__arg variables from async fn desugaring are accessible to the user

[nikomatsakis]

The fixes for #54716 and #60236 introduced __arg variables that, presently, are accessible to the user (playground):

#![feature(async_await)]

async fn foo(_: usize) {
    println!("{}", __arg0);
}

fn main() { }

This code compiles, but it should error.

[davidtwco]

A complete test case:

// aux-build:arc_wake.rs
// edition:2018

#![allow(unused_variables)]
#![feature(async_await, await_macro)]

extern crate arc_wake;

use arc_wake::ArcWake;
use std::future::Future;
use std::sync::Arc;
use std::task::Context;

struct EmptyWaker;

impl ArcWake for EmptyWaker {
    fn wake(self: Arc<Self>) {}
}

async fn foobar_async(x: u32, (a, _, _c): (u32, u32, u32), _: u32, _y: u32) {
    assert_eq!(__arg1, (1, 2, 3)); // this doesn't compile error, but should
    assert_eq!(__arg2, 4);
}

fn main() {
    let empty = Arc::new(EmptyWaker);
    let waker = ArcWake::into_waker(empty);
    let mut cx = Context::from_waker(&waker);

    let mut fut = Box::pin(foobar_async(0, (1, 2, 3), 4, 0));
    let _ = fut.as_mut().poll(&mut cx);
}