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rust-lang-nursery / rls


Repository for the Rust Language Server (aka RLS)


Build Status Build status

Rust Language Server (RLS)

This project is in the alpha stage of development. It is likely to be buggy in some situations; proceed with caution.

The RLS provides a server that runs in the background, providing IDEs, editors, and other tools with information about Rust programs. It supports functionality such as 'goto definition', symbol search, reformatting, and code completion, and enables renaming and refactorings.

The RLS gets its source data from the compiler and from Racer. Where possible it uses data from the compiler which is precise and complete. Where its not possible, (for example for code completion and where building is too slow), it uses Racer.

Since the Rust compiler does not yet support end-to-end incremental compilation, we can't offer a perfect experience. However, by optimising our use of the compiler and falling back to Racer, we can offer a pretty good experience for small to medium sized crates. As the RLS and compiler evolve, we'll offer a better experience for larger and larger crates.

The RLS is designed to be frontend-independent. We hope it will be widely adopted by different editors and IDEs. To seed development, we provide a reference implementation of an RLS frontend for Visual Studio Code.



Otherwise the RLS will not work very well.

Step 1: Clone and build the RLS

Since the RLS is closely linked to the compiler and is in active development, you'll need a recent nightly compiler to build it.

git clone
cd rls
cargo build --release

Step 2: Connect the RLS to your compiler

If you're using recent versions of rustup, you will also need to make sure that the compiler's dynamic libraries are available for the RLS to load. You can see where they are using:

rustc --print sysroot

This will show you where the compiler keeps the dynamic libs. In Windows, this will be in the bin directory under this path. On other platforms, it will be in the lib directory.

Next, you'll make the compiler available to the RLS:


On Windows, make sure this path (plus bin) is in your PATH. For example:

set PATH=%PATH%;C:\Users\appveyor\.multirust\toolchains\nightly-i686-pc-windows-gnu\bin


For Mac, you need to set the DYLD_LIBRARY_PATH. For example:

export DYLD_LIBRARY_PATH=/Users/jturner/.rustup/toolchains/nightly-x86_64-apple-darwin/lib


For Linux, this path is called LD_LIBRARY_PATH.

export LD_LIBRARY_PATH=/Users/jturner/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib

Step 3: Set your RLS_ROOT

Next, we'll set the RLS_ROOT environment variable to point to where we built the RLS:

export RLS_ROOT=/Source/rls

Step 4: Download standard library metadata

Finally, we need to get the metadata for the standard library. This lets us get additional docs and types for all of std. The command is currently only supported on the nightly compilers, though we hope to remove this restriction in the future.

rustup component add rust-analysis

If you've never set up Racer before, you may also need follow the Racer configuration steps

Note: in the future, we hope to move these setup steps to rustup, so that you only have one step to add IDE support.


Though the RLS is built to work with many IDEs and editors, we currently use VSCode to test the RLS (you can run the RLS manually with the cargo run command).

To run with VSCode, you'll need a recent VSCode version installed.

Next, you'll need to run the VSCode extension (for this step, you'll need a recent node installed:

git clone
cd rls_vscode
npm install
code .

VSCode will open into the rls_vscode project. From here, click the Debug button on the left-hand side (a bug with a line through it). Next, click the green triangle at the top. This will launch a new instance of VSCode with the rls_vscode plugin enabled. From there, you can open your Rust projects using the RLS.

You'll know it's working when you see this in the status bar at the bottom, with a spinning indicator:

RLS analysis: working /

Once you see:

RLS analysis: done

Then you have the full set of capabilities available to you. You can goto def, find all refs, rename, goto type, etc. Completions are also available using the heuristics that Racer provides. As you type, your code will be checked and error squiggles will be reported when errors occur. You can hover these squiggles to see the text of the error.


The RLS can be configured on a per-project basis by adding a file called rls.toml to the project root (i.e., next to Cargo.toml). Entries in this file will affect how the RLS operates and how it builds your project.

Currently we accept the following options:

  • build_lib (bool, defaults to false) checks the project as if you passed the --lib argument to cargo.
  • cfg_test (bool, defaults to true) checks the project as if you were running cargo test rather than cargo build. I.e., compiles (but does not run) test code.


You can look in the in this repo to learn more about contributing to this project.