ICE: "mut #0 has parameters, but no args were provided in instantiate" panic on completion popup with Arc<Mutex<T>>

rust-analyzer version
rust-analyzer 1.97.1 (8bab26f 2026-07-14)

rustc version
rustc 1.97.1 (8bab26f4f 2026-07-14)

editor or extension
Neovim

code snippet to reproduce:

use std::{
    sync::{Arc, Mutex},
    thread::{self, JoinHandle},
    time::Duration,
};

fn main() {
    let counter = Arc::new(Mutex::new(0));
    let mut handles: Vec<JoinHandle<()>> = vec![];

    for _ in 0..=10 {
        let counter = Arc::clone(&counter);
        let handle = thread::spawn(move || {
            let mut num = counter.lock().unwrap();
            *num += 1;
        });
        handles.push(handle);
    }

    for handle in handles {
        handle.join().unwrap();
        // println!("handle lengt: {}", handles.len());
        thread::sleep(Duration::from_millis(500));
    }

    println!("Result: {}", *counter.lock().unwrap());
}

summary
rust-analyzer panics with the error whenever the completion/signature popup
appears while typing in a function that uses Arc<Mutex<T>> across threads:

https://github.com/user-attachments/assets/47cf0a39-77dc-48fd-88c3-e3ce2e42077a

The report names the completion/signature popup as the failing entry point and provides a Rust reproducer for Neovim. First reproduce the panic with the supplied Arc<Mutex> snippet, then trace completion handling around the reported instantiate error. Done means the popup no longer panics for this code and the regression is covered by an appropriate test.