sasso-rails

Rails integration for sasso — a pure-Rust, zero-dependency SCSS/Sass → CSS compiler (a byte-for-byte dart-sass alternative).

Compiles your stylesheets into app/assets/builds/ for the Rails asset pipeline to serve. Propshaft-first (the Rails 8 default) and Sprockets-compatible.

Why this gem over the alternatives:

  • No Node.js. Unlike cssbundling-rails, there is no node/yarn/npx requirement — the compiler ships as a precompiled Ruby native gem.
  • In-process, no subprocess. Unlike dartsass-rails / tailwindcss-rails (which shell out to a standalone binary), sasso-rails calls Sasso.compile directly through the native extension — no process spawn, no IPC.
  • Pure Rust core. No libsass (deprecated), no Dart VM.

Installation

# Gemfile
gem "sasso-rails"

bundle install
bin/rails generate sasso:install

The installer scaffolds app/assets/stylesheets/application.scss, creates app/assets/builds/ (with a .keep), links the builds directory in a Sprockets manifest if one exists, and adds a watch process to Procfile.dev.

Reference the compiled CSS in your layout:

<%= stylesheet_link_tag "application" %>

Usage

bin/rails sasso:build     # compile once
bin/rails sasso:watch     # recompile on change (or run ./bin/dev)
bin/rails sasso:clobber   # remove generated CSS

sasso:build runs automatically before assets:precompile, so deploys (rails assets:precompile) regenerate the CSS before the pipeline fingerprints it. No extra deploy wiring needed.

Configuration

In config/application.rb (or an environment file):

# Map each entrypoint (under source_dir) to an output file (under build_dir).
config.sasso.builds = {
  "application.scss" => "application.css",
  "admin.scss"       => "admin.css",
}

# :expanded | :compressed. Default: :compressed in production, :expanded else.
config.sasso.style = :compressed

# Extra @use/@import include dirs. The entrypoint's own directory is always
# searched first, so sibling partials need no configuration.
config.sasso.load_paths = [Rails.root.join("vendor/styles").to_s]

# Defaults shown:
config.sasso.source_dir = "app/assets/stylesheets"
config.sasso.build_dir  = "app/assets/builds"

How it fits the pipeline

sasso:build writes plain CSS to app/assets/builds/. Both pipelines treat that directory as a source of ready-to-serve assets:

  • Propshaft (Rails 8 default) discovers app/assets/builds on its load path, fingerprints the CSS, and rewrites url(...) references — it performs no Sass compilation itself.
  • Sprockets serves the built CSS as a static asset (the installer adds //= link_tree ../builds to app/assets/config/manifest.js).

Notes & limitations

  • Source maps are not supported yet. The in-process compiler returns CSS only; there is no .css.map output in any environment.
  • builds is file-to-file. Each entry maps one input file to one output file. There is no directory-glob form (no "." => "."); list each entrypoint explicitly.
  • Load paths are explicit. Only the entrypoint's own directory (searched automatically) and config.sasso.load_paths are on the @use/@import search path. Gem-vendored or config.assets.paths stylesheets are NOT added automatically — list their directories in config.sasso.load_paths.
  • No double-minification. On a Sprockets app that sets config.assets.css_compressor, sasso-rails stays :expanded and lets the pipeline compress (unless you pin config.sasso.style). Propshaft does not compress, so production output is :compressed there.
  • Watch is a 1s mtime poll (dependency-free, no native fs-events); it covers the source dir and config.sasso.load_paths.

Versioning

This gem versions independently of the sasso compiler gem and pins it with a range (sasso >= 0.1.1, < 1). An app may pin a specific compiler version in its own Gemfile.

License

MIT, matching the Sass ecosystem. (The core sasso compiler crate remains dual-licensed MIT OR Apache-2.0.)