Crate onlyerror

source ·
Expand description

no_std-compatible derive macro for error handling.

onlyerror is comparable in feature set to the venerable thiserror crate with two major differences:

  1. The feature subset is highly restricted.
  2. Generally much faster compile times.

For more on compile times, see the myn benchmarks.


use onlyerror::Error;

#[derive(Debug, Error)]
pub enum HttpClientError {
    /// I/O error.
    Io(#[from] std::io::Error),

    /// Login error.
    #[error("Login error. Server message: `{0}`.")]

    /// Invalid header.
    #[error("Invalid header (expected {expected:?}, found {found:?}).")]
    InvalidHeader {
        expected: String,
        found: String,

    /// Unknown.

§DSL reference

The macro has a DSL modeled after thiserror, so it should feel familiar to anyone who has used it.

  • The macro derives an implementation for the Error trait.
  • Display is derived using the #[error("...")] attributes with a fallback to doc comments.
  • From is derived for each #[from] or #[source] attribute.

Error messages in #[error("...")] can reference enum variant fields by name (for struct-like variants) or by number (for tuple-like variants) using the std::fmt machinery.

It is recommended to use #[error("...")] when you need interpolation, otherwise use doc comments. Doing this will keep implementation details out of your documentation while making the error variants self-documenting.


  • Only enum types are supported by the Error macro.
  • Only inline string interpolations are supported by the derived Display impl.
  • Either all variants must be given an error message, or #[no_display] attribute must be set to enum with hand-written Display implementation
  • From impls are only derived for #[from] and #[source] attributes, not implicitly for any field names.
  • Backtrace is not supported.
  • #[error(transparent)] is not supported.

§Cargo features

To use onlyerror in a no_std environment, disable default features in your Cargo manifest.

As of writing, you must add #![feature(error_in_core)] to the top-level or file to enable the core::error module. This feature flag is only available on nightly compilers.

Derive Macros§