You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: rpn_function.rs
    path of file: ./repos/tikv/components/tidb_query_codegen/src
    
    the code of the file until where you have to start completion: // Copyright 2019 TiKV Project Authors. Licensed under Apache-2.0.

//! Implementation of the `rpn_fn` attribute macro.
//!
//! The macro should be applied to a function:
//!
//! ```ignore
//! #[rpn_fn]
//! fn foo(x: Option<&u32>) -> Result<Option<u8>> {
//!     Ok(None)
//! }
//! ```
//!
//! The expanded function implements an operation in the coprocessor.
//!
//! ## Arguments to macro
//!
//! If neither `varg` or `raw_varg` are supplied, then the generated arguments
//! follow from the supplied function's arguments. Each argument must have a
//! type `Option<&T>` for some `T`.
//!
    
    fn test_generic_generate_real_fn_trait_impl() {
        let gen = generic_fn();
        let expected: TokenStream = quote! {
            impl<'arg_, A: M, B, Arg0_: crate::function::RpnFnArg<Type = Option<&'arg_ A::X> > > Foo_Fn<A, B>
                for crate::function::Arg<Arg0_, crate::function::Null>
            where
                B: N<A>
            {
                default fn eval(
                    self,
                    ctx: &mut tidb_query_datatype::expr::EvalContext,
                    output_rows: usize,
                    args: &[crate::RpnStackNode<'_>],
                    extra: &mut crate::RpnFnCallExtra<'_>,
                    metadata: &(dyn std::any::Any + Send),
                ) -> tidb_query_common::Result<tidb_query_datatype::codec::data_type::VectorValue> {
                    let arg = &self;
                    let mut result = <B as EvaluableRet>::ChunkedType::with_capacity(output_rows);
                    for row_index in 0..output_rows {
                        let (arg0, arg) = arg.extract(row_index);
                        result.push(foo::<A, B>(arg0)?);
                    }
                    Ok(B::cast_chunk_into_vector_value(result))
                }
            }
        };
        assert_eq!(
            expected.to_string(),
            gen.generate_real_fn_trait_impl().to_string()
        );
    }