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.

A high-level overview of the architecture can be found here.

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.

Setup

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 rust-analysis rust-src

error: component 'rls' is unavailable for download (nightly)

The development of rustc's internals is quite fast paced. Downstream projects that rely on nightly internals, particularly clippy, can break fairly often because of this.

When such breakages occur the nightly release will be missing rls. This is a trade-off compared with the other option of just not publishing the night's release, but does avoid blocking the rust nightly releases for people that don't need clippy/rls.

To mitigate the issues we have:

• rustup will warn if the update is missing any components you currently have. This means you can no longer accidentally update to a no-rls release. Once rls is available again it'll update.
• rls, clippy are available on the stable channel. Meaning most developers installing for the first time should use stable.
• However, if you need latest nightly rls you can use https://rust-lang.github.io/rustup-components-history/ to find and install a dated nightly release ie rustup install nightly-2018-12-06.

Also see #641.

Running

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:

RLS

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.

Configuration

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) overrides build debounce duration (ms). This is otherwise automatically inferred by the latest build duration.
• 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
• crate_blacklist ([String], defaults to this list) allows to specify which crates should be skipped by the RLS. By default skips libraries that are of considerable size but which the user often may not be directly interested in, thus reducing the build latency.
• 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 & go-to-definition 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.
  • "on" Display the same diagnostics as command-line clippy invoked with no arguments (clippy::all unless overridden).
  • "opt-in" Only display the lints explicitly enabled in the code. Start by adding #![warn(clippy::all)] to the root of each crate you want linted.

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 (bool, defaults to true) show additional context in hover tooltips when available. This is often the local variable declaration. When set to false the content is only available when holding the ctrl key in some editors.

Troubleshooting

For tips on debugging and troubleshooting, see debugging.md.

Contributing

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

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