vendor/thiserror-1.0.64/README.md - toolchain/rustc - Git at Google

 derive(Error)
 =============

 [<img alt="github" src="https://img.shields.io/badge/github-dtolnay/thiserror-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/thiserror)
 [<img alt="crates.io" src="https://img.shields.io/crates/v/thiserror.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/thiserror)
 [<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-thiserror-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/thiserror)
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 This library provides a convenient derive macro for the standard library's
 [`std::error::Error`] trait.

 [`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html

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

 *Compiler support: requires rustc 1.56+*

 <br>

 ## Example

 ```rust
 use thiserror::Error;

 #[derive(Error, Debug)]
 pub enum DataStoreError {
     #[error("data store disconnected")]
     Disconnect(#[from] io::Error),
     #[error("the data for key `{0}` is not available")]
     Redaction(String),
     #[error("invalid header (expected {expected:?}, found {found:?})")]
     InvalidHeader {
         expected: String,
         found: String,
     },
     #[error("unknown data store error")]
     Unknown,
 }
 ```

 <br>

 ## Details

 - Thiserror deliberately does not appear in your public API. You get the same
   thing as if you had written an implementation of `std::error::Error` by hand,
   and switching from handwritten impls to thiserror or vice versa is not a
   breaking change.

 - Errors may be enums, structs with named fields, tuple structs, or unit
   structs.

 - A `Display` impl is generated for your error if you provide `#[error("...")]`
   messages on the struct or each variant of your enum, as shown above in the
   example.

   The messages support a shorthand for interpolating fields from the error.

     - `#[error("{var}")]`&ensp;⟶&ensp;`write!("{}", self.var)`
     - `#[error("{0}")]`&ensp;⟶&ensp;`write!("{}", self.0)`
     - `#[error("{var:?}")]`&ensp;⟶&ensp;`write!("{:?}", self.var)`
     - `#[error("{0:?}")]`&ensp;⟶&ensp;`write!("{:?}", self.0)`

   These shorthands can be used together with any additional format args, which
   may be arbitrary expressions. For example:

   ```rust
   #[derive(Error, Debug)]
   pub enum Error {
       #[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)]
       InvalidLookahead(u32),
   }
   ```

   If one of the additional expression arguments needs to refer to a field of the
   struct or enum, then refer to named fields as `.var` and tuple fields as `.0`.

   ```rust
   #[derive(Error, Debug)]
   pub enum Error {
       #[error("first letter must be lowercase but was {:?}", first_char(.0))]
       WrongCase(String),
       #[error("invalid index {idx}, expected at least {} and at most {}", .limits.lo, .limits.hi)]
       OutOfBounds { idx: usize, limits: Limits },
   }
   ```

 - A `From` impl is generated for each variant that contains a `#[from]`
   attribute.

   The variant using `#[from]` must not contain any other fields beyond the
   source error (and possibly a backtrace &mdash; see below). Usually `#[from]`
   fields are unnamed, but `#[from]` is allowed on a named field too.

   ```rust
   #[derive(Error, Debug)]
   pub enum MyError {
       Io(#[from] io::Error),
       Glob(#[from] globset::Error),
   }
   ```

 - The Error trait's `source()` method is implemented to return whichever field
   has a `#[source]` attribute or is named `source`, if any. This is for
   identifying the underlying lower level error that caused your error.

   The `#[from]` attribute always implies that the same field is `#[source]`, so
   you don't ever need to specify both attributes.

   Any error type that implements `std::error::Error` or dereferences to `dyn
   std::error::Error` will work as a source.

   ```rust
   #[derive(Error, Debug)]
   pub struct MyError {
       msg: String,
       #[source]  // optional if field name is `source`
       source: anyhow::Error,
   }
   ```

 - The Error trait's `provide()` method is implemented to provide whichever field
   has a type named `Backtrace`, if any, as a `std::backtrace::Backtrace`. Using
   `Backtrace` in errors requires a nightly compiler with Rust version 1.73 or
   newer.

