Blog post: Troubleshooting Rust Language Server


To understand how to troubleshoot the Rust Language Server (RLS), it helps to know what RLS is and how the components interact.

RLS is a Rust implementation of the Language Server Protocol (LSP). LSP is based on the client server architecture, and simplifies the way code editors and IDEs interact with a programming language.

The client in this instance is the official RLS extension. The server is rls which is another Rust command line tool like rustfmt, racer and rustsym. The extension is responsible for setting up and starting rls.

If you want to find out more I recommend checking out @nrc’s introductory blog post: What the RLS can do.

Whilst this post is tailored towards the VS Code extension, where possible I’ll let you know when you can run this against your own setup.

The up to date mantra

  • Always have the latest version of Rust nightly and RLS. rustup update
  • Keep Visual Studio Code and the Rust RLS extension up to date.

Increase logging level

To turn on the fire hose:
RUST_LOG=rls=debug code .

To reduce the noise you can drop down to informational:
RUST_LOG=rls=info code .

This tip assumes you have Visual Studio Code in your path so it can be launched from the command line.

Diagnostics will appear in the Output panel for the Rust Language Server (the drop down to the right of the panel).

This can also be used for other editors, just replace code with your own editor.

Additional configuration settings

In your user or workspace settings (settings.json) add the following configuration parameters:

    // Includes standard error from RLS in the Output panel. This isn't 
    // particular useful if you've already enabled logging at debug level.
    "rust-client.showStdErr": true,
    // This will also log everything that appears in the Output panel to a 
    // log file in the root of your workspace.
    "rust-client.logToFile": true,

Both of these will require a restart of the editor. If your LSP client implements the workspace/didChangeConfiguration method, then add the same keys to your client’s configuration file.

Alternatively send the abovementioned method and that example json fragment to RLS.

Where to go if you need more help?

@nrc has just written the definitive guide to debugging and troubleshooting RLS. I would recommend you also visit this article.

As always with any good open source project, help is always available via a new github issue. Alternative if you use irc, you can ask in #rust-dev-tools on the Mozilla network.

This post appears originally on