Ergonomic, static, contextual errors in Rust.

In a (typically larger) Rust program which is handling errors correctly, by default Rust does not capture the location (filename, line number and column number) where an Error was first emitted.

Further, a function typically has incomplete context around why it is accessing a resource (e.g. opening a file, establishing a network connection, locking a resource, etc.). A contextual error library provides convenient techniques for attaching context to an Error at various (and multiple) levels of the call stack.

If you use dynamic errors types (dyn core::error::Error) in Rust, there are many crates including anyhow that you can use for contextual error-handling.

But if you use static errors (no type erasure) for your libraries and/or applications, at least as of the time of this writing, there are no Rust solutions available which allow you to ergonomically (without significant repetitive boilerplate) and idiomatically (without significant changes to your existing Error types) benefit from contextual errors in your code.

In significant applications, considerable engineering time can be spent deducing the context of an emitted Error, not including the time to understand, fix and ship/redeploy the solution. ctx_error is designed, with proper use, to provide the necessary context with the Error, to increase the speed at which a bugfix can be implemented.

This library is can be updated to be #[no_std] compatible (this work has not yet been done).
Compile with cargo build --no-default-features to build in #[no_std] environments. When compiled for #[no_std], ctx_error will no longer support Backtraces (a std feature) or user context (append_ctx())--user context is implemented as a Vec<String> (also a std feature) to support user context from multiple levels of the call stack.

Let's imagine a project layout looking something like this:

src/
  +- error
  | +- init.rs
  | +- user_input.rs
  +- error.rs
  +- init.rs
  +- lib.rs

error.rs looks like:

// error.rs
pub mod init;
pub mod user_input;

use core::result::Result as CoreResult;

use thiserror::Error;

type Result<T, E = Error> = CoreResult<T, E>;

#[derive(Debug, Error)]
pub enum Error {
    #[error(transparent)]
    Init(#[from] init::Error),
    #[error(transparent)]
    Io(#[from] std::io::Error),
    #[error(transparent)]
    UserInput(#[from] user_input::Error),
}

and init.rs contains:

// error/init.rs
use core::result::Result as CoreResult;

use thiserror::Error;

type Result<T, E = Error> = CoreResult<T, E>;

#[derive(Debug, Error)]
pub enum Error {
    #[error("Environment variable {_0} not set")]
    EnvironmentVarNotSet(String),
    #[error(transparent)]
    UnreadableConfigFile(#[from] std::io::Error),
    // ...
}

Given the above project, main() might look something like:

// main.rs
use lib::{init, error::Result};

fn main() -> Result<()> {
    let config_reader = init::config_reader()?;
    let app = App::new(reader)?;
    app.run()?;
  
    Ok(())
}

And config_reader() might look like:

// init.rs
use std::io::Read;

use crate::error::init::Result;

pub fn config_reader() -> Result<impl Read> where
{
    // ...
    let config_reader = File::open(config_path)?;
    Ok(config_reader)
}

Above, init() returns an error::init::Error, but main() returns an error::Error. The init::Error returned by init() is converted to an Error via the From impl defined within Error. But the user sees no information on where the Err(Error::Init(UnreadableConfigFile(std::io::Error))) was emitted. In this trivial example, it could only have come from one place. But in a full application, a "File not found" error could be emitted from potentially dozens, hundreds or even thousands of locations, and without additional information, a lot of time will be spent tracking down the source of the error.

Now let's update the example with ctx_error. In the update, we make the following changes to each Error definiton module:

1. From your crate root folder type cargo add ctx_error. To each error module:
2. Add use ctx_error::ctx_error.
3. Add ctx_error!(, <optional: path to Parent Error (typically super::Error)>);
4. (Optional) Change Result alias' default Error type to CtxError.

ctx_error is designed to add minimal additional boilerplate to the Error definition pattern. It effectively creates a set of CtxErrors which wrap the locally defined Error with additional context information, without requiring each Error variant to carry (repetitive Context type information). By avoiding changing the definition of Errors, we also leave construction and usage of those Errors intact.

CtxError implements From converters for the local Error type, enabling seamless transition from Error -> corresponding CtxError using the try operator (?).

CtxError also implements core::error::Error, so it can be used anywhere any other core::error::Error can be used.

Updated error.rs (root error definition):

// error.rs
pub mod init;
pub mod user_input;

use core::result::Result as CoreResult;

// New!
use ctx_error::ctx_error;
use thiserror::Error;

//                   v-- new!
type Result<T, E = CtxError> = CoreResult<T, E>;

#[derive(Debug, Error)]
pub enum Error {
    #[error(transparent)]
    Init(#[from] init::Error),
    #[error(transparent)]
    Io(#[from] std::io::Error),
    #[error(transparent)]
    UserInput(#[from] user_input::Error),
}

// New!
ctx_error!(Error);

Updated init.rs sub-error definition:

// error/init.rs
use core::result::Result as CoreResult;

// New!
use ctx_error::ctx_error;
use thiserror::Error;

//                   v-- new!
type Result<T, E = CtxError> = CoreResult<T, E>;

#[derive(Debug, Error)]
pub enum Error {
    #[error("Environment variable {_0} not set")]
    EnvironmentVarNotSet(String),
    #[error(transparent)]
    UnreadableConfigFile(#[from] std::io::Error),
    // ...
}

// New!
ctx_error!(Error, super::Error);

Now running the same program gives a different result:

No such file or directory (os error 2)

becomes:

No such file or directory (os error 2) at src/init.rs:9:5

With RUST_BACKTRACE=1 cargo run:

No such file or directory (os error 2) at src/main.rs:7:5
Error Backtrace
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1: std::backtrace_rs::backtrace::trace_unsynchronized
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2: std::backtrace::Backtrace::create
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/backtrace.rs:331:13
... snip ...
  19: std::rt::lang_start_internal
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/rt.rs:143:20
  20: std::rt::lang_start
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/rt.rs:163:17
  21: _main

ctx_error provides append_ctx() method on all core::result::Result and all CtxErrors, so the Error can easily be associated with arbitrary user context. Imagine append_context() is used in init::config_reader() as follows:

// init.rs
use std::io::Read;

use crate::error::init::Result;

pub fn config_reader() -> Result<impl Read> where
{
  // ...
  //                                              v-- new!
  let config_reader = File::open(config_path).append_ctx(config_path)?;
  Ok(config_reader)
}

and main() is modified as follows:

// main.rs
use lib::error::Result;

fn main() -> Result<()> {
    //                                                     v-- new!
    let config_reader = lib::init::config_reader().append_context("Reading app config")?;
    let app = App::new(reader)?;
    app.run()?;
  
    Ok(())
}

Now the run output will change from:

No such file or directory (os error 2)

to (filename is exemplar)

No such file or directory (os error 2) at src/init.rs:9:5
Context (Display order: error origination site -> program entry point):
  1: config/my_app_config.json
  2: Reading app config

or, with RUST_BACKTRACE=1 cargo run:

No such file or directory (os error 2) at src/init.rs:9:5
Context (Display order: error origination site -> program entry point):
  1: config_files/my_app_config.json
  2: Reading app config
Error Backtrace
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1: std::backtrace_rs::backtrace::trace_unsynchronized
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2: std::backtrace::Backtrace::create
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/backtrace.rs:331:13
... snip ...
  19: std::rt::lang_start_internal
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/rt.rs:143:20
  20: std::rt::lang_start
             at /rustc/26b5599e4d6ed2b45152c60493c1788c0a27533d/library/std/src/rt.rs:163:17
  21: _main

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