Come up with regression test for #66286

[estebank]

Follow up to #66286 and #66390, as per #66390 (comment).

Minimize a test that exercises the same codepath as

extern crate wasm_bindgen;
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub extern fn third(a: Vec(u32)) -> u32 {
    return a[3]
}

[hellow554]

As already stated in #66286 (comment) the wasm_bindgen proc macro can be reduced to:

extern crate proc_macro;

use proc_macro::TokenStream;
use quote::ToTokens;

#[proc_macro_attribute]
pub fn wasm_bindgen(attr: TokenStream, input: TokenStream) -> TokenStream {
    let mut tokens = proc_macro2::TokenStream::new();
    let item = syn::parse::<syn::Item>(input).expect("parse");

    if let syn::Item::Fn(f) = item {
        f.to_tokens(&mut tokens);
    }

    tokens.into()
}

[camelid]

How do you write a UI test that depends on 3rd-party libraries?

[estebank]

@camelid https://github.com/rust-lang/rust/blob/5c6f7b32aab683276d4feb1810244ccf72bda736/src/test/ui/proc_macro.rs is an example of a test that relies on a proc_macro, and we can use @hellow554's snippet for the necessary proc_macro being tested.