--- source: tests/output.rs expression: "get_stdout(path, &[\"anyhow::Error\"])" --- anyhow Struct anyhow::Error rusty-man SYNOPSIS pub struct Error { /* fields omitted */ } DESCRIPTION The `Error` type, a wrapper around a dynamic error type. `Error` works a lot like `Box`, but with these differences: * `Error` requires that the error is `Send`, `Sync`, and `'static`. * `Error` guarantees that a backtrace is available, even if the underlying error type does not provide one. * `Error` is 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.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: ::ext_context at /git/anyhow/src/backtrace.rs:26 1: core::result::Result::map_err at /git/rustc/src/libcore/result.rs:596 2: anyhow::context:: for core::result::Result>::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<()> { ... } METHODS impl Error new pub fn new(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. msg pub fn msg(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: use anyhow::{Error, Result}; use futures::stream::{Stream, StreamExt, TryStreamExt}; async fn demo(stream: S) -> Result> where S: Stream, { stream .then(ffi::do_some_work) // returns Result .map_err(Error::msg) .try_collect() .await } context pub fn context(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 anyhow::Result; use std::fs::File; use std::path::Path; struct ParseError { line: usize, column: usize, } fn parse_impl(file: File) -> Result { ... } pub fn parse(path: impl AsRef) -> Result { 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) }) } chain pub fn chain(&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 { for cause in error.chain() { if let Some(io_error) = cause.downcast_ref::() { return Some(io_error.kind()); } } None } root_cause pub fn root_cause(&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]. is pub fn is(&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. downcast pub fn downcast(self) -> Result where     E: Display + Debug + Send + Sync + 'static, Attempt to downcast the error object to a concrete type. downcast_ref pub fn downcast_ref(&self) -> Option<&E> where     E: Display + Debug + Send + Sync + 'static, 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::() { Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)), None => Err(error), } downcast_mut pub fn downcast_mut(&mut self) -> Option<&mut E> where     E: Display + Debug + Send + Sync + 'static, Downcast this error object by mutable reference. Methods from Deref is pub fn is(&self) -> bool where     T: 'static + Error, Returns `true` if the boxed type is the same as `T` downcast_ref pub fn downcast_ref(&self) -> Option<&T> where     T: 'static + Error, Returns some reference to the boxed value if it is of type `T`, or `None` if it isn't. downcast_mut pub fn downcast_mut(&mut self) -> Option<&mut T> where     T: 'static + Error, Returns some mutable reference to the boxed value if it is of type `T`, or `None` if it isn't. IMPLEMENTATIONS Trait Implementations AsRef impl AsRef for Error AsRef impl AsRef for Error Debug impl Debug for Error Deref impl Deref for Error DerefMut impl DerefMut for Error Display impl Display for Error Drop impl Drop for Error From impl From for Box From impl From for Box From impl From for Error where     E: StdError + Send + Sync + 'static, Auto Trait Implementations RefUnwindSafe impl RefUnwindSafe for Error Send impl Send for Error Sync impl Sync for Error Unpin impl Unpin for Error UnwindSafe impl UnwindSafe for Error Blanket Implementations Any impl Any for T where     T: 'static + ?Sized, Borrow impl Borrow for T where     T: ?Sized, BorrowMut impl BorrowMut for T where     T: ?Sized, From impl From for T From impl From for T Into impl Into for T where     U: From, ToString impl ToString for T where     T: Display + ?Sized, TryFrom impl TryFrom for T where     U: Into, TryInto impl TryInto for T where     U: TryFrom,