deps
`frontend`	`frontend` module: convert errors to `wasm_bindgen::JsValue` and emit `console.error` logs in WASM/browser contexts	`wasm-bindgen`, `js-sys`, `serde-wasm-bindgen`
`turnkey`	`turnkey` module: `TurnkeyErrorKind`, `TurnkeyError`, `classify_turnkey_error` and conversions into `AppError`	—
`benchmarks`	Criterion benchmark suite and CI baseline tooling (local profiling only)	—

Baseline conversions (always available)

Without any feature flag, AppError already converts from:

Source	Target kind
`std::io::Error`	`Internal`
`String`	`BadRequest`

Recipes

REST API on Axum with problem+json, OpenAPI docs and tracing:

masterror = { version = "0.28", features = ["axum", "openapi", "serde_json", "tracing"] }

Database service with sqlx, migrations and metrics:

masterror = { version = "0.28", features = ["sqlx", "sqlx-migrate", "metrics", "tracing"] }

gRPC service:

masterror = { version = "0.28", features = ["tonic", "tracing", "backtrace"] }

Telegram bot with Mini App auth:

masterror = { version = "0.28", features = ["teloxide", "init-data", "reqwest", "tokio"] }

WASM front-end:

masterror = { version = "0.28", features = ["frontend", "serde_json"] }

no_std embedded or library target:

masterror = { version = "0.28", default-features = false }

Notes

All integration flags imply std except sqlx and sqlx-migrate, which stay std-independent at the flag level.
axum and actix pull serde_json transitively because their response bodies are JSON.
Feature flags never change the wire contract of ErrorResponse/ProblemJson fields that are already enabled — they only add capabilities (e.g. serde_json upgrades details from plain text to structured JSON) or trait implementations.

See also: Getting Started · Web Frameworks · Integrations · Observability · No-Std

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