allow aborting if we have unsupported configurations

[clux]

Good evening!
This is a feature request (AFAIKT there's no configuration option or duplicate issue for this).

In many cases where nightly rustfmt is applied by CI in public repositories, users will often run cargo fmt themselves directly or through their editor causing warnings arising from using the stable rustfmt where the nightly is required. E.g.:

$ cargo fmt
Warning: can't set `reorder_impl_items = true`, unstable features are only available in nightly channel.
Warning: can't set `overflow_delimited_expr = true`, unstable features are only available in nightly channel.
Warning: can't set `blank_lines_upper_bound = 2`, unstable features are only available in nightly channel.
Warning: can't set `inline_attribute_width = 80`, unstable features are only available in nightly channel.
Warning: can't set `comment_width = 110`, unstable features are only available in nightly channel.

The problem here is that cargo fmt will succeed regardless (and there is no way to avoid this). Succeeding here is strictly worse than not doing anything because it will reformat the entire file / project with the wrong settings (and it's not always clear to the user that this has happened).

(This is mostly a problem in projects that do not compile with nightly - and lack an override - but uses nightly for convenience tooling like rustfmt).

My thought is that there could maybe be a configuration for abort_on_unrecognised_options causing rustfmt to exit 1 where it normally would emit a warning.

[calebcartwright]

Love this idea, thanks for the suggestion!

[calebcartwright]

cc @ytmimi in case you'd have any interest working on this since it's something a bit different (no worries if not though!)

Main portions in the code to start digging into would be

rustfmt/src/bin/main.rs

Line 296 in f7c4a44

let (config, config_path) = load_config(None, Some(options.clone()))?;

and

rustfmt/src/config/mod.rs

Lines 327 to 350 in f7c4a44

	pub fn load_config<O: CliOptions>(
	file_path: Option<&Path>,
	options: Option<O>,
	) -> Result<(Config, Option<PathBuf>), Error> {
	let over_ride = match options {
	Some(ref opts) => config_path(opts)?,
	None => None,
	};
	let result = if let Some(over_ride) = over_ride {
	Config::from_toml_path(over_ride.as_ref()).map(|p| (p, Some(over_ride.to_owned())))
	} else if let Some(file_path) = file_path {
	Config::from_resolved_toml_path(file_path)
	} else {
	Ok((Config::default(), None))
	};
	result.map(|(mut c, p)| {
	if let Some(options) = options {
	options.apply_to(&mut c);
	}
	(c, p)
	})
	}

Thought some minor tweaks may be needed to track whether the config attempted to use a nightly option outside nightly

[ytmimi]

Thanks for thinking of me!

Also, I appreciate you pointing out a place to start. I'm eager to take a crack at it, and I'll let you know if I run into any problems or have any questions.