Support using error message from source

[MOZGIII]

I have a very simple enum that just wraps all other error types:

#[derive(Debug)]
pub enum SomeParseFromEnvError {
    InvalidLogFormat(ParseFromEnvError<InvalidLogFormat>),
    InvalidBool(ParseFromEnvError<ParseBoolError>),
    InvalidInfallible(ParseFromEnvError<Infallible>),
}

The Display implementation I have doesn't do anything fancy - it just relies the fmt directly to the wrapped value:

impl std::fmt::Display for SomeParseFromEnvError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            SomeParseFromEnvError::InvalidLogFormat(err) => err.fmt(f),
            SomeParseFromEnvError::InvalidBool(err) => err.fmt(f),
            SomeParseFromEnvError::InvalidInfallible(err) => err.fmt(f),
        }
    }
}

I need a simple way to derive the same behavior.
Annotation every field with #[error("{0})] is an option that I currently have to use, but it doesn't expand to the same code I write manually.
Also, #[error("{0})] above every field doesn't look good 😄

[dtolnay]

I would accept a PR to treat:

#[derive(Error, Debug)]
#[error(...)]
pub enum SomeParseFromEnvError {
    InvalidLogFormat(ParseFromEnvError<InvalidLogFormat>),
    InvalidBool(ParseFromEnvError<ParseBoolError>),
    InvalidInfallible(ParseFromEnvError<Infallible>),
}

as equivalent to:

#[derive(Error, Debug)]
pub enum SomeParseFromEnvError {
    #[error(...)]
    InvalidLogFormat(ParseFromEnvError<InvalidLogFormat>),
    #[error(...)]
    InvalidBool(ParseFromEnvError<ParseBoolError>),
    #[error(...)]
    InvalidInfallible(ParseFromEnvError<Infallible>),
}

Would that work for your use case? You would put #[error("{0}")] once at the top of the enum.

it doesn't expand to the same code I write manually.

Thanks, I filed #13 to track this separately.

[MOZGIII]

I guess #[error("{0}")] is the problem for me here, how about we introduce #[error_proxy_to_source] or sth?

Having to write it for every line may bothers me less than the this thing: ("{0}")

[dtolnay]

You can alternatively write #[error("{}", .0)] if that's less bothersome.

I think I wouldn't consider this annoying enough to introduce a new #[error_proxy_to_source] attribute that does the same thing.

[dtolnay]

Closing in favor of #18.