fack

Declarative Rust error derivation with no_std support and zero allocation in generated runtime code.

use fack::prelude::*;

#[derive(Error, Debug)]
#[error("file not found {path}")]
struct FileError {
    path: String,
}

Installation

[dependencies]
fack = "0.2.0"

Design

All derive behavior uses the single #[error(...)] helper namespace
Generated implementations use ::core unless an import root is selected
Format fields and source fields are resolved before expansion
fack-codegen exposes opaque parse, validate, and expansion stages for downstream tooling
Generated Display, Error, and From implementations allocate nothing

Formatting

Named and tuple fields can be captured directly. Explicit Rust format arguments are also supported.

use fack::prelude::*;

#[derive(Error, Debug)]
#[error("invalid input {input:?}")]
struct InputError {
    input: String,
}

#[derive(Error, Debug)]
#[error("invalid value {0}")]
struct ValueError(i32);

#[derive(Error, Debug)]
#[error("normalized {value}", value = input.trim())]
struct NormalizedError {
    input: String,
}

A custom formatter can be selected when a format string is not the right abstraction.

use fack::prelude::*;

fn render(error: &RenderedError, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
    let RenderedError { code } = error;
    write!(f, "rendered {code}")
}

#[derive(Error, Debug)]
#[error(display(render))]
struct RenderedError {
    code: u8,
}

Sources

#[error(source(field))] exposes the selected field as the ordinary source. Direct, boxed, optional, and optional boxed sources are supported.

use fack::prelude::*;

#[derive(Error, Debug)]
#[error("request failed")]
#[error(source(io))]
struct RequestError {
    io: std::io::Error,
    url: String,
}

#[error(transparent(field))] requires exactly one non-optional field. Display forwards to that field and source chaining forwards through its own Error::source implementation.

use fack::prelude::*;

#[derive(Error, Debug)]
#[error(transparent(0))]
struct Wrapper(std::io::Error);

Conversion

#[error(from)] requires exactly one field. It generates From<T> and makes that field the ordinary error source.

use fack::prelude::*;

#[derive(Error, Debug)]
enum AppError {
    #[error("io error")]
    #[error(from)]
    Io(std::io::Error),

    #[error("parse error")]
    #[error(from)]
    Parse(std::num::ParseIntError),
}

Generation options

Without an inline declaration, generated methods receive no explicit inline attribute. #[error(inline)] and #[error(inline(neutral))] emit ordinary #[inline]. The always and never strategies emit their corresponding Rust attributes.

use fack::prelude::*;

#[derive(Error, Debug)]
#[error(inline(never))]
#[error("rare error")]
struct RareError;

Generated paths use ::core by default. #[error(import(path))] selects a different root.

use fack::prelude::*;

#[derive(Error, Debug)]
#[error(import(::std))]
#[error("standard error")]
struct StdError;

Enum-wide generation options belong on the enum. Display, source, transparency, and conversion declarations belong on variants.

Code generation engine

fack-codegen is a regular no_std library rather than a proc-macro crate. Its supported API keeps compiler representations opaque while allowing downstream tools to choose staged or one-shot generation.

let target = fack_codegen::Target::input(&input)?;
let validated = target.validate()?;
let tokens = validated.expand()?;

let tokens = fack_codegen::generate(&input)?;

Workspace

fack provides the user-facing facade
fack-core provides the blanket error capability
fack-macro provides the derive macro entry point
fack-codegen provides the staged generation engine

License

GPL-3.0

This program is free software. You can redistribute it and modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License or any later version.

See LICENSE.md for the full text.

fack-codegen 0.2.0