JANUARY 8, 2016

YouCompleteMe, Rust

YouCompleteMe now supports Rust auto-completion and GoTo. Rust semantic analysis is provided by racerd, a JSON/HTTP server powered by racer. YCM with Rust provides a superior experience to current vim-racer, emacs-racer, and other racer-based plugins.

YouCompleteMe

For the uninitiated, YouCompleteMe is a fast, fuzzy, as-you-type code-completion engine built originally for Vim. YCM runs on Mac, Linux, and Windows and includes completion engines for C/C++, ObjC, Python, C#, Go, TypeScript, JavaScript, and now Rust. YCM additionally provides an identifier based completion engine to supplement semantic completers and provide completions for languages without native support.

The YCM core, ycmd, exists as an independent project; clients for Vim, Emacs, and Atom all share the same infrastructure. The complete list of known clients is found in the ycmd README. If a ycmd client does not exist for $EDITOR, integrating with ycmd is as simple as integrating with a REST API. The ycmd example client shows how it’s done.

Rust

Rust is a systems programming language that runs blazingly fast, prevents segfaults, and guarantees thread safety.

Support for Rust is now available in YCM. The result is a powerful development environment providing completions and GoTo in your favorite editor. Examples below assume your editor is Vim. For other editors, please reference their YCM client.

Completions

Semantic completions in YCM are provided when a semantic trigger, ., or ::, is detected. In the provided example, you can see a completion menu immediately appear after typing std::, std::collections::, and Vec::; no hotkey was necessary. As you continue to type, YCM fuzzy-filters available completions using your input to narrow the completion list.

GoTo

The YCM Rust Completer provides the GoTo subcommand. GoTo attempts to find where the identifier under the cursor is defined. If successful, a buffer is opened for the file containing the definition, and the cursor is placed on the definition.

The complete GoTo command in Vim is :YcmCompleter GoTo, and I highly recommend mapping it to some hotkey. Example:

nnoremap <Leader>] :YcmCompleter GoTo<CR>

YCM Configuration (Vim)

Rust completions and GoTo from the current crate and its dependencies will be available without any additional configuration. For completions in the standard library, a single variable g:ycm_rust_src_path must be defined in your .vimrc.

" Naturally, this needs to be set to wherever your rust
" source tree resides.
let g:ycm_rust_src_path = '/usr/local/rust/rustc-1.5.0/src'

Differences from vim-racer, emacs-racer, and others

Since racerd, and subsequently YCM’s Rust Completer are powered by racer, the same code-completion and find-definition features are available. The addition of YCM to this equation provides a massive quality-of-life improvement over the existing plugins:

No hotkey required for completions
Fuzzy search of available completions
Identifier-based completion engine to supplement rust completions
Performance: Completions are cached within YCM, and latency with racerd is typically only a few ms once files are cached from disk. This is discussed more in the racerd section following.

racerd

racerd is suitable for providing completions and find-definition support for any Rust IDE and is not constrained to YCM. It provides several benefits over integrating racer via the command line or with the racer daemon flag.

Persistent file caching: When starting a new racer process for every completion, or when running racer in daemon mode, the cache is thrown out after each operation. Racerd keeps this cache between requests. This gives a nice performance boost since files only need to be read from disk once.
Support for dirty buffers: Racerd’s JSON API supports a buffers field - a list of file paths (which need not exist) and associated contents which is added to the racer cache before performing a query. This feature eliminates the need for temporary files.
HTTP/JSON API: Very effective method for integrating a semantic completion engine. Just about every language has a built-in or library for querying such an API, and it is extremely performant on localhost (a few ms for completing out of standard library). The process is long lived; there is no process startup overhead for each query. Racerd also has extensive API documentation.

Giving Thanks

YouCompleteMe would not be the fantastic project it is, both technically and as a positive open-source community, without the work of @Valloric and the YCM team @micbou, @oblitum, @vheon, and @puremourning. racerd and YCM’s Rust completer would not have been possible without @phildawes’ fantastic racer library. Thanks to @birkenfeld for his assistance implementing some changes in racer to make racerd possible. Thanks to @reem whom I pestered with far too many questions in #iron. Finally, thanks to the YCM team for reading drafts of this post.

JOE WILM