no-std-thiserror 0.1.0

thiserror but no_std compatible
Documentation 
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#![cfg_attr(not(feature = "std"), no_std)]
cfg_if::cfg_if! {
    if #[cfg(not(feature = "std"))] {
        extern crate alloc;
        use alloc::string::{String, ToString};
        use alloc::format;
        use alloc::borrow::ToOwned;

        mod io {
            use thiserror::Error;

            #[derive(Error, Debug)]
            #[error("test")]
            pub struct Error;

            impl Error {
                pub fn new(kind: ErrorKind, s: &str) -> Self {
                    Self
                }
            }

            pub enum ErrorKind {
                Other
            }
        }
    } else {
        use std::error::Error as _;
        use std::io;
    }
}

use anyhow::anyhow;
use thiserror::Error;

#[test]
fn test_transparent_struct() {
    #[derive(Error, Debug)]
    #[error(transparent)]
    struct Error(ErrorKind);

    #[derive(Error, Debug)]
    enum ErrorKind {
        #[error("E0")]
        E0,
        #[error("E1")]
        E1(#[from] io::Error),
    }

    let error = Error(ErrorKind::E0);
    assert_eq!("E0", error.to_string());

    cfg_if::cfg_if! {
        if #[cfg(feature = "std")] {
            assert!(error.source().is_none());
        }
    }

    let io = io::Error::new(io::ErrorKind::Other, "oh no!");
    let error = Error(ErrorKind::from(io));
    assert_eq!("E1", error.to_string());

    cfg_if::cfg_if! {
        if #[cfg(feature = "std")] {
            error.source().unwrap().downcast_ref::<io::Error>().unwrap();
        }
    }
}

#[test]
fn test_transparent_enum() {
    #[derive(Error, Debug)]
    enum Error {
        #[error("this failed")]
        This,
        #[error(transparent)]
        Other(anyhow::Error),
    }

    let error = Error::This;
    assert_eq!("this failed", error.to_string());

    let error = Error::Other(anyhow!("inner").context("outer"));
    assert_eq!("outer", error.to_string());

    cfg_if::cfg_if! {
        if #[cfg(feature = "std")] {
            assert_eq!("inner", error.source().unwrap().to_string());
        }
    }
}

#[test]
fn test_anyhow() {
    #[derive(Error, Debug)]
    #[error(transparent)]
    struct Any(#[from] anyhow::Error);

    let error = Any::from(anyhow!("inner").context("outer"));
    assert_eq!("outer", error.to_string());

    cfg_if::cfg_if! {
        if #[cfg(feature = "std")] {
            assert_eq!("inner", error.source().unwrap().to_string());
        }
    }
}

#[test]
fn test_non_static() {
    #[derive(Error, Debug)]
    #[error(transparent)]
    struct Error<'a> {
        inner: ErrorKind<'a>,
    }

    #[derive(Error, Debug)]
    enum ErrorKind<'a> {
        #[error("unexpected token: {:?}", token)]
        Unexpected { token: &'a str },
    }

    let error = Error {
        inner: ErrorKind::Unexpected { token: "error" },
    };
    assert_eq!("unexpected token: \"error\"", error.to_string());

    cfg_if::cfg_if! {
        if #[cfg(feature = "std")] {
            assert!(error.source().is_none());
        }
    }
}