RustOwl Configuration (Emacs)

Table Of Contents

• RustOwl Configuration (Emacs)
  • Table Of Contents
  • Quick Start
  • Basic Setup
  • What the package does
  • Defaults and variables
  • How highlighting works
  • Enabling / Disabling
  • Examples
  • Troubleshooting

Quick Start

1. Install RustOwl (see docs/installation.md) and install the Emacs package via elpaca or use-package.
2. Open a Rust Cargo workspace in Emacs.
3. Place the cursor on a variable and wait ~2 seconds (default) to see overlays with lifetime and borrow info.

This document describes how to use RustOwl from Emacs. It mirrors the detail level of docs/neovim-configuration.md and is derived from rustowl.el.

Basic Setup

Install the rustowl executable (see docs/installation.md). Install the Emacs package via elpaca or use-package.

Elpaca example:

(elpaca
  (rustowl
    :host github
    :repo "cordx56/rustowl"))

Use-package example:

(use-package rustowl
  :after lsp-mode)

What the package does

• Registers an LSP client for rust-mode, rust-ts-mode, and rustic-mode using rustowl as the executable.
• Sends rustowl/analyze on save (see rustowl-enable-analyze-on-save).
• Sends rustowl/cursor when the cursor has been idle for rustowl-cursor-timeout seconds (default 2.0 seconds) and applies underlines via overlays.

Defaults and variables

• rustowl-cursor-timeout (float) — default: 2.0 seconds. Idle time before the cursor request is sent.
• rustowl-cursor-timer — internal timer variable managed by the package.

How highlighting works

• The package sends rustowl/cursor with position and document URI. The server responds with decorations (type, range, hover text, overlapped).
• For each non-overlapped decoration, the package maps types to underline colors:
  • lifetime → #00cc00 (green)
  • imm_borrow → #0000cc (blue)
  • mut_borrow → #cc00cc (purple)
  • move / call → #cccc00 (yellow)
  • outlive → #cc0000 (red)
• Underlines are implemented via overlays with face (:underline (:color <color> :style wave)).

Enabling / Disabling

• enable-rustowl-cursor — enable cursor-based highlighting for current buffer (adds post-command-hook).
• disable-rustowl-cursor — disable cursor-based highlighting (removes hook and cancels timer).
• Cursor highlighting is automatically enabled for Rust buffers via rust-mode-hook, rust-ts-mode-hook, and rustic-mode-hook.

Examples

To disable analyze-on-save globally, remove the hooks or call:

(remove-hook 'rust-mode-hook #'enable-rustowl-cursor)

To customize the timeout in your config:

(setq rustowl-cursor-timeout 1.0) ; 1 second

Troubleshooting

• If overlays don't appear, ensure lsp-mode is active and rustowl server is reachable.
• Use M-x rustowl-clear-overlays to clear overlays if they persist.

See docs/lsp-spec.md for the LSP request/response shapes.