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!):

NOTE: You must enable the "fancy" crate feature to get fancy report output like in the screenshots above. You should only do this in your toplevel crate, as the fancy feature pulls in a number of dependencies that libraries and such might not want.

Table of Contents

• About
• Features
• Installing
• Example
• Using
  • ... in libraries
  • ... in application code
  • ... in main()
  • ... diagnostic code URLs
  • ... snippets
  • ... multiple related errors
  • ... delayed source code
  • ... handler options
  • ... dynamic diagnostics
• Acknowledgements
• License

Features

• Generic Diagnostic protocol, compatible (and dependent on) std::error::Error.
• Unique error codes on every Diagnostic.
• Custom links to get more details on error codes.
• Super handy derive macro for defining diagnostic metadata.
• Replacements for anyhow/eyre types Result, Report and the miette! macro for the anyhow!/eyre! macros.
• Generic support for arbitrary SourceCodes for snippet data, with default support for Strings included.

The miette crate also comes bundled with a default ReportHandler 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
• Turns diagnostic codes into links in supported terminals.

Installing

$ cargo add miette

If you want to use the fancy printer in all these screenshots:

$ cargo add miette --features fancy

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),
    url(docsrs),
    help("try doing it better next time?")
)]
struct MyBad {
    // The Source that we're gonna be printing snippets out of.
    // This can be a String if you don't have or care about file names.
    #[source_code]
    src: NamedSource,
    // Snippets and highlights can be included in the diagnostic!
    #[label("This bit here")]
    bad_bit: SourceSpan,
}

/*
Now let's define a function!

Use this `Result` type (or its expanded version) as the return type
throughout your app (but NOT your libraries! Those should always return
concrete types!).
*/
use miette::{NamedSource, Result};
fn this_fails() -> Result<()> {
    // 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: NamedSource::new("bad_file.rs", src),
        bad_bit: (9, 4).into(),
    })?;

    Ok(())
}

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

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

    Ok(())
}

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

Using

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

#[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,

    #[error(transparent)]
    // Use `#[diagnostic(transparent)]` to wrap another [`Diagnostic`]. You won't see labels otherwise
    #[diagnostic(transparent)]
    AnotherError(#[from] AnotherError),
}

#[derive(Error, Diagnostic, Debug)]
#[error("another error")]
pub struct AnotherError {
   #[label("here")]
   pub at: SourceSpan
}

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 Report 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: Report and IntoDiagnostic. They work in tandem to make it easy to convert regular std::error::Errors into Diagnostics. Additionally, there's a Result type alias that you can use to be more terse.

When dealing with non-Diagnostic types, you'll want to .into_diagnostic() them:

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

pub fn some_tool() -> Result<Version> {
    Ok("1.2.x".parse().into_diagnostic()?)
}

miette also includes an anyhow/eyre-style Context/WrapErr traits that you can import to add ad-hoc context messages to your Diagnostics, as well, though you'll still need to use .into_diagnostic() to make use of it:

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

pub fn some_tool() -> Result<Version> {
    Ok("1.2.x"
        .parse()
        .into_diagnostic()
        .wrap_err("Parsing this tool's semver version failed.")?)
}

To construct your own simple adhoc error use the [miette!] macro:

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

pub fn some_tool() -> Result<Version> {
    let version = "1.2.x";
    Ok(version
        .parse()
        .map_err(|_| miette!("Invalid version {}", version))?)
}

There are also similar [bail!] and [ensure!] macros.

... in main()

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

NOTE: You must enable the "fancy" crate feature to get fancy report output like in the screenshots here.** You should only do this in your toplevel crate, as the fancy feature pulls in a number of dependencies that libraries and such might not want.

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

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

Please note: in order to get fancy diagnostic rendering with all the pretty colors and arrows, you should install miette with the fancy feature enabled:

miette = { version = "X.Y.Z", features = ["fancy"] }

... diagnostic code URLs

miette supports providing a URL for individual diagnostics. This URL will be displayed as an actual link in supported terminals, like so:

To use this, you can add a url() sub-param to your #[diagnostic] attribute:

use miette::Diagnostic;
use thiserror::Error;

#[derive(Error, Diagnostic, Debug)]
#[error("kaboom")]
#[diagnostic(
    code(my_app::my_error),
    // You can do formatting!
    url("https://my_website.com/error_codes#{}", self.code().unwrap())
)]
struct MyErr;

Additionally, if you‘re developing a library and your error type is exported from your crate’s top level, you can use a special url(docsrs) option instead of manually constructing the URL. This will automatically create a link to this diagnostic on docs.rs, so folks can just go straight to your (very high quality and detailed!) documentation on this diagnostic:

use miette::Diagnostic;
use thiserror::Error;

#[derive(Error, Diagnostic, Debug)]
#[diagnostic(
    code(my_app::my_error),
    // Will link users to https://docs.rs/my_crate/0.0.0/my_crate/struct.MyErr.html
    url(docsrs)
)]
#[error("kaboom")]
struct MyErr;

... snippets

Along with its general error handling and reporting features, miette also includes facilities for adding error spans/annotations/labels 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 SourceCode and, along with the latter, gives miette all the information it needs to pretty-print some snippets! You can also use your own Into<SourceSpan> types as label spans.

