Suggestion: Better support for collecting all trailing arguments

[sffc]

Related: #5305, probably others.

I'm working on a tool cg2exec which takes one positional argument and then forwards all the rest to a subcommand. In the future, it might take flags, but for now it is just the one positional argument. I haven't found a stock Clap setup that has the behavior I'm seeking.

I want the following calls to succeed:

cg2exec groupname pwd
cg2exec groupname echo hello
cg2exec groupname ls -l
cg2exec groupname ls --human-readable
cg2exec groupname rm -- -myfile.txt
cg2exec -- -groupname echo hello

I want the following to fail:

# For forwards compatibility with future flag:
cg2exec --flag groupname echo hello

# Because this is garbage input:
cg2exec groupname --flag echo hello
cg2exec groupname -- echo hello

I'm not aware of a Clap configuration with this behavior.

I've tried:

#[derive(Parser, Debug)]
#[command(version, about = "Runs a program with a specific control group")]
struct Cli {
	/// Name of the control group.
	#[arg()]
	cgroup: String,

	/// The subcommand to run.
	#[arg()]
	cmd: OsString,

	/// Arguments to the subcommand.
	#[arg(allow_hyphen_values(true))]
	args: Vec<OsString>,
}

This works for every case except cg2exec groupname rm -- -myfile.txt, because it seems clap decided to parse -- using its own meaning rather than dumping it into the args vector. It works when the -- is the second argument in args, but not the first.

I've also tried

#[derive(Parser, Debug)]
#[command(version, about = "Runs a program with a specific control group")]
struct Cli {
	/// Name of the control group.
	#[arg()]
	cgroup: String,

	/// The subcommand to run with its arguments.
	#[arg(allow_hyphen_values(true), required = true)]
	cmd_args: Vec<OsString>,
}

This one allows cg2exec groupname --flag echo hello, which I had wanted to reject. I guess I could add a custom validator to reject the vector if the first argument starts with a hyphen.

For now I'm thinking of using clap_lex manually (octave-online/cg2tools#3), as suggested in #5305 (reply in thread), since this is not a very complicated CLI, but it would be nice to be able to reduce lines of code and have everything "just work" by using stock clap.

[epage]

To me, the right solution is your cmd_args one and then validate the first value.

#[derive(Parser, Debug)]
#[command(version, about = "Runs a program with a specific control group")]
struct Cli {
	/// Name of the control group.
	#[arg()]
	cgroup: String,

	/// The subcommand to run with its arguments.
	#[arg(allow_hyphen_values(true), required = true)]
	cmd_args: Vec<OsString>,
}