Formal parameter attributes won't be passed into proc-macro

[Hexilee]

As this example repo

This is the example proc-macro:

extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn rename_params(_args: TokenStream, input: TokenStream) -> TokenStream {
    println!("{}", &input);
    input
}

This is the test:

#![feature(param_attrs)]
use params_attribute_example::rename_params;

#[rename_params]
fn hello(a: i32, #[val] b: i32) {}

Run cargo expand --test rename-params，both of println result and expand result are:

fn hello(a: i32, b: i32) {}

Formal parameter attributes disappear in proc-macro.

Toolchain: nightly-x86_64-apple-darwin(8a58268b5 2019-07-31)

[Centril]

cc @c410-f3r @petrochenkov #60406

[Centril]

I have a fix in #63212.