Cannot define a custom error using thiserror crate

[Luni-4]

I'm trying to define a custom error through thiserror crate using this code

use thiserror::Error;

///Error types
#[derive(Debug, Error)]
pub enum Error {
    /// License not parsed correctly.
    #[error("License not parsed correctly")]
    InvalidLicense(#[from] license::ParseError),
}

/// A specialized `Result` type.
pub type Result<T> = ::std::result::Result<T, Error>;

but it seems that ParseError cannot be retrieved using #[from] attribute

[evenorog]

Thanks, will fix this when error is available in core.

rust-lang/rust#103765

[Luni-4]

Out of curiosity, why do we need to wait until that issue will be closed? Can we use for now an intermediate solution? I am going to take minijinja as direct example since I have used that crate recently 😄

Here how an error is handled

An I can add #[from] minijinja::Error to my custom error.

[evenorog]

I have never used thiserror so I don't know how it works, but I'm assuming #[from] requires the Error trait to be implemented for this to work? Or does it require another trait to be implemented?

This library is no-std so I currently can't implement Error without adding additional feature gating for std and alloc.

[Luni-4]

Ahh, if this library is considering even a no-std environment we cannot implement thiserror because it's a derive macro for std::error::Error trait. There is a workaround but they advise against using that.

In your opinion, what would a possible hack be to fix my problem? For now, I have introduced custom errors through map_err, but it does not seem a good solution eh eh

As an alternative, I might also try snafu, an error handling library which also considers no-std environment

[evenorog]

I'm only guessing but if it is the from attribute that fails, maybe you can implement the From trait manually?

impl From<ParseError> for Error {
   fn from(e: ParseError) -> Error {
       Error::InvalidLicense(e)
   }
}

Or is it the error attribute that fails?

[Luni-4]

Yep, in that case works, just wanted to use the #[from] to simplify the code a bit, but it's okay for me this solution. Thanks a lot!

[Luni-4]

The error I get with the #[from] attribute is:

src/error.rs:20:22
    |
20  |     InvalidLicense(#[from] license::ParseError),
    |                      ^^^^ method cannot be called on `&ParseError` due to unsatisfied trait bounds
    |
   ::: /home/.cargo/registry/src/index.crates.io-6f17d22bba15001f/license-3.2.0/src/lib.rs:159:1
    |
159 | pub struct ParseError(());
    | ---------------------
    | |
    | doesn't satisfy `ParseError: AsDynError<'_>`
    | doesn't satisfy `ParseError: StdError`
    |
    = note: the following trait bounds were not satisfied:
            `ParseError: StdError`
            which is required by `ParseError: AsDynError<'_>`
            `&ParseError: StdError`
            which is required by `&ParseError: AsDynError<'_>`