generated zsh completion fails when `source <()`'d

[drahnr]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Rust Version

rustc 1.58.1 (db9d1b20b 2022-01-20)

Clap Version

3.1.2

Minimal reproducible code

use clap::{AppSettings, Arg, Command, PossibleValue, Subcommand, ValueHint};
use clap_complete::{generate, Generator, Shell};
use std::{io, collections::{HashSet, HashMap}};

fn build_cli() -> Command<'static> {
    Command::new(env!("CARGO_PKG_NAME"))
        .subcommand_required(true)
        .subcommand(
            Command::new("foo")
            .arg(
                Arg::new("foo")
                    .long("foo")
                    .help("foos for real")
                    .possible_value(PossibleValue::new("7"))
                    .possible_value(PossibleValue::new("seven"))
            ),
        )
        .subcommand(
            Command::new("comp")
            .arg(
                Arg::new("shell")
                    .long("shell")
                    .help("shell to generate for")
                    .possible_values(Shell::possible_values()),
            ),
        )
}

fn print_completions<G: Generator>(gen: G, cmd: &mut Command) {
    generate(gen, cmd, cmd.get_name().to_string(), &mut io::stdout());
}

fn main() {
    let app = build_cli();
    let matches = app.get_matches();

    match matches.subcommand() {
        Some(("comp", args)) => {
            if let Ok(shell) = args.value_of_t::<Shell>("shell") {
                let mut cmd = build_cli();
                eprintln!("Generating completion file for {}...", shell);
                print_completions(dbg!(shell), &mut cmd);
                return;
            }
            eprintln!("nope");
        }
        Some(("foo", _args)) => {
            println!("foo");
            return;
        }
        _ => {}
    }
    build_cli().print_long_help().unwrap();
}

Steps to reproduce the bug with the above code

source <(cargo run -- comp --shell zsh)

Actual Behaviour

errors out and the completion does not work with the binary afterwards

error:

# source <(cargo run -- comp --shell zsh)
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/clap-completion-zoink comp --shell zsh`
Generating completion file for zsh...
[src/main.rs:42] shell = Zsh

_arguments:comparguments:325: can only be called from completion function

Expected Behaviour

should source the completion and from thereon complete the execution

Additional Context

An almost minimal, reproducible example can be checked out at https://github.com/drahnr/clap-completion-zoink

Encountered in the wild on https://github.com/soywod/himalaya and reported as volta-cli/volta#496

Debug Output

No response

[jjmark15]

did you find a way to avoid the issue?

[drahnr]

The generated file is meant to be written to a file, assuming the above project is foo, _foo would be the correct file name which has to reside in $fpath.

There also seems to be an artifact when writing to a file via foo comp --shell zsh > _foo:

<snip>
_cargo-spellcheck "$@"

which doesn't seem to be an ordinary linebreak when openeing an editor 🤷

---

Quick fix:

Delete the comment #compinit foo marker at the top and the last line _foo and it then should load fine, unless you hit #3022