Default value that panics only if neither arg nor env provided

[SamuelMarks]

Please complete the following tasks

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

Clap Version

4.4.0

Describe your use case

use clap::{Parser, CommandFactory};

use const_format::concatcp;
use clap::Parser;

#[derive(Parser)]
struct Cli {
    #[arg(long, env = "ROOT", default_value_t = String::from(concatcp!(
    (
    (if cfg!(windows) {
    std::env::var("UserProfile")
    } else {
    std::env::var("HOME")
    })
    .unwrap()
    ),
    std::path::MAIN_SEPARATOR_STR, "stuff",
    std::path::MAIN_SEPARATOR_STR, "name")))]
    root: String,
}

Describe the solution you'd like

Given a system without a home directory (e.g., could not be obtained from std::env::home_dir, dirs, directories, or other crate), it should first:

• If arg is provided, use this
• Else if env var is provided, use this
• Finally, panic (if no home dir is available in environment) xor use home dir (if available)

Alternatives, if applicable

No response

Additional Context

No response

[epage]

First, I'm going to reinterpret "panic" as "error" seeing as we'd refer not to panic for user errors.

This runs a bit counter to the meaning of a default value. My first reaction is that this should be handled in the code outside of clap because its so outside of the common case. The second thought I had is you could allow for ~/ in the user input (even though most shells will expand it for you) and a value_parser could then expand it and error if there isn't a home directory. If neither works, I would lean towards #3476. Overall we have other stuff tracking that though, so I'm going to close in favor of one of these existing options.

[SamuelMarks]

Sounds good I'll keep an eye on that issue. In the meantime I've put $HOME as placeholder and later on in my code I'll validate that if the value still startswith $HOME then look for homedir and panic if not found.