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---
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source: tests/output.rs
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expression: "get_stdout(path, &[\"anyhow\"])"
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---
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anyhow Module anyhow rusty-man
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DESCRIPTION
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[[github]][1] [[crates-io]][2] [[docs-rs]][3]
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This library provides `anyhow::Error`, a trait object based error type for easy idiomatic
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error handling in Rust applications.
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# Details
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* Use `Result<T, anyhow::Error>`, or equivalently `anyhow::Result<T>`, as the return type of
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any fallible function.
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Within the function, use `?` to easily propagate any error that implements the
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`std::error::Error` trait.
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`use anyhow::Result;
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fn get_cluster_info() -> Result<ClusterMap> {
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let config = std::fs::read_to_string("cluster.json")?;
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let map: ClusterMap = serde_json::from_str(&config)?;
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Ok(map)
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}`
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* Attach context to help the person troubleshooting the error understand where things went
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wrong. A low-level error like “No such file or directory” can be annoying to debug without
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more context about what higher level step the application was in the middle of.
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`use anyhow::{Context, Result};
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fn main() -> Result<()> {
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...
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it.detach().context("Failed to detach the important thing")?;
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let content = std::fs::read(path)
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.with_context(|| format!("Failed to read instrs from {}", path))?;
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...
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}`
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`Error: Failed to read instrs from ./path/to/instrs.json
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Caused by:
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No such file or directory (os error 2)`
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* Downcasting is supported and can be by value, by shared reference, or by mutable reference
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as needed.
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`// If the error was caused by redaction, then return a
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// tombstone instead of the content.
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match root_cause.downcast_ref::<DataStoreError>() {
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Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
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None => Err(error),
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}`
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* If using the nightly channel, a backtrace is captured and printed with the error if the
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underlying error type does not already provide its own. In order to see backtraces, they
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must be enabled through the environment variables described in [`std::backtrace`][1]:
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* If you want panics and errors to both have backtraces, set `RUST_BACKTRACE=1`;
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* If you want only errors to have backtraces, set `RUST_LIB_BACKTRACE=1`;
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* If you want only panics to have backtraces, set `RUST_BACKTRACE=1` and
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`RUST_LIB_BACKTRACE=0`.
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The tracking issue for this feature is [rust-lang/rust#53487][2].
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[1] https://doc.rust-lang.org/std/backtrace/index.html#environment-variables
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[2] https://github.com/rust-lang/rust/issues/53487
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* Anyhow works with any error type that has an impl of `std::error::Error`, including ones
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defined in your crate. We do not bundle a `derive(Error)` macro but you can write the impls
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yourself or use a standalone macro like [thiserror][1].
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`use thiserror::Error;
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#[derive(Error, Debug)]
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pub enum FormatError {
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#[error("Invalid header (expected {expected:?}, got {found:?})")]
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InvalidHeader {
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expected: String,
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found: String,
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},
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#[error("Missing attribute: {0}")]
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MissingAttribute(String),
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}`
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[1] https://github.com/dtolnay/thiserror
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* One-off error messages can be constructed using the `anyhow!` macro, which supports string
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interpolation and produces an `anyhow::Error`.
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`return Err(anyhow!("Missing attribute: {}", missing));`
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A `bail!` macro is provided as a shorthand for the same early return.
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`bail!("Missing attribute: {}", missing);`
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# No-std support
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In no_std mode, the same API is almost all available and works the same way. To depend on
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Anyhow in no_std mode, disable our default enabled “std” feature in Cargo.toml. A global
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allocator is required.
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`[dependencies]
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anyhow = { version = "1.0", default-features = false }`
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Since the `?`-based error conversions would normally rely on the `std::error::Error` trait
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which is only available through std, no_std mode will require an explicit
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`.map_err(Error::msg)` when working with a non-Anyhow error type inside a function that
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returns Anyhow’s error type.
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[1] https://github.com/dtolnay/anyhow
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[2] https://crates.io/crates/anyhow
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[3] https://docs.rs/anyhow
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MACROS
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anyhow
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Construct an ad-hoc error from a string or existing non-`anyhow` error value.
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bail
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Return early with an error.
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ensure
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Return early with an error if a condition is not satisfied.
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STRUCTS
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Chain
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Iterator of a chain of source errors.
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Error
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The `Error` type, a wrapper around a dynamic error type.
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TRAITS
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Context
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Provides the `context` method for `Result`.
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TYPEDEFS
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Result
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`Result<T, Error>`
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