Crate miette

you run miette? You run her code like the software? Oh. Oh! Error code for coder! Error code for One Thousand Lines!

About

miette is a diagnostic library for Rust. It includes a series of traits/protocols that allow you to hook into its error reporting facilities, and even write your own error reports! It lets you define error types that can print out like this (or in any format you like!):

Table of Contents

• About
• Features
• Installing
• Example
• Usage
  • … in libraries
  • … in application code
  • … in main()
  • … snippets
• Acknowledgements
• License

Features

• Generic Diagnostic protocol, compatible (and dependent on) std::error::Error.
• Unique error codes on every Diagnostic.
• Super handy derive macro for defining diagnostic metadata.
• Lightweight anyhow/eyre-style error wrapper type, DiagnosticReport, which can be returned from main.
• Generic support for arbitrary Sources for snippet data, with default support for Strings included.

The miette crate also comes bundles with a default DiagnosticReportPrinter with the following features:

• Fancy graphical diagnostic output, using ANSI/Unicode text
• single- and multi-line highlighting support
• Screen reader/braille support, gated on NO_COLOR, and other heuristics.
• Fully customizable graphical theming (or overriding the printers entirely).
• Cause chain printing

Installing

Using cargo-edit:

$ cargo add miette

Example

/*
You can derive a Diagnostic from any `std::error::Error` type.

`thiserror` is a great way to define them, and plays nicely with `miette`!
*/
use miette::{Diagnostic, SourceSpan};
use thiserror::Error;

#[derive(Error, Debug, Diagnostic)]
#[error("oops!")]
#[diagnostic(
    code(oops::my::bad),
    help("try doing it better next time?"),
)]
struct MyBad {
    // The Source that we're gonna be printing snippets out of.
    src: String,
    // Snippets and highlights can be included in the diagnostic!
    #[snippet(src, "This is the part that broke")]
    snip: SourceSpan,
    #[highlight(snip)]
    bad_bit: SourceSpan,
}

/*
Now let's define a function!

Use this DiagnosticResult type (or its expanded version) as the return type
throughout your app (but NOT your libraries! Those should always return concrete
types!).
*/
use miette::DiagnosticResult;
fn this_fails() -> DiagnosticResult<()> {
    // You can use plain strings as a `Source`, or anything that implements
    // the one-method `Source` trait.
    let src = "source\n  text\n    here".to_string();
    let len = src.len();

    Err(MyBad {
        src,
        snip: ("bad_file.rs", 0, len).into(),
        bad_bit: ("this bit here", 9, 4).into(),
    })?;

    Ok(())
}

/*
Now to get everything printed nicely, just return a DiagnosticResult<()>
and you're all set!

Note: You can swap out the default reporter for a custom one using `miette::set_reporter()`
*/
fn pretend_this_is_main() -> DiagnosticResult<()> {
    // kaboom~
    this_fails()?;

    Ok(())
}

And this is the output you’ll get if you run this program:

Usage

… in libraries

miette is fully compatible with library usage. Consumers who don’t know about, or don’t want, miette features can safely use its error types as regular std::error::Error.

We highly recommend using something like thiserror to define unique error types and error wrappers for your library.

While miette integrates smoothly with thiserror, it is not required. If you don’t want to use the Diagnostic derive macro, you can implement the trait directly, just like with std::error::Error.

// lib/error.rs
use thiserror::Error;
use miette::Diagnostic;

#[derive(Error, Diagnostic, Debug)]
pub enum MyLibError {
    #[error(transparent)]
    #[diagnostic(code(my_lib::io_error))]
    IoError(#[from] std::io::Error),

    #[error("Oops it blew up")]
    #[diagnostic(code(my_lib::bad_code))]
    BadThingHappened,
}

Then, return this error type from all your fallible public APIs. It’s a best practice to wrap any “external” error types in your error enum instead of using something like eyre in a library.

… in application code

Application code tends to work a little differently than libraries. You don’t always need or care to define dedicated error wrappers for errors coming from external libraries and tools.

For this situation, miette includes two tools: DiagnosticReport and IntoDiagnostic. They work in tandem to make it easy to convert regular std::error::Errors into Diagnostics. Additionally, there’s a DiagnosticResult type alias that you can use to be more terse:

// my_app/lib/my_internal_file.rs
use miette::{IntoDiagnostic, DiagnosticResult};
use semver::Version;

pub fn some_tool() -> DiagnosticResult<Version> {
    Ok("1.2.x".parse().into_diagnostic("my_app::semver::parse_error")?)
}

… in main()

main() is just like any other part of your application-internal code. Use DiagnosticResult as your return value, and it will pretty-print your diagnostics automatically.

use miette::{DiagnosticResult, IntoDiagnostic};
use semver::Version;

fn pretend_this_is_main() -> DiagnosticResult<()> {
    let version: Version = "1.2.x".parse().into_diagnostic("my_app::semver::parse_error")?;
    println!("{}", version);
    Ok(())
}

… snippets

Along with its general error handling and reporting features, miette also includes facilities for adding error spans and annotations/highlights to your output. This can be very useful when an error is syntax-related, but you can even use it to print out sections of your own source code!

