I do not expect to accept this PR in the current form:

1. I do not think that every error type constructed by SNAFU should have this functionality (increasing the amount of code generated).
2. Putting sources in Display, even through the alternate formatter, counteracts the guidance to not do so.

Instead, I could instead see this being an alternate to Report for the cases where you need a single-line error. For example:

tracing::warn!("{}", ReportOneline::new(err));

Implementation-wise, this should use CleanedErrorText (when the right feature(s) are enabled) to avoid repeating the error text from errors that already don't follow the guidelines.

but without needing to take ownership of the error (which Report requireds)

It does not:

use snafu::prelude::*;

#[derive(Debug, Snafu)]
struct ExampleError;

fn main() {
    let e = ExampleError;
    let r = snafu::Report::from_error(&e);
    println!("{r}");
}

I'd expect a hypothetical ReportOneline to behave similarly.