Trait Context

pub trait Context<T, E>: Sealed {
    // Required methods
    fn wrap_err<D>(self, msg: D) -> Result<T, Report>
       where D: Display + Send + Sync + 'static;
    fn wrap_err_with<D, F>(self, f: F) -> Result<T, Report>
       where D: Display + Send + Sync + 'static,
             F: FnOnce() -> D;
    fn context<D>(self, msg: D) -> Result<T, Report>
       where D: Display + Send + Sync + 'static;
    fn with_context<D, F>(self, f: F) -> Result<T, Report>
       where D: Display + Send + Sync + 'static,
             F: FnOnce() -> D;
}

Compatibility re-export of WrapErr for interop with anyhow Provides the wrap_err() method for Result.

This trait is sealed and cannot be implemented for types outside of miette.

Example

use miette::{WrapErr, IntoDiagnostic, Result};
use std::{fs, path::PathBuf};

pub struct ImportantThing {
    path: PathBuf,
}

impl ImportantThing {
    pub fn detach(&mut self) -> Result<()> {...}
}

pub fn do_it(mut it: ImportantThing) -> Result<Vec<u8>> {
    it.detach().wrap_err("Failed to detach the important thing")?;

    let path = &it.path;
    let content = fs::read(path)
        .into_diagnostic()
        .wrap_err_with(|| format!(
            "Failed to read instrs from {}",
            path.display())
        )?;

    Ok(content)
}

When printed, the outermost error would be printed first and the lower level underlying causes would be enumerated below.

Error: Failed to read instrs from ./path/to/instrs.json

Caused by:
    No such file or directory (os error 2)

Wrapping Types That Do Not Implement Error

For example &str and Box<dyn Error>.

Due to restrictions for coherence Report cannot implement From for types that don’t implement Error. Attempts to do so will give "this type might implement Error in the future" as an error. As such, wrap_err(), which uses From under the hood, cannot be used to wrap these types. Instead we encourage you to use the combinators provided for Result in std/core.

For example, instead of this:

use std::error::Error;
use miette::{WrapErr, Report};

fn wrap_example(err: Result<(), Box<dyn Error + Send + Sync + 'static>>)
    -> Result<(), Report>
{
    err.wrap_err("saw a downstream error")
}

We encourage you to write this:

use miette::{miette, Report, WrapErr};
use std::error::Error;

fn wrap_example(err: Result<(), Box<dyn Error + Send + Sync + 'static>>) -> Result<(), Report> {
    err.map_err(|e| miette!(e))
        .wrap_err("saw a downstream error")
}

Effect on Downcasting

After attaching a message of type D onto an error of type E, the resulting miette::Error may be downcast to D or to E.

That is, in codebases that rely on downcasting, miette’s wrap_err() supports both of the following use cases:

• Attaching messages whose type is insignificant onto errors whose type is used in downcasts.

  In other error libraries whose wrap_err() is not designed this way, it can be risky to introduce messages to existing code because new message might break existing working downcasts. In miette, any downcast that worked before adding the message will continue to work after you add a message, so you should freely wrap errors wherever it would be helpful.

  use miette::{WrapErr, Result};
  
  fn do_it() -> Result<()> {
      helper().wrap_err("Failed to complete the work")?;
      ...
  }
  
  fn main() {
      let err = do_it().unwrap_err();
      if let Some(e) = err.downcast_ref::<SuspiciousError>() {
          // If helper() returned SuspiciousError, this downcast will
          // correctly succeed even with the message in between.
      }
  }

• Attaching message whose type is used in downcasts onto errors whose type is insignificant.

  Some codebases prefer to use machine-readable messages to categorize lower level errors in a way that will be actionable to higher levels of the application.

  use miette::{WrapErr, Result};
  
  fn do_it() -> Result<()> {
      helper().wrap_err(HelperFailed)?;
      ...
  }
  
  fn main() {
      let err = do_it().unwrap_err();
      if let Some(e) = err.downcast_ref::<HelperFailed>() {
          // If helper failed, this downcast will succeed because
          // HelperFailed is the message that has been attached to
          // that error.
      }
  }

Required Methods

fn wrap_err<D>(self, msg: D) -> Result<T, Report>

where D: Display + Send + Sync + 'static,

Wrap the error value with a new adhoc error

fn wrap_err_with<D, F>(self, f: F) -> Result<T, Report>

where D: Display + Send + Sync + 'static, F: FnOnce() -> D,

Wrap the error value with a new adhoc error that is evaluated lazily only once an error does occur.

fn context<D>(self, msg: D) -> Result<T, Report>

where D: Display + Send + Sync + 'static,

Compatibility re-export of wrap_err() for interop with anyhow

fn with_context<D, F>(self, f: F) -> Result<T, Report>

where D: Display + Send + Sync + 'static, F: FnOnce() -> D,

Compatibility re-export of wrap_err_with() for interop with anyhow

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementations on Foreign Types

impl<T, E> WrapErr<T, E> for Result<T, E>

where E: Diag + Send + Sync + 'static,

fn wrap_err<D>(self, msg: D) -> Result<T, Report>

where D: Display + Send + Sync + 'static,

fn wrap_err_with<D, F>(self, msg: F) -> Result<T, Report>

where D: Display + Send + Sync + 'static, F: FnOnce() -> D,

fn context<D>(self, msg: D) -> Result<T, Report>

where D: Display + Send + Sync + 'static,

fn with_context<D, F>(self, msg: F) -> Result<T, Report>

where D: Display + Send + Sync + 'static, F: FnOnce() -> D,

Implementors