Function emit 

pub fn emit<'files, F: Files<'files> + ?Sized, W: GeneralWrite + ?Sized>(
    writer: &mut W,
    config: &Config,
    files: &'files F,
    diagnostic: &Diagnostic<F::FileId>,
) -> Result<(), Error>

👎Deprecated since 0.13.0: Use emit_to_write_style instead or depending on the writer use emit_to_io_write or emit_to_string

Emit a diagnostic using the given writer, context, config, and files.

The return value covers all error cases. These error case can arise if:

• a file was removed from the file database.
• a file was changed so that it is too small to have an index
• IO fails