async-rust-lsp

A standalone LSP server that provides real-time diagnostics for async Rust antipatterns — focusing on patterns that clippy and rust-analyzer miss.

Problem

Existing Rust tooling has a blind spot for tokio-specific async antipatterns:

clippy only checks std::sync::Mutex across .await, not tokio::sync::Mutex
rust-analyzer has no plugin system for custom diagnostics
Runtime tools (tokio-console) only help during execution, not during editing

This LSP fills the gap with real-time editor feedback for async lock patterns.

Current rules

`async-rust/mutex-across-await`

Warns when tokio::sync::Mutex or RwLock guards are held across .await points — a pattern that can deadlock under tokio's cooperative scheduling.

// BAD — guard lives across the await
let guard = mutex.lock().await;
do_something(&guard);
some_future.await; // WARNING: deadlock risk

// OK — guard scoped before the await
let value = {
    let guard = mutex.lock().await;
    guard.clone()
};
some_future.await; // fine

The rule tracks guard liveness through:

drop(guard) calls (including inside conditional branches)
let shadowing that kills the guard binding
Block scoping (guard dropped at end of block)

Installation

cargo install --git https://github.com/rgbkrk/async-rust-lsp

Or build from source:

git clone https://github.com/rgbkrk/async-rust-lsp
cd async-rust-lsp
cargo build --release
# binary at ./target/release/async-rust-lsp

Editor setup

Neovim (nvim-lspconfig)

local lspconfig = require('lspconfig')
local configs = require('lspconfig.configs')

if not configs.async_rust_lsp then
  configs.async_rust_lsp = {
    default_config = {
      cmd = { 'async-rust-lsp' },
      filetypes = { 'rust' },
      root_dir = lspconfig.util.root_pattern('Cargo.toml'),
    },
  }
end

lspconfig.async_rust_lsp.setup {}

VS Code

Use a generic LSP client extension and add to .vscode/settings.json:

{
  "lsp-client.servers": [
    {
      "name": "async-rust-lsp",
      "command": "async-rust-lsp",
      "filetypes": ["rust"]
    }
  ]
}

Zed

Add to ~/.config/zed/settings.json:

{
  "lsp": {
    "async-rust-lsp": {
      "binary": {
        "path": "async-rust-lsp"
      }
    }
  }
}

Claude Code

claude lsp add --name async-rust-lsp --command async-rust-lsp

Diagnostics appear automatically in Claude's context when editing .rs files.

Logging

The server logs to $TMPDIR/async-rust-lsp.log (never stdout/stderr, which are reserved for LSP stdio protocol).

tail -f "$TMPDIR/async-rust-lsp.log"
RUST_LOG=debug async-rust-lsp  # verbose logging

Architecture

┌─────────────┐    LSP/stdio    ┌──────────────────┐
│   Editor    │ <─────────────> │  async-rust-lsp  │
│ (or Claude) │                 │                  │
└─────────────┘                 │  tree-sitter-rust│
                                │  + custom rules  │
                                └──────────────────┘

Built with:

tower-lsp — async LSP framework
tree-sitter-rust — incremental Rust parser

Origin

Born from a real deadlock in the nteract desktop daemon. See nteract/desktop#1614. The gap in static analysis for tokio async patterns is well-known but no one has filled it with an LSP yet.

License

MIT

async-rust-lsp 0.2.2