n0-error

An Rust error library that supports tracking the call-site location of errors.

This crate provides a StackError trait and proc macro to ergonomically work with nested enum and struct errors, while allowing to track the call-site location for the full error chain.

It also has a AnyError type that works similar to anyhow errors while keeping the location metadata of StackError errors accessible through the full error chain.

Example

use n0_error::{e, ensure, Result, StackResultExt, stack_error};

/// The `stack_error` macro controls how to turn our enum into a `StackError`.
///
/// * `add_meta` adds a field to all variants to track the call-site error location
/// * `derive` adds `#[derive(StackError)]`
/// * `from_sources` creates `From` impls for the error sources
#[stack_error(derive, add_meta, from_sources)]
enum MyError {
    /// We can define the error message with the `error` attribute
    #[error("invalid input")]
    InvalidInput { source: InvalidInput },
    /// Or we can define a variant as `transparent`, which forwards the Display impl to the error source.
    #[error(transparent)]
    Io {
        /// For sources that do not implement `StackError`, we have to mark the source as `std_err`.
        #[error(std_err)]
        source: std::io::Error,
    },
}

/// We can use the [`stack_error`] macro on structs as well.
#[stack_error(derive, add_meta)]
#[error("wanted {expected} but got {actual}")]
struct InvalidInput {
    expected: u32,
    actual: u32,
}

fn validate_input(input: u32) -> Result<(), InvalidInput> {
    if input != 23 {
        // With the `e` macro we can construct an error directly without spelling out the `meta` field:
        return Err(e!(InvalidInput { expected: 12, actual: input }))
    }
    /// There's also `bail` and `ensure` macros that expand to include the `meta` field:
    n0_error::ensure!(input == 23, InvalidInput { expected: 23, actual: input });
    Ok(())
}

/// The `Result` type defaults to `AnyError` for the error variant.
///
/// Errors types using the derive macro convert to `AnyError`, and we can add additional context
/// with the result extensions.
fn process(input: u32) -> Result<()> {
    validate_input(input).context("failed to process input")?;
    Ok(())
}

The error returned from process would look like this in the alternate display format:

failed to process input: invalid input: wanted 23 but got 13

and like this in the debug format with RUST_BACKTRACE=1 or RUST_ERROR_LOCATION=1:

failed to process input (examples/basic.rs:61:17)
Caused by:
    invalid input (examples/basic.rs:36:5)
    wanted 23 but got 13 (examples/basic.rs:48:13)

Details

• All errors using the macro implement the StackError trait, which exposes call-site metadata for where the error occurred. Its source method returns references which may be other stack errors, allowing access to location data for the entire error chain.
• The proc macro can add a meta field to structs or enum variants. This field stores call-site metadata accessed through the StackError trait.
  • Call-site metadata in the meta field is collected only if RUST_BACKTRACE=1 or RUST_ERROR_LOCATION=1 env variable is set. Otherwise, it is disabled to avoid runtime overhead.
  • The declarative macro e! provides an ergonomic way to construct such errors without explicitly setting the field. The crate's ensure and bail macro also do this.
• The crate includes an AnyError type, similar to anyhow::Error. When created from a StackError, the call-site metadata is preserved. AnyError is recommended for applications and tests, while libraries should use concrete derived errors.
  • All stack errors convert to AnyError, so they can be propagated to such results with ?.
  • For std errors, use std_context or anyerr to convert to AnyError. For stack errors, use context or simply propagate with ?.
  • Result extension traits provide conversions between results with StackErrors, std::error::Errors to AnyError, with support for attaching context.
• Both AnyError and all errors using the StackError derive feature consistent, structured output that includes location metadata when available.

Feature flags

• anyhow (off by default): Enables From<anyhow::Error> for AnyError

License

Copyright 2025 N0, INC.

This project is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

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