README.md - platform/external/rust/crates/anyhow - Git at Google

 Anyhow&ensp;¯\\\_(°ペ)\_/¯
 ==========================

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 This library provides [`anyhow::Error`][Error], a trait object based error type
 for easy idiomatic error handling in Rust applications.

 [Error]: https://docs.rs/anyhow/1.0/anyhow/struct.Error.html

 ```toml
 [dependencies]
 anyhow = "1.0"
 ```

 *Compiler support: requires rustc 1.34+*

 <br>

 ## Details

 - Use `Result<T, anyhow::Error>`, or equivalently `anyhow::Result<T>`, as the
   return type of any fallible function.

   Within the function, use `?` to easily propagate any error that implements the
   `std::error::Error` trait.

   ```rust
   use anyhow::Result;

   fn get_cluster_info() -> Result<ClusterMap> {
       let config = std::fs::read_to_string("cluster.json")?;
       let map: ClusterMap = serde_json::from_str(&config)?;
       Ok(map)
   }
   ```

 - Attach context to help the person troubleshooting the error understand where
   things went wrong. A low-level error like "No such file or directory" can be
   annoying to debug without more context about what higher level step the
   application was in the middle of.

   ```rust
   use anyhow::{Context, Result};

   fn main() -> Result<()> {
       ...
       it.detach().context("Failed to detach the important thing")?;

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

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

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

 - Downcasting is supported and can be by value, by shared reference, or by
   mutable reference as needed.

   ```rust
   // 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),
   }
   ```

 - If using the nightly channel, or stable with `features = ["backtrace"]`, a
   a backtrace is captured and printed with the error if the underlying error
   type does not already provide its own. In order to see backtraces, they must
   be enabled through the environment variables described in [`std::backtrace`]:

   - 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`.

   The tracking issue for this feature is [rust-lang/rust#53487].

   [`std::backtrace`]: https://doc.rust-lang.org/std/backtrace/index.html#environment-variables
   [rust-lang/rust#53487]: https://github.com/rust-lang/rust/issues/53487

 - Anyhow works with any error type that has an impl of `std::error::Error`,
   including ones defined in your crate. We do not bundle a `derive(Error)` macro
   but you can write the impls yourself or use a standalone macro like
   [thiserror].

   ```rust
   use thiserror::Error;

   #[derive(Error, Debug)]
   pub enum FormatError {
       #[error("Invalid header (expected {expected:?}, got {found:?})")]
       InvalidHeader {
           expected: String,
           found: String,
       },
       #[error("Missing attribute: {0}")]
       MissingAttribute(String),
   }
   ```

 - One-off error messages can be constructed using the `anyhow!` macro, which
   supports string interpolation and produces an `anyhow::Error`.

   ```rust
   return Err(anyhow!("Missing attribute: {}", missing));
   ```

   A `bail!` macro is provided as a shorthand for the same early return.

   ```rust
   bail!("Missing attribute: {}", missing);
   ```

 <br>

 ## No-std support

 In no_std mode, the same API is almost all available and works the same way. To
 depend on Anyhow in no_std mode, disable our default enabled "std" feature in
 Cargo.toml. A global allocator is required.

 ```toml
 [dependencies]
 anyhow = { version = "1.0", default-features = false }
 ```

 Since the `?`-based error conversions would normally rely on the
 `std::error::Error` trait which is only available through std, no_std mode will
 require an explicit `.map_err(Error::msg)` when working with a non-Anyhow error
 type inside a function that returns Anyhow's error type.

 <br>

 ## Comparison to failure

 The `anyhow::Error` type works something like `failure::Error`, but unlike
 failure ours is built around the standard library's `std::error::Error` trait
 rather than a separate trait `failure::Fail`. The standard library has adopted
 the necessary improvements for this to be possible as part of [RFC 2504].

 [RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md

 <br>

 ## Comparison to thiserror

 Use Anyhow if you don't care what error type your functions return, you just
 want it to be easy. This is common in application code. Use [thiserror] if you
 are a library that wants to design your own dedicated error type(s) so that on
 failures the caller gets exactly the information that you choose.

 [thiserror]: https://github.com/dtolnay/thiserror

 <br>

 #### License

 <sup>
 Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
 2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
 </sup>

 <br>

 <sub>
 Unless you explicitly state otherwise, any contribution intentionally submitted
 for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
 be dual licensed as above, without any additional terms or conditions.
 </sub>
	Anyhow&ensp;¯\\\_(°ペ)\_/¯
	==========================

	[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/anyhow-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/anyhow)
	[<img alt="crates.io" src="https://img.shields.io/crates/v/anyhow.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/anyhow)
	[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-anyhow-66c2a5?style=for-the-badge&labelColor=555555&logoColor=white&logo=data:image/svg+xml;base64,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" height="20">](https://docs.rs/anyhow)
	[<img alt="build status" src="https://img.shields.io/github/workflow/status/dtolnay/anyhow/CI/master?style=for-the-badge" height="20">](https://github.com/dtolnay/anyhow/actions?query=branch%3Amaster)

	This library provides [`anyhow::Error`][Error], a trait object based error type
	for easy idiomatic error handling in Rust applications.

	[Error]: https://docs.rs/anyhow/1.0/anyhow/struct.Error.html

