Crate error_chain

A library for consistent and reliable error handling

This crate defines an opinionated strategy for error handling in Rust, built on the following principles:

• No error should ever be discarded. This library primarily makes it easy to "chain" errors with the chain_err method.
• Introducing new errors is trivial. Simple errors can be introduced at the error site with just a string.
• Handling errors is possible with pattern matching.
• Conversions between error types are done in an automatic and consistent way - From conversion behavior is never specified explicitly.
• Errors implement Send.
• Errors carry backtraces.

Similar to other libraries like error-type and quick-error, this library defines a macro, error_chain! that declares the types and implementation boilerplate necessary for fulfilling a particular error-hadling strategy. Most importantly it defines a custom error type (called Error by convention) and the From conversions that let the try! macro and ? operator work.

This library differs in a few ways from previous error libs:

• Instead of defining the custom Error type as an enum, it is a struct containing an ErrorKind (which defines the description and display methods for the error), an opaque, optional, boxed std::error::Error + Send + 'static object (which defines the cause, and establishes the links in the error chain), and a Backtrace.
• The macro additionally defines a trait, by convention called ChainErr, that defines a chain_err method. This method on all std::error::Error + Send + 'static types extends the error chain by boxing the current error into an opaque object and putting it inside a new concrete error.
• It provides automatic From conversions between other error types defined by the error_chain! that preserve type information, and facilitate seamless error composition and matching of composed errors.
• It provides automatic From conversions between any other error type that hides the type of the other error in the cause box.
• It collects a single backtrace at the earliest opportunity and propagates it down the stack through From and ChainErr conversions.

To accomplish its goals it makes some tradeoffs:

• The split between the Error and ErrorKind types can make it slightly more cumbersome to instantiate new (unchained) errors errors, requiring an Into or From conversion; as well as slightly more cumbersome to match on errors with another layer of types to match.
• Because the error type contains std::error::Error + Send + 'static objects, it can't implement PartialEq for easy comparisons.

Declaring error types

Generally, you define one family of error types per crate, though it's also perfectly fine to define error types on a finer-grained basis, such as per module.

Assuming you are using crate-level error types, typically you will define an errors module and inside it call error_chain!:

error_chain! {
    // The type defined for this error. These are the conventional
    // and recommended names, but they can be arbitrarily chosen.
    types {
        Error, ErrorKind, ChainErr, Result;
    }

    // Automatic conversions between this error chain and other
    // error chains. In this case, it will e.g. generate an
    // `ErrorKind` variant called `Dist` which in turn contains
    // the `rustup_dist::ErrorKind`, with conversions from
    // `rustup_dist::Error`.
    //
    // This section can be empty.
    links {
        rustup_dist::Error, rustup_dist::ErrorKind, Dist;
        rustup_utils::Error, rustup_utils::ErrorKind, Utils;
    }

    // Automatic conversions between this error chain and other
    // error types not defined by the `error_chain!`. These will be
    // boxed as the error cause and wrapped in a new error with,
    // in this case, the `ErrorKind::Temp` variant.
    //
    // This section can be empty.
    foreign_links {
        temp::Error, Temp,
        "temporary file error";
    }

    // Define additional `ErrorKind` variants. The syntax here is
    // the same as `quick_error!`, but the `from()` and `cause()`
    // syntax is not supported.
    errors {
        InvalidToolchainName(t: String) {
            description("invalid toolchain name")
            display("invalid toolchain name: '{}'", t)
        }
    }
}

This populates the the module with a number of definitions, the most important of which are the Error type and the ErrorKind type. They look something like the following:

use std::error::Error as StdError;
use std::sync::Arc;

#[derive(Debug)]
pub struct Error(pub ErrorKind,
                 pub Option<Box<StdError + Send>>,
                 pub Arc<error_chain::Backtrace>);

impl Error {
    pub fn kind(&self) -> &ErrorKind { ... }
    pub fn into_kind(self) -> ErrorKind { ... }
    pub fn iter(&self) -> error_chain::ErrorChainIter { ... }
    pub fn backtrace(&self) -> &error_chain::Backtrace { ... }
}

impl StdError for Error { ... }
impl Display for Error { ... }

#[derive(Debug)]
pub enum ErrorKind {
    Msg(String),
    Dist(rustup_dist::ErrorKind),
    Utils(rustup_utils::ErrorKind),
    Temp,
    InvalidToolchainName(String),
}

This is the basic error structure. You can see that ErrorKind has been populated in a variety of ways. All ErrorKinds get a Msg variant for basic errors. When strings are converted to ErrorKinds they become ErrorKind::Msg. The "links" defined in the macro are expanded to Dist and Utils variants, and the "foreign links" to the Temp variant.

Both types come with a variety of From conversiaos as well: Error can be created from ErrorKind, from &str and String, and from the "link" and "foreign_link" error types. ErrorKind can be created from the corresponding ErrorKinds of the link types, as wall as from &str and String.

into() and From::from are used heavily to massage types into the right shape. Which one to use in any specific case depends on the influence of type inference, but there are some patterns that arise frequently.

Returning new errors

Introducing new error chains, with a string message:

fn foo() -> Result<()> {
    Err("foo error!".into())
}

Introducing new error chains, with an ErrorKind:

fn foo() -> Result<()> {
    Err(ErrorKind::FooError.into())
}

Note that the return type is is the typedef Result, which is defined by the macro as pub type Result<T> = ::std::result::Result<T, Error>. Note that in both cases .into() is called to convert a type into the Error type: both strings and ErrorKind have From conversions to turn them into Error.

When the error is emitted inside a try! macro or behind the ? operator, then the explicit conversion isn't needed, since the behavior of try! will automatically convert Err(ErrorKind) to Err(Error). So the below is equivalent to the previous:

fn foo() -> Result<()> {
    Ok(try!(Err(ErrorKind::FooError)))
}

fn bar() -> Result<()> {
    Ok(try!(Err("bogus!")))

Chaining errors

To extend the error chain:

use errors::ChainErr;
try!(do_something().chain_err(|| "something went wrong"));

chain_err can be called on any Result type where the contained error type implements std::error::Error + Send + 'static. If the Result is an Err then chain_err evaluates the closure, which returns some type that can be converted to ErrorKind, boxes the original error to store as the cause, then returns a new error containing the original error.

Foreign links

Errors that do not conform to the same conventions as this library can still be included in the error chain. They are considered "foreign errors", and are declared using the foreign_links block of the error_chain! macro. Errors are automatically created from foreign errors by the try! macro.

Foreign links and regular links have one crucial difference: From conversions for regular links do not introduce a new error into the error chain, while conversions for foreign links always introduce a new error into the error chain. So for the example above all errors deriving from the temp::Error type will be presented to the user as a new ErrorKind::Temp variant, and the cause will be the original temp::Error error. In contrast, when rustup_utils::Error is converted to Error the two ErrorKinds are converted between each other to create a new Error but the old error is discarded; there is no "cause" created from the original error.

Backtraces

The earliest non-foreign error to be generated creates a single backtrace, which is passed through all From conversions and chain_err invocations of compatible types. To read the backtrace just call the backtrace() method.

Iteration

The iter method returns an iterator over the chain of error boxes.

Macros

error_chain
quick_error

Structs

Backtrace	Representation of an owned and self-contained backtrace.
ErrorChainIter