Argument completion in bash doesn't work for aliases with arguments

[DemvSteinbrink]

Description

For aliases, the bash completion is broken. It only completes one element and it's always completed to the first argument in the alias.

Steps to Reproduce the Problem

1. Create an alias gp = ["git", "push"]
2. source <(COMPLETE=bash jj)
3. Type jj gp and then hit <Tab>

Expected Behavior

Completion behaves the same as if I had typed jj git push, i.e. it would add -- to my prompt.

Actual Behavior

The completion adds push to my prompt. It even replaces partial input with push, i.e. jj gp --he + <Tab> becomes jj gp push.

Specifications

• Platform: Linux Debian Sid
• Version: jj 0.25.0-041c4fecb77434dd6720e7d7f1ce48d9575ac5f7

[yuja]

Perhaps, it is the problem of _CLAP_COMPLETE_INDEX. Alias completion works only on fish right now.

jj/cli/src/cli_util.rs

Lines 3390 to 3402 in 646bdab

	let arg_index: Option<usize> = env::var("_CLAP_COMPLETE_INDEX")
	.ok()
	.and_then(|s| s.parse().ok());
	let resolved_aliases = if let Some(index) = arg_index {
	// As of clap_complete 4.5.38, zsh completion script doesn't pad an
	// empty arg at the complete position. If the args doesn't include a
	// command name, the default command would be expanded at that
	// position. Therefore, no other command names would be suggested.
	// TODO: Maybe we should instead expand args[..index] + [""], adjust
	// the index accordingly, strip the last "", and append remainder?
	let pad_len = usize::saturating_sub(index + 1, orig_args.len());
	let padded_args = orig_args.chain(iter::repeat(OsString::new()).take(pad_len));
	expand_args(ui, app, padded_args, config)?