Slow MBE expansion with `async-std`

use async_std::io;
use async_std::net::{TcpListener, TcpStream};
use async_std::prelude::*;
use async_std::task;

async fn process(stream: TcpStream) -> io::Result<()> {
    println!("Accepted from: {}", stream.peer_addr()?);

    let mut reader = stream.clone();
    let mut writer = stream;
    io::copy(&mut reader, &mut writer).await?;

    Ok(())
}

fn main() -> io::Result<()> {
    task::block_on(async {
        let listener = TcpListener::bind("127.0.0.1:8080").await?;
        println!("Listening on {}", listener.local_addr()?);

        let mut incoming = listener.incoming();

        while let Some(stream) = incoming.next().await {
            let stream = stream?;
            task::spawn(async {
                process(stream).await.unwrap();
            });
        }
        Ok(())
    })
}

(cargo add async-std)

17606ms - import_map_query
    17605ms - collect_import_map
        16270ms - collect_trait_assoc_items
                1ms - crate_def_map:wait (3080 calls)
              622ms - file_item_tree_query (1539 calls)
             5273ms - macro_expand (1539 calls)
             7458ms - parse_macro_expansion (2987 calls)
            0   - PerNs::filter_visibility (244 calls)
            0   - PerNs::iter_items (200 calls)
         1060ms - collect_trait_assoc_items (27 calls)
          275ms - crate_def_map:wait (1 calls)
        0   - fst_path (292 calls)

38442ms - handle_hover
    11300ms - classify_name_ref @ next
        11300ms - Semantics::analyze_impl
            11300ms - infer:wait @ main
                0   - SourceBinder::to_module_def (1 calls)
                0   - crate_def_map:wait (1 calls)
        0   - crate_def_map:wait (21 calls)
        0   - descend_into_macros (1 calls)
     5908ms - parse_macro_expansion (1532 calls)
    21234ms - ???

28820ms - handle_hover
        0   - classify_name_ref (1 calls)
        0   - crate_def_map:wait (24 calls)
        0   - descend_into_macros (1 calls)
    17672ms - parse_macro_expansion (4016 calls)
    11148ms - ???

27826ms - handle_hover
        0   - classify_name_ref (1 calls)
        0   - crate_def_map:wait (21 calls)
        0   - descend_into_macros (1 calls)
    17895ms - parse_macro_expansion (4337 calls)
     9930ms - ???

Reproduce the slowdown with the async-std example and profiling data in the issue. Start by investigating import_map_query, collect_trait_assoc_items, macro_expand, parse_macro_expansion, and handle_hover. Done means the reported expansion and hover latency is reduced and the relevant profiling measurements improve.