Struct Error

struct Error { ... }

The Error type, a wrapper around a dynamic error type.

Error works a lot like Box<dyn std::error::Error>, but with these differences:


Display representations

When you print an error object using "{}" or to_string(), only the outermost underlying error or context is printed, not any of the lower level causes. This is exactly as if you had called the Display impl of the error from which you constructed your anyhow::Error.

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

To print causes as well using anyhow's default formatting of causes, use the alternate selector "{:#}".

Failed to read instrs from ./path/to/instrs.json: No such file or directory (os error 2)

The Debug format "{:?}" includes your backtrace if one was captured. Note that this is the representation you get by default if you return an error from fn main instead of printing it explicitly yourself.

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

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

and if there is a backtrace available:

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

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

Stack backtrace:
   0: <E as anyhow::context::ext::StdError>::ext_context
             at /git/anyhow/src/backtrace.rs:26
   1: core::result::Result<T,E>::map_err
             at /git/rustc/src/libcore/result.rs:596
   2: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::with_context
             at /git/anyhow/src/context.rs:58
   3: testing::main
             at src/main.rs:5
   4: std::rt::lang_start
             at /git/rustc/src/libstd/rt.rs:61
   5: main
   6: __libc_start_main
   7: _start

To see a conventional struct-style Debug representation, use "{:#?}".

Error {
    context: "Failed to read instrs from ./path/to/instrs.json",
    source: Os {
        code: 2,
        kind: NotFound,
        message: "No such file or directory",
    },
}

If none of the built-in representations are appropriate and you would prefer to render the error and its cause chain yourself, it can be done something like this:

use anyhow::{Context, Result};

fn main() {
    if let Err(err) = try_main() {
        eprintln!("ERROR: {}", err);
        err.chain().skip(1).for_each(|cause| eprintln!("because: {}", cause));
        std::process::exit(1);
    }
}

fn try_main() -> Result<()> {
    # const IGNORE: &str = stringify! {
    ...
    # };
    # Ok(())
}

Implementations

impl crate::Error

fn new<E>(error: E) -> Self
where
    E: StdError + Send + Sync + 'static

Create a new error object from any error type.

The error type must be threadsafe and 'static, so that the Error will be as well.

If the error type does not provide a backtrace, a backtrace will be created here to ensure that a backtrace exists.

fn msg<M>(message: M) -> Self
where
    M: Display + Debug + Send + Sync + 'static

Create a new error object from a printable error message.

If the argument implements std::error::Error, prefer Error::new instead which preserves the underlying error's cause chain and backtrace. If the argument may or may not implement std::error::Error now or in the future, use anyhow!(err) which handles either way correctly.

Error::msg("...") is equivalent to anyhow!("...") but occasionally convenient in places where a function is preferable over a macro, such as iterator or stream combinators:

# mod ffi {
#     pub struct Input;
#     pub struct Output;
#     pub async fn do_some_work(_: Input) -> Result<Output, &'static str> {
#         unimplemented!()
#     }
# }
#
# use ffi::{Input, Output};
#
use anyhow::{Error, Result};
use futures::stream::{Stream, StreamExt, TryStreamExt};

async fn demo<S>(stream: S) -> Result<Vec<Output>>
where
    S: Stream<Item = Input>,
{
    stream
        .then(ffi::do_some_work) // returns Result<Output, &str>
        .map_err(Error::msg)
        .try_collect()
        .await
}

fn context<C>(self: Self, context: C) -> Self
where
    C: Display + Send + Sync + 'static

Wrap the error value with additional context.

For attaching context to a Result as it is propagated, the [Context][crate::Context] extension trait may be more convenient than this function.

The primary reason to use error.context(...) instead of result.context(...) via the Context trait would be if the context needs to depend on some data held by the underlying error:

# use std::fmt::{self, Debug, Display};
#
# type T = ();
#
# impl std::error::Error for ParseError {}
# impl Debug for ParseError {
#     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
#         unimplemented!()
#     }
# }
# impl Display for ParseError {
#     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
#         unimplemented!()
#     }
# }
#
use anyhow::Result;
use std::fs::File;
use std::path::Path;

struct ParseError {
    line: usize,
    column: usize,
}

fn parse_impl(file: File) -> Result<T, ParseError> {
    # const IGNORE: &str = stringify! {
    ...
    # };
    # unimplemented!()
}

pub fn parse(path: impl AsRef<Path>) -> Result<T> {
    let file = File::open(&path)?;
    parse_impl(file).map_err(|error| {
        let context = format!(
            "only the first {} lines of {} are valid",
            error.line, path.as_ref().display(),
        );
        anyhow::Error::new(error).context(context)
    })
}

fn backtrace(self: &Self) -> &std::backtrace::Backtrace

Get the backtrace for this Error.