   ```rust
   use std::backtrace::Backtrace;

   #[derive(Error, Debug)]
   pub struct MyError {
       msg: String,
       backtrace: Backtrace,  // automatically detected
   }
   ```

 - If a field is both a source (named `source`, or has `#[source]` or `#[from]`
   attribute) *and* is marked `#[backtrace]`, then the Error trait's `provide()`
   method is forwarded to the source's `provide` so that both layers of the error
   share the same backtrace. The `#[backtrace]` attribute requires a nightly
   compiler with Rust version 1.73 or newer.


   ```rust
   #[derive(Error, Debug)]
   pub enum MyError {
       Io {
           #[backtrace]
           source: io::Error,
       },
   }
   ```

 - For variants that use `#[from]` and also contain a `Backtrace` field, a
   backtrace is captured from within the `From` impl.

   ```rust
   #[derive(Error, Debug)]
   pub enum MyError {
       Io {
           #[from]
           source: io::Error,
           backtrace: Backtrace,
       },
   }
   ```

 - Errors may use `error(transparent)` to forward the source and Display methods
   straight through to an underlying error without adding an additional message.
   This would be appropriate for enums that need an "anything else" variant.

   ```rust
   #[derive(Error, Debug)]
   pub enum MyError {
       ...

       #[error(transparent)]
       Other(#[from] anyhow::Error),  // source and Display delegate to anyhow::Error
   }
   ```

   Another use case is hiding implementation details of an error representation
   behind an opaque error type, so that the representation is able to evolve
   without breaking the crate's public API.

   ```rust
   // PublicError is public, but opaque and easy to keep compatible.
   #[derive(Error, Debug)]
   #[error(transparent)]
   pub struct PublicError(#[from] ErrorRepr);

   impl PublicError {
       // Accessors for anything we do want to expose publicly.
   }

   // Private and free to change across minor version of the crate.
   #[derive(Error, Debug)]
   enum ErrorRepr {
       ...
   }
   ```

 - See also the [`anyhow`] library for a convenient single error type to use in
   application code.

   [`anyhow`]: https://github.com/dtolnay/anyhow

 <br>

 ## Comparison to anyhow

 Use thiserror if you care about designing your own dedicated error type(s) so
 that the caller receives exactly the information that you choose in the event of
 failure. This most often applies to library-like code. 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-like code.

 [Anyhow]: https://github.com/dtolnay/anyhow

 <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>
	derive(Error)
	=============

	[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/thiserror-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/thiserror)
	[<img alt="crates.io" src="https://img.shields.io/crates/v/thiserror.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/thiserror)
	[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-thiserror-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/thiserror)
	[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/thiserror/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/thiserror/actions?query=branch%3Amaster)

	This library provides a convenient derive macro for the standard library's
	[`std::error::Error`] trait.

	[`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html

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

	Compiler support: requires rustc 1.56+

	<br>

	## Example

	```rust
	use thiserror::Error;

	#[derive(Error, Debug)]
	pub enum DataStoreError {
	#[error("data store disconnected")]
	Disconnect(#[from] io::Error),
	#[error("the data for key `{0}` is not available")]
	Redaction(String),
	#[error("invalid header (expected {expected:?}, found {found:?})")]
	InvalidHeader {
	expected: String,
	found: String,
	},
	#[error("unknown data store error")]
	Unknown,
	}
	```

	<br>

	## Details

	- Thiserror deliberately does not appear in your public API. You get the same
	thing as if you had written an implementation of `std::error::Error` by hand,
	and switching from handwritten impls to thiserror or vice versa is not a
	breaking change.

	- Errors may be enums, structs with named fields, tuple structs, or unit
	structs.

	- A `Display` impl is generated for your error if you provide `#[error("...")]`
	messages on the struct or each variant of your enum, as shown above in the
	example.

	The messages support a shorthand for interpolating fields from the error.

