Implement Command::raw_arg on Windows

[m-haug]

Is your feature request related to a problem? Please describe.
In watchexec/watchexec#613, it was discovered that CMD does not handle quoted arguments properly. This is known behaviour and led to the inclusion of a CommandExt::raw_arg method in std.

Describe the solution you'd like
For parity with std, it would be nice to implement Command::raw_arg on Windows.

Describe alternatives you've considered
As a workaround, users can create a StdCommand and convert this to tokio's Command using the provided From implementation. However, this means two additional imports (StdCommand and CommandExt), and might potentially confuse users. It's also not as nice to use since method chaining is no longer possible.

Additional context
I've included three example programs to demonstrate the issue when using tokio, a solution using only std, and the workaround described above.

Problematic tokio code:

use std::process::Output;

use tokio::process::Command;

#[tokio::main]
async fn main() {
    let output = Command::new("cmd.exe")
        .arg("/C")
        .arg(r#""C:\Users\Markus Haug\scoop\apps\tectonic\current\tectonic.exe" -X build --open"#)
        .output()
        .await;

    if let Ok(Output { stderr, .. }) = output {
        let stderr = String::from_utf8(stderr).unwrap();
        eprintln!("{stderr}")
    }
}

Pure std solution:

use std::os::windows::process::CommandExt;
use std::process::Command;
use std::process::Output;

#[tokio::main]
async fn main() {
    let output = Command::new("cmd.exe")
        .arg("/C")
        .raw_arg(
            r#""C:\Users\Markus Haug\scoop\apps\tectonic\current\tectonic.exe" -X build --open"#,
        )
        .output();

    if let Ok(Output { stderr, .. }) = output {
        let stderr = String::from_utf8(stderr).unwrap();
        eprintln!("{stderr}")
    }
}

Workaround using conversion from StdCommand:

use std::os::windows::process::CommandExt;
use std::process::Command as StdCommand;
use std::process::Output;

use tokio::process::Command as TokioCommand;

#[tokio::main]
async fn main() {
    let mut command = StdCommand::new("cmd.exe");
    command.arg("/C").raw_arg(
        r#""C:\Users\Markus Haug\scoop\apps\tectonic\current\tectonic.exe" -X build --open"#,
    );

    let output = TokioCommand::from(command).output().await;

    if let Ok(Output { stderr, .. }) = output {
        let stderr = String::from_utf8(stderr).unwrap();
        eprintln!("{stderr}")
    }
}

[joriskleiber]

I already implemented that (#5704) and it’s an unstable feature since tokio v1.29.0. But I don’t know why it doesn’t show up in the docs.

[m-haug]

Apologies, I did only search for raw_arg in issues, not PRs. Good to see that this is already implemented.

I think it's not listed on docs.rs because it's #[cfg(windows)] and, therefore, not built when generating documentation. Comparing the cfg_windows macro, it seems to include explicit provisions to include the item in documentation builds.

So, now it's just a matter of waiting for tokio to increase its MSRV to 1.62.

[Darksonn]

Closing as completed by #5704. I opened #5862 for the docs issue.