In-process SCSS / Sass → CSS compilation for Ruby, backed by sasso — a pure-Rust, dependency-free dart-sass alternative that targets byte-for-byte parity with current dart-sass. Shipped as a native extension (Rust via magnus

• rb-sys); no Node, no subprocess, no Dart VM.

This gem is the Ruby binding. The compiler core lives in the separate momiji-rs/sasso repo (crate on crates.io); this gem pins it exactly and releases on its own cadence.

# Gemfile
gem "sasso"

$ bundle install

Precompiled native gems ship for common platforms (Linux gnu/musl, macOS, and Windows on x86_64/arm64), so no Rust toolchain is needed. On other platforms the gem compiles from source via cargo (needs a Rust toolchain).

require "sasso"

Sasso.compile_string("$c: #336699; a { color: $c; &:hover { color: red } }")
# => "a {\n  color: #336699;\n}\na:hover {\n  color: red;\n}\n"

# Minified:
Sasso.compile_string("a { b: 1px }", style: :compressed)   # => "a{b:1px}"

# Indented .sass syntax:
Sasso.compile_string("a\n  b: 1px\n", indented: true)

# A file (syntax inferred from the extension; diagnostics get the file name):
Sasso.compile("app/assets/stylesheets/application.scss",
              style: :compressed, load_paths: ["vendor/stylesheets"])

A compile failure raises Sasso::CompileError (a Sasso::Error < StandardError) whose #message is the compiler's full diagnostic — the same text the sasso CLI prints.

begin
  Sasso.compile_string("a { b: 1px + 1em }")
rescue Sasso::CompileError => e
  warn e.message
end

Using a Ruby web framework? These drop-in gems build on this one and compile your Sass/SCSS in-process (no Node, no Dart, no subprocess), byte-for-byte identical to dart-sass — typically ~6–7× faster per compile than the Node sass default:

• Rails (Propshaft + Sprockets) — sasso-rails
• Bridgetown — bridgetown-sasso
• Hanami (2.1+) — hanami-sasso

The core passes 100% of the attempted official sass-spec suite byte-for-byte against dart-sass; see the core repo. For the inputs below, sasso's output is byte-identical to sass-embedded (dart-sass).

Because sasso compiles in-process (a direct Rust call — no subprocess, no IPC, no Dart VM), it avoids the per-call protocol roundtrip of sass-embedded and the process-spawn cost paid by any out-of-process compiler.

Compiling the same SCSS (variables, nesting, @for, math) on an Apple M2 Max, Ruby 3.4.1:

Parenthesised values are how much slower the other gem is than sasso.

• Per-request compiling (e.g. a Sinatra route): in-process latency is ~13 µs vs ~129 µs for sass-embedded's pipe roundtrip to its Dart subprocess.
• One-shot builds (e.g. rails assets:precompile): the dominant cost is the ~38 ms Dart subprocess spawn, which sasso does not pay (~1 ms cold).

The engine is also heavily perf-tuned (a scoped bump arena, reference-counted values). Numbers are representative of one machine; run your own with your stylesheets.

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