feat: expose positional to the public api

[thesayyn]

Currently there is no way to read positional arguments for type that implement the invoke function from the StarlarkValue trait.

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[thesayyn]

Current workaround is to do something like this to get args as a tuple.

        let __args: [_; 1usize] = starlark::__derive_refs::parse_args::parse_signature(
            &starlark::__derive_refs::sig::parameter_spec("get_memory", &[], &[], true, &[], false),
            args,
            eval.heap(),
        )?;
        let positionals: UnpackList<Value<'v>> =
            starlark::__derive_refs::parse_args::check_required("args", __args[0usize])?;

[linabutler]

I had to do a bit of spelunking in starlark-rust and buck2 to find this, but you can use ParametersSpec to extract positionals from an Arguments<'v>.

Here's a snippet from one of my custom types that implements StarlarkValue::invoke():

use starlark::eval::{Arguments, Evaluator, ParametersSpec, ParametersSpecParam};

#[starlark_value(type = FUNCTION_TYPE)]
impl<'v> StarlarkValue<'v> for RequestWithMethod {
    fn invoke(
        &self,
        _this: Value<'v>,
        arguments: &Arguments<'v, '_>,
        eval: &mut Evaluator<'v, '_, '_>,
    ) -> starlark::Result<Value<'v>> {
        let spec = ParametersSpec::<Value<'v>>::new_parts(
            "request", // Function name.
            [
                // Positional-only arguments, equivalent to
                // `#[starlark(require = named)]`.
            ],
            [
                // Positional or named arguments.
                ("url", ParametersSpecParam::Required),
            ],
            false, // Whether to accept `*args`.
            [
                // Named-only arguments, equivalent to annotating a param with
                // `#[starlark(require = named)]` in a `#[starlark_module]`.
                ("headers", ParametersSpecParam::Optional),
                ("body", ParametersSpecParam::Optional),
            ],
            false, // Whether to accept `**kwargs`.
        );

        let (url, headers, body) = spec.parser(arguments, eval, |parser, _eval| {
            // Unpacking a required argument.
            let url = parser.next::<UnpackUrl<'_>>()?;
            // Unpacking optional arguments.
            let headers = parser.next_opt::<UnpackHeaders<'_>>()?.unwrap_or_default();
            let body = parser
                .next_opt::<UnpackBytes<'_>>()?;
            Ok((url, headers, body))
        })?;

        Ok(eval.heap().alloc(Request {
            method: self.method,
            url,
            headers,
            body,
        }))
    }
}

[linabutler]

(It would be lovely to be able to use arguments.positional() directly, though; I was just sharing my workaround because I ran into the same thing you did! 😊)