In order for the backtrace to be meaningful, one of the two environment variables RUST_LIB_BACKTRACE=1 or RUST_BACKTRACE=1 must be defined and RUST_LIB_BACKTRACE must not be 0. Backtraces are somewhat expensive to capture in Rust, so we don't necessarily want to be capturing them all over the place all the time.

  • If you want panics and errors to both have backtraces, set RUST_BACKTRACE=1;
  • If you want only errors to have backtraces, set RUST_LIB_BACKTRACE=1;
  • If you want only panics to have backtraces, set RUST_BACKTRACE=1 and RUST_LIB_BACKTRACE=0.

Stability

Standard library backtraces are only available when using Rust ≥ 1.65. On older compilers, this function is only available if the crate's "backtrace" feature is enabled, and will use the backtrace crate as the underlying backtrace implementation. The return type of this function on old compilers is &(impl Debug + Display).

[dependencies]
anyhow = { version = "1.0", features = ["backtrace"] }

fn chain(self: &Self) -> Chain<'_>

An iterator of the chain of source errors contained by this Error.

This iterator will visit every error in the cause chain of this error object, beginning with the error that this error object was created from.

Example

use anyhow::Error;
use std::io;

pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
    for cause in error.chain() {
        if let Some(io_error) = cause.downcast_ref::<io::Error>() {
            return Some(io_error.kind());
        }
    }
    None
}

fn root_cause(self: &Self) -> &dyn StdError + 'static

The lowest level cause of this error — this error's cause's cause's cause etc.

The root cause is the last error in the iterator produced by [chain()][Error::chain].

fn is<E>(self: &Self) -> bool
where
    E: Display + Debug + Send + Sync + 'static

Returns true if E is the type held by this error object.

For errors with context, this method returns true if E matches the type of the context C or the type of the error on which the context has been attached. For details about the interaction between context and downcasting, see here.

fn downcast<E>(self: Self) -> Result<E, Self>
where
    E: Display + Debug + Send + Sync + 'static

Attempt to downcast the error object to a concrete type.

fn downcast_ref<E>(self: &Self) -> Option<&E>
where
    E: Display + Debug + Send + Sync + 'static

Downcast this error object by reference.

Example

# use anyhow::anyhow;
# use std::fmt::{self, Display};
# use std::task::Poll;
#
# #[derive(Debug)]
# enum DataStoreError {
#     Censored(()),
# }
#
# impl Display for DataStoreError {
#     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
#         unimplemented!()
#     }
# }
#
# impl std::error::Error for DataStoreError {}
#
# const REDACTED_CONTENT: () = ();
#
# let error = anyhow!("...");
# let root_cause = &error;
#
# let ret =
// If the error was caused by redaction, then return a tombstone instead
// of the content.
match root_cause.downcast_ref::<DataStoreError>() {
    Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
    None => Err(error),
}
# ;

fn downcast_mut<E>(self: &mut Self) -> Option<&mut E>
where
    E: Display + Debug + Send + Sync + 'static

Downcast this error object by mutable reference.

impl AsRef for crate::Error

fn as_ref(self: &Self) -> &dyn StdError + Send + Sync + 'static

impl AsRef for crate::Error

fn as_ref(self: &Self) -> &dyn StdError + 'static

impl Debug for crate::Error

fn fmt(self: &Self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result

impl Deref for crate::Error

fn deref(self: &Self) -> &<Self as >::Target

impl DerefMut for crate::Error

fn deref_mut(self: &mut Self) -> &mut <Self as >::Target

impl Display for crate::Error

fn fmt(self: &Self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result

impl Drop for crate::Error

fn drop(self: &mut Self)

impl Freeze for Error

impl RefUnwindSafe for crate::Error

impl Send for Error

impl Sync for Error

impl Unpin for Error

impl UnwindSafe for crate::Error

impl<E> From for crate::Error

fn from(error: E) -> Self

impl<P, T> Receiver for Error

impl<T> Any for Error

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for Error

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for Error

fn borrow_mut(self: &mut Self) -> &mut T

impl<T> From for Error

fn from(t: never) -> T

impl<T> From for Error

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> ToString for Error

fn to_string(self: &Self) -> String

impl<T, U> Into for Error

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

impl<T, U> TryFrom for Error

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto for Error

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>