	```toml
	[dependencies]
	anyhow = "1.0"
	```

	Compiler support: requires rustc 1.34+

	<br>

	## Details

	- Use `Result<T, anyhow::Error>`, or equivalently `anyhow::Result<T>`, as the
	return type of any fallible function.

	Within the function, use `?` to easily propagate any error that implements the
	`std::error::Error` trait.

	```rust
	use anyhow::Result;

	fn get_cluster_info() -> Result<ClusterMap> {
	let config = std::fs::read_to_string("cluster.json")?;
	let map: ClusterMap = serde_json::from_str(&config)?;
	Ok(map)
	}
	```

	- Attach context to help the person troubleshooting the error understand where
	things went wrong. A low-level error like "No such file or directory" can be
	annoying to debug without more context about what higher level step the
	application was in the middle of.

	```rust
	use anyhow::{Context, Result};

	fn main() -> Result<()> {
	...
	it.detach().context("Failed to detach the important thing")?;

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

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

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

	- Downcasting is supported and can be by value, by shared reference, or by
	mutable reference as needed.

	```rust
	// 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),
	}
	```

	- If using the nightly channel, or stable with `features = ["backtrace"]`, a
	a backtrace is captured and printed with the error if the underlying error
	type does not already provide its own. In order to see backtraces, they must
	be enabled through the environment variables described in [`std::backtrace`]:

	- 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`.

	The tracking issue for this feature is [rust-lang/rust#53487].

	[`std::backtrace`]: https://doc.rust-lang.org/std/backtrace/index.html#environment-variables
	[rust-lang/rust#53487]: https://github.com/rust-lang/rust/issues/53487

	- Anyhow works with any error type that has an impl of `std::error::Error`,
	including ones defined in your crate. We do not bundle a `derive(Error)` macro
	but you can write the impls yourself or use a standalone macro like
	[thiserror].

	```rust
	use thiserror::Error;

	#[derive(Error, Debug)]
	pub enum FormatError {
	#[error("Invalid header (expected {expected:?}, got {found:?})")]
	InvalidHeader {
	expected: String,
	found: String,
	},
	#[error("Missing attribute: {0}")]
	MissingAttribute(String),
	}
	```

	- One-off error messages can be constructed using the `anyhow!` macro, which
	supports string interpolation and produces an `anyhow::Error`.

	```rust
	return Err(anyhow!("Missing attribute: {}", missing));
	```

	A `bail!` macro is provided as a shorthand for the same early return.

	```rust
	bail!("Missing attribute: {}", missing);
	```

	<br>

	## No-std support

	In no_std mode, the same API is almost all available and works the same way. To
	depend on Anyhow in no_std mode, disable our default enabled "std" feature in
	Cargo.toml. A global allocator is required.

	```toml
	[dependencies]
	anyhow = { version = "1.0", default-features = false }
	```

	Since the `?`-based error conversions would normally rely on the
	`std::error::Error` trait which is only available through std, no_std mode will
	require an explicit `.map_err(Error::msg)` when working with a non-Anyhow error
	type inside a function that returns Anyhow's error type.

	<br>

	## Comparison to failure

	The `anyhow::Error` type works something like `failure::Error`, but unlike
	failure ours is built around the standard library's `std::error::Error` trait
	rather than a separate trait `failure::Fail`. The standard library has adopted
	the necessary improvements for this to be possible as part of [RFC 2504].

	[RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md

	<br>

	## Comparison to thiserror

	Use Anyhow if you don't care what error type your functions return, you just
	want it to be easy. This is common in application code. Use [thiserror] if you
	are a library that wants to design your own dedicated error type(s) so that on
	failures the caller gets exactly the information that you choose.

	[thiserror]: https://github.com/dtolnay/thiserror

	<br>

	#### License

	<sup>
	Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
	2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
	</sup>

	<br>

	<sub>
	Unless you explicitly state otherwise, any contribution intentionally submitted
	for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
	be dual licensed as above, without any additional terms or conditions.
	</sub>