Provides the context method for Result.

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

Example

use anyhow::{Context, Result};
use std::fs;
use std::path::PathBuf;

pub struct ImportantThing {
    path: PathBuf,
}

impl ImportantThing {
    # const IGNORE: &'static str = stringify! {
    pub fn detach(&mut self) -> Result<()> {...}
    # };
    # fn detach(&mut self) -> Result<()> {
    #     unimplemented!()
    # }
}

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

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

    Ok(content)
}

When printed, the outermost context 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)

Refer to the Display representations documentation for other forms in which this context chain can be rendered.

Effect on downcasting

After attaching context of type C onto an error of type E, the resulting anyhow::Error may be downcast to C or to E.

That is, in codebases that rely on downcasting, Anyhow's context supports both of the following use cases:

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

  In other error libraries whose context is not designed this way, it can be risky to introduce context to existing code because new context might break existing working downcasts. In Anyhow, any downcast that worked before adding context will continue to work after you add a context, so you should freely add human-readable context to errors wherever it would be helpful.

  # use anyhow::bail;
  # use thiserror::Error;
  #
  # #[derive(Error, Debug)]
  # #[error("???")]
  # struct SuspiciousError;
  #
  # fn helper() -> Result<()> {
  #     bail!(SuspiciousError);
  # }
  #
  use anyhow::{Context, Result};
  
  fn do_it() -> Result<()> {
      helper().context("Failed to complete the work")?;
      # const IGNORE: &str = stringify! {
      ...
      # };
      # unreachable!()
  }
  
  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 context in between.
          # return;
      }
      # panic!("expected downcast to succeed");
  }
  

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

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

  # use anyhow::bail;
  # use thiserror::Error;
  #
  # #[derive(Error, Debug)]
  # #[error("???")]
  # struct HelperFailed;
  #
  # fn helper() -> Result<()> {
  #     bail!("no such file or directory");
  # }
  #
  use anyhow::{Context, Result};
  
  fn do_it() -> Result<()> {
      helper().context(HelperFailed)?;
      # const IGNORE: &str = stringify! {
      ...
      # };
      # unreachable!()
  }
  
  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 context that has been attached to
          // that error.
          # return;
      }
      # panic!("expected downcast to succeed");
  }