rust-lang/rust-analyzer · error

failed to spawn thread

Error message

failed to spawn thread

What it means

An `expect` on `stdx::thread::Builder::spawn` in `FlycheckHandle::spawn`: the dedicated flycheck worker thread fails to launch. `std::thread::spawn` essentially only fails on resource exhaustion (thread/process limits, memory), so this indicates the process hit an OS limit while starting the check runner; the input at fault is the OS environment, not flycheck configuration.

Source

Thrown at crates/rust-analyzer/src/flycheck.rs:249

    ) -> FlycheckHandle {
        let actor = FlycheckActor::new(
            id,
            generation.load(Ordering::Relaxed),
            sender,
            config,
            config_json,
            sysroot_root,
            workspace_root,
            manifest_path,
            ws_target_dir,
            ws_build_dir,
            toolchain_version,
        );
        let (sender, receiver) = unbounded::<StateChange>();
        let thread =
            stdx::thread::Builder::new(stdx::thread::ThreadIntent::Worker, format!("Flycheck{id}"))
                .spawn(move || actor.run(receiver))
                .expect("failed to spawn thread");
        FlycheckHandle { id, generation, sender, _thread: thread }
    }

    /// Schedule a re-start of the cargo check worker to do a workspace wide check.
    pub(crate) fn restart_workspace(&self, saved_file: Option<AbsPathBuf>) {
        let generation = self.generation.fetch_add(1, Ordering::Relaxed) + 1;
        self.sender
            .send(StateChange::Restart {
                generation,
                scope: FlycheckScope::Workspace,
                saved_file,
                target: None,
            })
            .unwrap();
    }

    /// Schedule a re-start of the cargo check worker to do a package wide check.
    pub(crate) fn restart_for_package(

View on GitHub (pinned to e8f7e90aa3)

Solutions

1. Check `ulimit -u` / cgroup pids.max and raise the thread or process limit for the rust-analyzer process
2. Reduce concurrent flycheck workspaces so fewer worker threads are needed
3. Free memory or reduce memory pressure; thread creation fails under exhaustion
4. Convert the `expect` into a recoverable error so a failed spawn disables flycheck instead of crashing the server

When it happens

Trigger: Thrown at crates/rust-analyzer/src/flycheck.rs:249 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of rust-lang/rust-analyzer@e8f7e90aa3 (2026-09-03). Data as JSON: /api/errors/3741c9265e50f932. Report an issue: GitHub.