facet-miette

Crates.io	facet-miette
lib.rs	facet-miette
version	0.42.0
created_at	2025-12-29 20:37:43.573939+00
updated_at	2026-01-06 14:55:01.01111+00
description	Derive miette::Diagnostic for facet types - rich error reporting with source spans
homepage
repository	https://github.com/facet-rs/facet
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id	2011302
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Amos Wenger (fasterthanlime)

documentation

README

facet-miette

Derive miette::Diagnostic for your error types using facet's plugin system. Get rich error reporting with source spans, error codes, help text, and more.

Usage

use facet::Facet;
use facet_miette as diagnostic;
use miette::SourceSpan;

#[derive(Facet, Debug)]
#[facet(derive(Error, facet_miette::Diagnostic))]
pub enum ParseError {
    /// Unexpected token in input
    #[facet(diagnostic::code = "parse::unexpected_token")]
    #[facet(diagnostic::help = "Check for typos or missing delimiters")]
    UnexpectedToken {
        #[facet(diagnostic::source_code)]
        src: String,
        #[facet(diagnostic::label = "this token was unexpected")]
        span: SourceSpan,
    },

    /// End of file reached unexpectedly
    #[facet(diagnostic::code = "parse::unexpected_eof")]
    UnexpectedEof,
}

Attributes

Container/Variant Level

#[facet(diagnostic::code = "my_lib::error_code")] - Error code for this diagnostic
#[facet(diagnostic::help = "Helpful message")] - Help text shown to user
#[facet(diagnostic::url = "https://...")] - URL for more information
#[facet(diagnostic::severity = "warning")] - Severity: "error", "warning", or "advice"

Field Level

#[facet(diagnostic::source_code)] - Field containing the source text (must impl SourceCode)
#[facet(diagnostic::label = "description")] - Field is a span to highlight with label
#[facet(diagnostic::related)] - Field contains related diagnostics (iterator)

Integration with facet-error

Typically you'll use both Error and Diagnostic together:

use facet::Facet;
use facet_miette as diagnostic;

#[derive(Facet, Debug)]
#[facet(derive(Error, facet_miette::Diagnostic))]
pub enum MyError {
    /// Something went wrong while processing
    #[facet(diagnostic::code = "my_error::processing")]
    #[facet(diagnostic::help = "Try again with different input")]
    ProcessingError,
}

The Error derive (from facet-error) generates Display and Error implementations from doc comments, while Diagnostic adds miette's rich error reporting features.