Allow rust-analyzer to run on single file for completion

Create a rust-project.json in your directory

{
    "sysroot_src": "path/to/the/library",
    "crates": [
        {
            "root_module": "main.rs",
            "edition": "2021",
            "deps": []
        }
    ]
}

Here, i manually defined rustup home. By default it's ~/.rustup

export RUSTUP_HOME="${XDG_DATA_HOME:-$HOME/.local/share}/rustup"

Define crates deps inside a single file.

//! ```cargo
//! [dependencies]
//! clap = { version = "4.2", features = ["derive"] }
//! ```

extern crate clap;

use clap::Parser;

#[derive(Parser, Debug)]
#[clap(version)]
struct Args {
    #[arg(short, long, default_value = "PwnWriter")]
    pub name: String,
}

fn main() {
    let args = Args::parse();
    println!("{}", args.name);
}