To achieve this, miette defines its own lightweight SourceSpan type. This is a basic byte-offset and length into an associated Source and, along with the latter, gives miette all the information it needs to pretty-print some snippets!

The easiest way to define errors like this is to use the derive(Diagnostic) macro:

use miette::{Diagnostic, SourceSpan};
use thiserror::Error;

#[derive(Diagnostic, Debug, Error)]
#[error("oops")]
#[diagnostic(code(my_lib::random_error))]
pub struct MyErrorType {
    // The `Source` that miette will use.
    src: String,

    // A snippet that points to `src`, our `Source`. The filename can be
    // provided at the callsite.
    #[snippet(src, "This is the snippet")]
    snip: SourceSpan,

    // A highlight for the `snip` snippet we defined above. This will
    // underline/mark the specific code inside the larger snippet context.
    //
    // The label is provided using `SourceSpan`'s label.
    #[highlight(snip)]
    err_span: SourceSpan,
}

Acknowledgements

miette was not developed in a void. It owes enormous credit to various other projects and their authors:

• anyhow and color-eyre: these two enormously influential error handling libraries have pushed forward the experience of application-level error handling and error reporting. miette’s DiagnosticReport type is an attempt at a very very rough version of their Report types.
• thiserror for setting the standard for library-level error definitions, and for being the inspiration behind miette’s derive macro.
• rustc and @estebank for their state-of-the-art work in compiler diagnostics.
• ariadne for pushing forward how pretty these diagnostics can really look!

License

miette is released to the Rust community under the Apache license 2.0.

It also includes some code taken from eyre, and some from thiserror, also under the Apache License. Some code is taken from ariadne, which is MIT licensed.

Structs

DiagnosticError

Convenience Diagnostic that can be used as an “anonymous” wrapper for Errors. This is intended to be paired with IntoDiagnostic.

DiagnosticReport

When used with ?/From, this will wrap any Diagnostics and, when formatted with Debug, will fetch the current DiagnosticReportPrinter and use it to format the inner Diagnostic.

DiagnosticSnippet

A snippet from a Source to be displayed with a message and possibly some highlights.

GraphicalReportPrinter

A DiagnosticReportPrinter that displays a given crate::DiagnosticReport in a quasi-graphical way, using terminal colors, unicode drawing characters, and other such things.

GraphicalTheme

Theme used by crate::GraphicalReportPrinter to render fancy crate::Diagnostic reports.

JokePrinter

Literally what it says on the tin.

MietteSpanContents

Basic implementation of the SpanContents trait, for convenience.

NarratableReportPrinter

DiagnosticReportPrinter that renders plain text and avoids extraneous graphics. It’s optimized for screen readers and braille users, but is also used in any non-graphical environments, such as non-TTY output.

SourceOffset

Newtype that represents the ByteOffset from the beginning of a Source

SourceSpan

Span within a Source with an associated message.

ThemeCharacters

Characters to be used when drawing when using crate::GraphicalReportPrinter.

ThemeStyles

Styles for various parts of graphical rendering for the crate::GraphicalReportPrinter.

Enums

MietteError

Error enum for miette. Used by certain operations in the protocol.

Severity

Diagnostic severity. Intended to be used by DiagnosticReportPrinters to change the way different Diagnostics are displayed.

Traits

Diagnostic

Adds rich metadata to your Error that can be used by DiagnosticReportPrinter to print really nice and human-friendly error messages.

DiagnosticReportPrinter

Protocol for Diagnostic handlers, which are responsible for actually printing out Diagnostics.

IntoDiagnostic

Convenience trait that adds a .into_diagnostic() method that converts a type to a Result<T, DiagnosticError>.

Source

Represents a readable source of some sort.

SpanContents

Contents of a Source covered by SourceSpan.

Functions

set_printer

Set the global DiagnosticReportPrinter that will be used when you report using crate::DiagnosticReport.

Type Definitions

ByteOffset

“Raw” type for the byte offset from the beginning of a Source.

DiagnosticResult

Convenience alias. This is intended to be used as the return type for main()

Derive Macros

Diagnostic