#[repr(transparent)]pub struct Error { /* private fields */ }Expand description
The Error type, a wrapper around a dynamic error type.
Error works a lot like Box<dyn std::error::Error>, but with these
differences:
Errorrequires that the error isSend,Sync, and'static.Errorguarantees that a backtrace is available, even if the underlying error type does not provide one.Erroris represented as a narrow pointer — exactly one word in size instead of two.
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.jsonTo 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: _startTo 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<()> {
...
}Implementations
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.
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:
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
}Wrap the error value with additional context.
For attaching context to a Result as it is propagated, the
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 anyhow::Result;
use std::fs::File;
use std::path::Path;
struct ParseError {
line: usize,
column: usize,
}
fn parse_impl(file: File) -> Result<T, ParseError> {
...
}
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)
})
}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=1andRUST_LIB_BACKTRACE=0.
Stability
Standard library backtraces are only available on the nightly channel. Tracking issue: rust-lang/rust#53487.
On stable 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.
[dependencies]
anyhow = { version = "1.0", features = ["backtrace"] }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
}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().
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.
Attempt to downcast the error object to a concrete type.
Downcast this error object by reference.
Example
// 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),
}