Auto-context for simple pass-through errors (like thiserror)

[hgomersall]

I've found that thiserror and snafu fill two distinct roles for me. In the case when I have complex errors that take extra arguments, SNAFU is fantastic. However, in the case when module errors are simple wrappers around library errors, thiserror is simpler to use and syntactically tighter. It's also the case that one often gets name collisions with the intermediate structure and the library error, so this doesn't work (which is often frustrating):

use a_lib::{library_function, LibraryError};

#[derive(Debug, Snafu)]
pub enum MyError {
    #[snafu(display("Error : {}", source))]
    LibraryError { source: LibraryError },
}
fn do_something -> Result<(), MyError>
{
    library_function.context(LibraryError)?;
    Ok(())
}

It occurred to me that the SNAFU could fill both roles if in the simple case, the context could be inferred, much like with thiserror. Obviously, this depends on being able to disambiguate it, but that would still include many situations.

The solution might even be to include thiserror like syntax for pass-through errors:

use a_lib::{library_function, LibraryError};
use std::io::{self, Write};

#[derive(Debug, Snafu)]
pub enum MyError {
    #[snafu(display("Error : {}", source))]
    LibraryError { #[from] LibraryError },
    #[snafu(display("Error number: {}, source: {}", number, source))]
    AMoreComplicatedError { source: io::Error, number: usize },
}
fn do_something() -> Result<(), MyError>
{
    library_function?;
    io::stdout().write_all(b"hello world").context( AMoreComplicatedError { number: 10 })?;
    Ok(())
}

[hgomersall]

There's likely a few errors in that code, but I think the point is apparent.

[shepmaster]

You can do that in SNAFU today:

#[derive(Debug, Snafu)]
pub enum MyError {
    #[snafu(display("Error : {}", source))]
    #[snafu(context(false))] // <--- Here
    LibraryError { source: LibraryError },

    #[snafu(display("Error number: {}, source: {}", number, source))]
    AMoreComplicatedError { source: io::Error, number: usize },
}

#199 tracks making this work as LibraryError(LibraryError)

[hgomersall]

My bad. So LibraryError(LibraryError) is currently not supported?

[hgomersall]

Also, does it still create the hidden context structs?

Thanks!

[shepmaster]

So LibraryError(LibraryError) is currently not supported?

That is correct.

does it still create the hidden context structs?

It does not.

[hgomersall]

Great, thank you!