Explicitly requests the "-Z unstable-options" flag if someone attempts to use
a cargo option gated by it. This enhances discoverability, particularly in the
instance where the user is on the nightly compiler but isn't using the flag.
let opt = $matches.opt_present($option_name);
if !$allow_unstable && opt {
return Err(format!(
- "The \"{}\" flag is only accepted on the nightly compiler",
+ "The \"{}\" flag is only accepted on the nightly compiler with -Z unstable-options",
$option_name
));
}