#error-handling #error-reporting #hacktoberfest2021 #compiler #fancy


Fancy diagnostic reporting library and protocol for us mere mortals who aren’t compiler hackers

45 releases (25 stable)

new 5.1.0 Jun 25, 2022
4.7.1 May 15, 2022
4.3.0 Mar 27, 2022
3.3.0 Jan 8, 2022
0.13.0 Aug 21, 2021

#8 in Rust patterns

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166,136 downloads per month
Used in 155 crates (77 directly)




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


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!):

Hi! miette also includes a screen-reader-oriented diagnostic printer that's enabled in various situations, such as when you use NO_COLOR or CLICOLOR settings, or on CI. This behavior is also fully configurable and customizable. For example, this is what this particular diagnostic will look like when the narrated printer is enabled:
Error: Received some bad JSON from the source. Unable to parse.
Caused by: missing field `foo` at line 1 column 1700
Begin snippet for https://api.nuget.org/v3/registration5-gz-semver2/json.net/index.json starting
at line 1, column 1659
snippet line 1: gs":["json"],"title":"","version":"1.0.0"},"packageContent":"https://api.nuget.o
highlight starting at line 1, column 1699: last parsing location
diagnostic help: This is a bug. It might be in ruget, or it might be in the
source you're using, but it's definitely a bug and should be reported.
diagnostic error code: ruget::api::bad_json

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


  • 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.


Using cargo-edit:

$ cargo add miette

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

$ cargo add miette --features fancy


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)]
    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.
    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(),


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
fn pretend_this_is_main() -> Result<()> {
    // kaboom~


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

Narratable printout:
Error: Types mismatched for operation.
Diagnostic severity: error
Begin snippet starting at line 1, column 1
snippet line 1: 3 + "5"
label starting at line 1, column 1: int
label starting at line 1, column 1: doesn't support these values.
label starting at line 1, column 1: string
diagnostic help: Change int or string to be the right types and try again.
diagnostic code: nu::parser::unsupported_operation
For more details, see https://docs.rs/nu-parser/0.1.0/nu-parser/enum.ParseError.html#variant.UnsupportedOperation


... 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;
use thiserror::Error;

#[derive(Error, Diagnostic, Debug)]
pub enum MyLibError {
    IoError(#[from] std::io::Error),

    #[error("Oops it blew up")]

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> {

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> {
        .wrap_err("Parsing this tool's semver version failed.")?)

... 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);

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:

 Example showing the graphical report printer for miette
pretty-printing an error code. The code is underlined and followed by text
saying to 'click here'. A hover tooltip shows a full-fledged URL that can be
Ctrl+Clicked to open in a browser.
This feature is also available in the narratable printer. It will add a line
after printing the error code showing a plain URL that you can visit.

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

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

#[derive(Error, Diagnostic, Debug)]
    // 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)]
    // Will link users to https://docs.rs/my_crate/0.0.0/my_crate/struct.MyErr.html
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)]
pub struct MyErrorType {
    // The `Source` that miette will use.
    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
    snip4: Option<SourceSpan>,
... help text

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


~31K SLoC