Warn on unknown settings when the first positional is an argument

[cole-h]

Motivation

We should warn when unknown settings are passed on the command line.

Context

While debugging an unrelated issue, I noticed that one of the flags I was passing wasn't valid for that version, but Nix never warned me of this fact. When I tested a similar invocation with the legacy interface, I was immediately warned about this unknown setting.

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[fricklerhandwerk]

Does this also affect nix-build and all other stable commands?

[cole-h]

OK, I updated the tests to test both clis and ensure that only one line of "unknown setting" is output.

Here's what I found while investigating this:

• The warning on the old cli comes from the call to initialFlagsProcessed
• Because the new cli uses subcommands (i.e. nix eval), eval is treated as an arg, which hits this branch before seeing any other options on the command line:

  nix/src/libutil/args.cc

  Lines 339 to 342 in bbe780b

  	if (!argsSeen) {
  	argsSeen = true;
  	initialFlagsProcessed();
  	}

• If we unconditionally call initialFlagsProcessed after all the flags and args have been processed, the issue goes away and we are always (at least in every scenario I tested) warned about unknown args/flags.

---

You can actually reproduce this "silence" on the old cli with this:

$ nix-instantiate '{}' --max-jobs 0 --option foobar baz --expr

'{}' is an "arg", and the very first thing, so even though there's an invalid option specified later (foobar), this function doesn't care anymore because after parsing '{}', it has argsSeen == true, and will never attempt to call initialFlagsProcessed again.

[cole-h]

Before:

$ nix-instantiate --option foobar baz --expr '{}'
warning: unknown setting 'foobar'
$ nix-instantiate '{}' --option foobar baz --expr
$ nix eval --expr '{}' --option foobar baz
{ }

After:

$ nix-instantiate --option foobar baz --expr '{}'
warning: unknown setting 'foobar'
$ nix-instantiate '{}' --option foobar baz --expr
warning: unknown setting 'foobar'
$ nix eval --expr '{}' --option foobar baz
warning: unknown setting 'foobar'
{ }