Dynamic completion for powershell repeats last word when completing next argument

[jakobhellermann]

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• [x] I have searched the discussions
• [x] I have searched the open and rejected issues

Rust Version

rustc 1.88.0-nightly (27d6200a7 2025-05-06)

Clap Version

4.5.38

Minimal reproducible code

use clap::{CommandFactory as _, Parser};

#[derive(Parser, Debug)]
#[command(version, name = "demo")]
struct Args {}

fn main() {
    clap_complete::env::CompleteEnv::with_factory(Args::command).complete();
}

Steps to reproduce the bug with the above code

cargo install --path . --example demo
Invoke-Expression (& { (env COMPLETE=powershell C:/Users/user/.cargo/bin/demo | Out-String) })

demo --help <TAB>

Actual Behaviour

demo --help --help

Expected Behaviour

demo --help --

Additional Context

This is because the powershell EnvCompleter determines the argument index based on the parsed args vec, which does not contain an empty element at the end: https://github.com/clap-rs/clap/blob/e2188d9af318f3287c1c5a52cba6b9dfebe7bb75/clap_complete/src/env/shells.rs#L322

Register-ArgumentCompleter comes with $cursorPosition, but that gives you the actual column index in the command, which you can't trivially turn into an argument index.

Possible Solutions

1. in the powershell glue code, select the correct word index based on $commandAst.ToString() and $cursorPosition and pass that via _CLAP_COMPLETE_INDEX
2. pass the raw $commandAst.ToString() and $cursorPosition to the binary and determine the argument index there
3. If we're at the end of the text, pass an extra empty argument to the binary: & {completer} -- $commandAst ''. That way it computes the argument index correctly.

Or maybe something I've missed.

Debug Output

No response