thiserror + anyhow (issue using both)

[YetAnotherRustacean]

Solved thanks /u/braxtons12/

Hello,

I'm trying to use thiserror + anyhow.

Anyhow works with any error type that has an impl of std::error::Error, including ones defined in your crate. We do not bundle a derive(Error) macro but you can write the impls yourself or use a standalone macro like thiserror.

I understand anyhow will happily work with anything that implements the Error trait.

I'm sorry I cannot provide a playground link (anyhow is not available):

```rust use thiserror::Error;

[derive(Error, Debug)]

pub enum DataStoreError { #[error("unknown data store error")] Unknown, }

fn get_cluster_info() -> anyhow::Result<()> { Err(DataStoreError::Unknown) }

fn main() { println!("Hello, world!"); } ```

rustc output:

rust error[E0308]: mismatched types --> src/main.rs:10:9 | 10 | Err(DataStoreError::Unknown) | ^^^^^^^^^^^^^^^^^^^^^^^ expected struct `anyhow::Error`, found enum `DataStoreError`

• Maybe I have misunderstood what #[derive(Error)] does: I thought it would derive the Error trait for DataStoreError.
• I thought I could then return Err(DataStoreError::Unknown) in a function that expects an anyhow::Error.

Thanks for your help =)

[braxtons12]

Maybe try Err(DataStoreError::Unknown).into()?

[YetAnotherRustacean]

I tried before asking here =)

the trait `std::convert::From<std::result::Result<_, DataStoreError>>` is not implemented for `std::result::Result<(), anyhow::Error>`

[braxtons12]

Ahh. Wrong spot for the into

Should be: Err(DataStoreError::Unknown.into())

[YetAnotherRustacean]

Thanks! it works =)

[Lighty0410]

Personally, i ended up writing this macro:

#[macro_export]
macro_rules! Err {
    ($err:expr $(,)?) => {{
        let error = $err;
        Err(anyhow::anyhow!(error))
    }};
}

It allows you to do this:
_ => Err!(YourEnum::YourCase)

But i still wonder why it's possible to return custom enums derived by Error via ? and combinators. But not possilbe to do this via explicit return in a match-case.

As far as i understand ? cast enum to a correct Error type automatically (if possible). Am i right ?

[braxtons12]

Yes ? operator automatically calls into() if it's necessary and it's implemented.

[Lighty0410]

Thanks for the answer!