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Rust Language Server (RLS)

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 it is 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.


Step 1: Install rustup

You can install rustup on many platforms. This will help us quickly install the RLS and its dependencies.

If you already have rustup installed, update to ensure you have the latest rustup and compiler:

rustup update

If you're going to use the VSCode extension, you can skip step 2.

Step 2: Install the RLS

Once you have rustup installed, run the following commands:

rustup component add rls-preview rust-analysis rust-src

Note (nightly only)

Sometimes the rls-preview component is not included in a nightly build due to certain issues. To see if the component is included in a particular build and what to do if it's not, check #641.


The RLS is built to work with many IDEs and editors, we mostly use VSCode to test the RLS. The easiest way is to use the published extension.

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

RLS: working ◐

Once you see:


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; using the Visual Studio Code extension this will be done via the workspace settings file settings.json.

Other editors will have their own way of sending the workspace/DidChangeConfiguration method.

Entries in this file will affect how the RLS operates and how it builds your project.

Currently we accept the following options:

  • unstable_features (bool, defaults to false) enables unstable features. Currently no option requires this flag.
  • sysroot (String, defaults to "") if the given string is not empty, use the given path as the sysroot for all rustc invocations instead of trying to detect the sysroot automatically
  • target (String, defaults to "") if the given string is not empty, use the given target triple for all rustc invocations
  • wait_to_build (u64, defaults to 1500) time in milliseconds between receiving a change notification and starting build
  • all_targets (bool, defaults to true) checks the project as if you were running cargo check --all-targets. I.e., check all targets and integration tests too
  • use_crate_blacklist (bool, defaults to true) if disabled, also indexes data from the blacklisted crates
  • build_on_save (bool, defaults to false) toggles whether the RLS should perform continuous analysis or only after a file is saved
  • features ([String], defaults to empty) list of Cargo features to enable
  • all_features (bool, defaults to false) enables all Cargo features
  • no_default_features (bool, defaults to false) disables default Cargo features
  • racer_completion (bool, defaults to true) enables code completion using racer (which is, at the moment, our only code completion backend). Also enables hover tooltips to fall back to racer when save-analysis data is unavailable.
  • clippy_preference (String, defaults to "opt-in") controls eagerness of clippy diagnostics when available. Valid values are (case-insensitive):
    • "off" Disable clippy lints.
    • "opt-in" Clippy lints are shown when crates specify #![warn(clippy)].
    • "on" Clippy lints enabled for all crates in workspace.

and the following unstable options:

  • build_lib (bool, defaults to false) checks the project as if you passed the --lib argument to cargo. Mutually exclusive with, and preferred over, build_bin.
  • build_bin (String, defaults to "") checks the project as if you passed -- bin <build_bin> argument to cargo. Mutually exclusive with build_lib.
  • cfg_test (bool, defaults to false) checks the project as if you were running cargo test rather than cargo build. I.e., compiles (but does not run) test code.
  • full_docs (bool, defaults to false) instructs rustc to populate the save-analysis data with full source documentation. When set to false, only the first paragraph is recorded. This option currently has little to no effect on hover tooltips. The save-analysis docs are only used if source extraction fails. This option has no effect on the standard library.
  • show_hover_context show additional context in hover tooltips when available. This is often the local variable declaration. When set to false the content is only availabe when holding the ctrl key in some editors.


For tips on debugging and troubleshooting, see


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

If you want to implement RLS support in an editor, see