	- `#[error("{var}")]`&ensp;⟶&ensp;`write!("{}", self.var)`
	- `#[error("{0}")]`&ensp;⟶&ensp;`write!("{}", self.0)`
	- `#[error("{var:?}")]`&ensp;⟶&ensp;`write!("{:?}", self.var)`
	- `#[error("{0:?}")]`&ensp;⟶&ensp;`write!("{:?}", self.0)`

	These shorthands can be used together with any additional format args, which
	may be arbitrary expressions. For example:

	```rust
	#[derive(Error, Debug)]
	pub enum Error {
	#[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)]
	InvalidLookahead(u32),
	}
	```

	If one of the additional expression arguments needs to refer to a field of the
	struct or enum, then refer to named fields as `.var` and tuple fields as `.0`.

	```rust
	#[derive(Error, Debug)]
	pub enum Error {
	#[error("first letter must be lowercase but was {:?}", first_char(.0))]
	WrongCase(String),
	#[error("invalid index {idx}, expected at least {} and at most {}", .limits.lo, .limits.hi)]
	OutOfBounds { idx: usize, limits: Limits },
	}
	```

	- A `From` impl is generated for each variant that contains a `#[from]`
	attribute.

	The variant using `#[from]` must not contain any other fields beyond the
	source error (and possibly a backtrace — see below). Usually `#[from]`
	fields are unnamed, but `#[from]` is allowed on a named field too.

	```rust
	#[derive(Error, Debug)]
	pub enum MyError {
	Io(#[from] io::Error),
	Glob(#[from] globset::Error),
	}
	```

	- The Error trait's `source()` method is implemented to return whichever field
	has a `#[source]` attribute or is named `source`, if any. This is for
	identifying the underlying lower level error that caused your error.

	The `#[from]` attribute always implies that the same field is `#[source]`, so
	you don't ever need to specify both attributes.

	Any error type that implements `std::error::Error` or dereferences to `dyn
	std::error::Error` will work as a source.

	```rust
	#[derive(Error, Debug)]
	pub struct MyError {
	msg: String,
	#[source] // optional if field name is `source`
	source: anyhow::Error,
	}
	```

	- The Error trait's `provide()` method is implemented to provide whichever field
	has a type named `Backtrace`, if any, as a `std::backtrace::Backtrace`. Using
	`Backtrace` in errors requires a nightly compiler with Rust version 1.73 or
	newer.

	```rust
	use std::backtrace::Backtrace;

	#[derive(Error, Debug)]
	pub struct MyError {
	msg: String,
	backtrace: Backtrace, // automatically detected
	}
	```

	- If a field is both a source (named `source`, or has `#[source]` or `#[from]`
	attribute) and is marked `#[backtrace]`, then the Error trait's `provide()`
	method is forwarded to the source's `provide` so that both layers of the error
	share the same backtrace. The `#[backtrace]` attribute requires a nightly
	compiler with Rust version 1.73 or newer.


	```rust
	#[derive(Error, Debug)]
	pub enum MyError {
	Io {
	#[backtrace]
	source: io::Error,
	},
	}
	```

	- For variants that use `#[from]` and also contain a `Backtrace` field, a
	backtrace is captured from within the `From` impl.

	```rust
	#[derive(Error, Debug)]
	pub enum MyError {
	Io {
	#[from]
	source: io::Error,
	backtrace: Backtrace,
	},
	}
	```

	- Errors may use `error(transparent)` to forward the source and Display methods
	straight through to an underlying error without adding an additional message.
	This would be appropriate for enums that need an "anything else" variant.

	```rust
	#[derive(Error, Debug)]
	pub enum MyError {
	...

	#[error(transparent)]
	Other(#[from] anyhow::Error), // source and Display delegate to anyhow::Error
	}
	```

	Another use case is hiding implementation details of an error representation
	behind an opaque error type, so that the representation is able to evolve
	without breaking the crate's public API.

	```rust
	// PublicError is public, but opaque and easy to keep compatible.
	#[derive(Error, Debug)]
	#[error(transparent)]
	pub struct PublicError(#[from] ErrorRepr);

	impl PublicError {
	// Accessors for anything we do want to expose publicly.
	}

	// Private and free to change across minor version of the crate.
	#[derive(Error, Debug)]
	enum ErrorRepr {
	...
	}
	```

	- See also the [`anyhow`] library for a convenient single error type to use in
	application code.

	[`anyhow`]: https://github.com/dtolnay/anyhow

	<br>

	## Comparison to anyhow

	Use thiserror if you care about designing your own dedicated error type(s) so
	that the caller receives exactly the information that you choose in the event of
	failure. This most often applies to library-like code. 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-like code.

	[Anyhow]: https://github.com/dtolnay/anyhow

	<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>