Feature Flags en

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.

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