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.