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.
    #[source_code]
    src: String,

    // This will underline/mark the specific code inside the larger
    // snippet context.
    #[label = "This is the highlight"]
    err_span: SourceSpan,

    // You can add as many labels as you want.
    // They'll be rendered sequentially.
    #[label("This is bad")]
    snip2: (usize, usize), // `(usize, usize)` is `Into<SourceSpan>`!

    // Snippets can be optional, by using Option:
    #[label("some text")]
    snip3: Option<SourceSpan>,

    // with or without label text
    #[label]
    snip4: Option<SourceSpan>,
}

... help text

miette provides two facilities for supplying help text for your errors:

The first is the #[help()] format attribute that applies to structs or enum variants:

use miette::Diagnostic;
use thiserror::Error;

#[derive(Debug, Diagnostic, Error)]
#[error("welp")]
#[diagnostic(help("try doing this instead"))]
struct Foo;

The other is by programmatically supplying the help text as a field to your diagnostic:

use miette::Diagnostic;
use thiserror::Error;

#[derive(Debug, Diagnostic, Error)]
#[error("welp")]
#[diagnostic()]
struct Foo {
    #[help]
    advice: Option<String>, // Can also just be `String`
}

let err = Foo {
    advice: Some("try doing this instead".to_string()),
};

... multiple related errors

miette supports collecting multiple errors into a single diagnostic, and printing them all together nicely.

To do so, use the #[related] tag on any IntoIter field in your Diagnostic type:

use miette::Diagnostic;
use thiserror::Error;

#[derive(Debug, Error, Diagnostic)]
#[error("oops")]
struct MyError {
    #[related]
    others: Vec<MyError>,
}

... delayed source code

Sometimes it makes sense to add source code to the error message later. One option is to use with_source_code() method for that:

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

#[derive(Diagnostic, Debug, Error)]
#[error("oops")]
#[diagnostic()]
pub struct MyErrorType {
    // Note: label but no source code
    #[label]
    err_span: SourceSpan,
}

fn do_something() -> miette::Result<()> {
    // This function emits actual error with label
    return Err(MyErrorType {
        err_span: (7..11).into(),
    })?;
}

fn main() -> miette::Result<()> {
    do_something().map_err(|error| {
        // And this code provides the source code for inner error
        error.with_source_code(String::from("source code"))
    })
}

Also source code can be provided by a wrapper type. This is especially useful in combination with related, when multiple errors should be emitted at the same time:

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

#[derive(Diagnostic, Debug, Error)]
#[error("oops")]
#[diagnostic()]
pub struct InnerError {
    // Note: label but no source code
    #[label]
    err_span: SourceSpan,
}

#[derive(Diagnostic, Debug, Error)]
#[error("oops: multiple errors")]
#[diagnostic()]
pub struct MultiError {
    // Note source code by no labels
    #[source_code]
    source_code: String,
    // The source code above is used for these errors
    #[related]
    related: Vec<InnerError>,
}

fn do_something() -> Result<(), Vec<InnerError>> {
    Err(vec![
        InnerError {
            err_span: (0..6).into(),
        },
        InnerError {
            err_span: (7..11).into(),
        },
    ])
}

fn main() -> miette::Result<()> {
    do_something().map_err(|err_list| MultiError {
        source_code: "source code".into(),
        related: err_list,
    })?;
    Ok(())
}

... Diagnostic-based error sources.

When one uses the #[source] attribute on a field, that usually comes from thiserror, and implements a method for [std::error::Error::source]. This works in many cases, but it's lossy: if the source of the diagnostic is a diagnostic itself, the source will simply be treated as an std::error::Error.

While this has no effect on the existing reporters, since they don't use that information right now, APIs who might want this information will have no access to it.

If it's important for you for this information to be available to users, you can use #[diagnostic_source] alongside #[source]. Not that you will likely want to use both:

use miette::Diagnostic;
use thiserror::Error;

#[derive(Debug, Diagnostic, Error)]
#[error("MyError")]
struct MyError {
    #[source]
    #[diagnostic_source]
    the_cause: OtherError,
}

#[derive(Debug, Diagnostic, Error)]
#[error("OtherError")]
struct OtherError;

... handler options

MietteHandler is the default handler, and is very customizable. In most cases, you can simply use MietteHandlerOpts to tweak its behavior instead of falling back to your own custom handler.

Usage is like so:

miette::set_hook(Box::new(|_| {
    Box::new(
        miette::MietteHandlerOpts::new()
            .terminal_links(true)
            .unicode(false)
            .context_lines(3)
            .tab_width(4)
            .build(),
    )
}))

See the docs for MietteHandlerOpts for more details on what you can customize!

... dynamic diagnostics

If you...

• ...don't know all the possible errors upfront
• ...need to serialize/deserialize errors then you may want to use miette!, diagnostic! macros or MietteDiagnostic directly to create diagnostic on the fly.

let source = "2 + 2 * 2 = 8".to_string();
let report = miette!(
  labels = vec[
      LabeledSpan::at(12..13, "this should be 6"),
  ],
  help = "'*' has greater precedence than '+'",
  "Wrong answer"
).with_source_code(source);
println!("{:?}", report)

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