Module snafu::guide::troubleshooting::missing_field_source

Missing field source / IntoError is not implemented

This error is encountered in multi-module / multi-file projects when the error type is defined in one module and constructed in another.

Failing Example

project_error.rs

use snafu::prelude::*;
use std::io;

#[derive(Debug, Snafu)]
#[snafu(visibility(pub(crate)))]
pub enum ProjectError {
    #[snafu(display("Unable to read configuration from {path}"))]
    IOConfigError {
        path: &'static str,
        source: io::Error,
    },
}

main.rs

mod project_error;

use project_error::ProjectError;
use snafu::prelude::*;
use std::fs;

const CONFIG_PATH: &str = "/etc/example/conf.conf";

pub fn read_config() -> Result<String, ProjectError> {
    fs::read_to_string(CONFIG_PATH).context(ProjectError::IOConfigError { path: CONFIG_PATH })
}

pub fn main() {
    println!("{}", read_config().unwrap());
}

Errors

error[E0063]: missing field `source` in initializer of `ProjectError`
  --> examples/scratch.rs:23:45
   |
23 |     fs::read_to_string(CONFIG_PATH).context(ProjectError::IOConfigError { path: CONFIG_PATH })
   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `source`

and

error[E0277]: the trait bound `ProjectError: IntoError<_>` is not satisfied
  --> examples/scratch.rs:23:45
   |
23 |     fs::read_to_string(CONFIG_PATH).context(ProjectError::IOConfigError { path: CONFIG_PATH })
   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `IntoError<_>` is not implemented for `ProjectError`

Solution

Replace the ProjectError::IOConfigError in the read_config() function with project_error::IOConfigSnafu.

Explanation

This works because the #[derive(Snafu)] macro creates the context selector type IoConfigSnafu:

#[derive(Debug, Snafu)]
pub enum ProjectError {
    IOConfigError {
        source: io::Error,
        path: &'static str,
    },
}

// some details removed
struct IOConfigSnafu<P> {
    path: P,
}

// Some impls for the IOConfigError struct

See what the Snafu macro generates section of the guide for more details.

When you use ProjectError::IOConfigError, you’re referencing the enum variant, not the struct that you need. Replacing ProjectError::IOConfigError with project_error::IOConfigSnafu fixes this problem.