value_parser For Option<PathBuf>

[mikeyT42]

I am having a problem running my program with a value parser that I've made. I get no compile time errors with the value parser, but I do get a runtime error.

The value_parser I have defined is to validate the file type of a path argument.

#[derive(Args, Debug)]
struct BuildArgs {
    #[arg(short, long, value_parser = path_is_xp)]
    source: PathBuf,
    #[arg(short, long, value_parser = optional_path_is_xslt)]
    target: Option<PathBuf>,
    #[arg(short, long)]
    verbose: Option<bool>,
}
// path_is_xp definition here... omitted for brevity.
fn optional_path_is_xslt(s: &str) -> Result<Option<PathBuf>, String> {
    let p: PathBuf = PathBuf::from_str(s)
        .map_err(|_| format!("{s} is not a path."))?;
    let extension = match p.extension() {
        Some(e) => e,
        None => return Err(format!("'{s}' has no extension."))
    };
    if extension == "xsl" || extension == "xslt" {
        Ok(Some(p))
    } else {
        Err(format!("'{s}' is not an XSLT file."))
    }
}

My runtime error is this:
thread 'main' panicked at src/main.rs:26:13: Mismatch between definition and access of 'target'. Could not downcast to std::path::PathBuf, need to downcast to core::option::Option<std::path::PathBuf>

I can't figure out why. I am returning an Option<PathBuf> from the value parser.

What am I doing wrong here?

[mikeyT42]

Well, I resolved this. I simply opted to remove the Option<PathBuf> from the return result and made it Result<PathBuf, String> and it runs perfectly fine. The value is still an option inside the BuildArgs struct.

[epage]

clap uses the Option for whether the argument is present or not and then wants a value parser for the inner type. There are ways to have a value parser return an Option if that ever becomes of interest.