sasso

A pure-Rust SCSS → CSS compiler — a from-scratch dart-sass alternative. Zero runtime dependencies, wasm-friendly, usable as a library and a CLI, and designed to match current dart-sass byte-for-byte on the subset it implements.

Status: v0.x, maturing fast. Compiles real-world SCSS and indented .sass byte-identically to dart-sass 1.100, and passes 100% of the attempted official sass-spec suite (13,896 / 13,896, zero failures) — tracked as a ratchet (see Conformance for exactly what that denominator means). What remains is real-world breadth and hardening, not spec coverage.

Why another Sass compiler?

grass is the incumbent Rust implementation and a strong one (it compiles Bootstrap/Bulma byte-accurately and is ~2× faster than dart-sass). But it is pinned to dart-sass 1.54.3 (mid-2022) and predates the CSS Color Level 4 overhaul, so it diverges from current dart-sass on, e.g., fractional color channels (rgb(63.75, 127.5, 191.25) vs rounded hex) and emits hex where dart-sass now keeps rgb()/hsl() forms. sasso targets current dart-sass exactly, with a span-first parser, a modern color model, and a zero-dependency, sandbox-friendly core. See docs/GRASS_LANDSCAPE.md for the full analysis.

Features (this slice)

• $variables, lexical scoping, !default, !global
• Nesting, the & parent selector (with selector-list multiplication), and combinator normalization (>, +, ~)
• #{} interpolation in selectors, property names and values
• // (stripped) and /* */ (preserved) comments
• Numbers with units and unit arithmetic ($pad * 2 → 16px)
• A full color model with fractional channels + author-spelling preservation (red, #336699, rgb()/hsl() round-trip unchanged)
• Color functions: rgb/rgba/hsl/hsla/mix/lighten/darken/ percentage (+ red/green/blue/alpha)
• @import partial inlining through a pluggable Importer (CSS imports pass through)
• expanded and compressed output styles
• Verbatim preservation of CSS functions it doesn't own (calc, var, clamp, translateX, …)

Since this slice was written the ratchet has added a great deal more — @mixin/@function, control flow (@if/@each/@for/@while), @extend and %placeholders, a calc() engine, the CSS unit system + math functions, full CSS Color 4 color spaces (oklch/lab/color()…), structured @media/@supports, maps, the @use/@forward module system (built-in sass:* modules + user files), and the indented .sass syntax. The compiler now passes 100% of the attempted sass-spec suite (13,896 / 13,896, zero failures) byte-for-byte against dart-sass 1.100 — 11,405 byte-exact CSS outputs plus 2,491 error specs it correctly rejects (see Conformance).

Install

CLI — prebuilt binaries. Every release ships static binaries for Linux (gnu + musl), macOS and Windows (x86_64 / aarch64), built with cargo-dist. Grab one from the Releases page, or:

$ curl -fsSL https://github.com/momiji-rs/sasso/releases/latest/download/sasso-installer.sh | sh   # Linux/macOS
$ cargo binstall sasso        # fetch the prebuilt binary
$ cargo install sasso         # build from source (needs a Rust toolchain)

Library — crates.io.

$ cargo add sasso

WebAssembly — npm. A tiny, dependency-free wasm build for JS build tools and the browser (no wasm-bindgen, no native add-ons):

$ npm install @momiji-rs/sasso

import { compile } from "@momiji-rs/sasso";
compile("a { color: #ffffff }", { style: "compressed" }); // a{color:#fff}

Library usage

use sasso::{compile, Options, OutputStyle};

let scss = "$c: #336699; .a { color: $c; &:hover { color: lighten($c, 10%); } }";
let css = compile(scss, &Options::default()).unwrap();
assert!(css.contains("a:hover"));

// Minified:
let min = compile(scss, &Options::default().with_style(OutputStyle::Compressed)).unwrap();

@import resolution is controlled by an Importer you supply, so file access stays on your side of any sandbox:

use sasso::{compile, Importer, Options};

struct MyFs;
impl Importer for MyFs {
    fn resolve(&self, path: &str) -> Option<String> {
        std::fs::read_to_string(format!("scss/_{path}.scss")).ok()
    }
}
let css = compile("@import \"base\";", &Options::default().with_importer(&MyFs)).unwrap();

A ready-made FsImporter is provided for standalone/CLI use.

CLI usage

$ cargo install --path .            # installs the `sasso` binary
$ sasso input.scss              # CSS to stdout (expanded)
$ sasso --style=compressed input.scss
$ sasso -I scss/ main.scss      # add @import load paths
$ echo '.a{color:red}' | sasso --stdin

Conformance

The official sass-spec suite is the parity oracle. The harness in spec/ runs the compiler against every spec case and reports a pass rate; we ratchet it upward over time.

Passing = byte-exact CSS output match plus error specs the compiler correctly rejects — the standard sass-spec conformance metric (the harness checks that an error spec errors; the error message is tracked separately as a non-gating metric). The 8 excluded cases are tagged :todo for dart-sass itself upstream — dart-sass doesn't pass them either; sasso matches dart-sass's actual behaviour on all 8 regardless.

Strict input validation, too. Matching dart-sass means rejecting what dart-sass rejects, not just reproducing its output. sasso errors — rather than silently accepting — on an invalid hex literal (#00000), out-of-grammar rgb()/hsl() arguments, a duplicate @mixin/@function parameter, a misplaced @content/@extend, a style rule or declaration inside a @function body, a malformed :nth-child() / empty :not() selector, a bad @charset or @at-root (…) query, and more — each with dart-sass's exact message.

Run it yourself:

$ spec/fetch.sh                                      # clone the suite
$ cargo build --release
$ SASS_BIN=target/release/sasso python3 spec/run_spec.py

Performance

sasso is a native, in-process library — no subprocess, no Node, no Dart VM — so startup is effectively free, which dominates when a build compiles many files. On an Apple M2 Max it is the fastest of the three engines measured, beating dart-sass by 19–30× end-to-end and leading grass (the incumbent Rust compiler) by ~2.3–2.9×:

¹ derived (cold − startup) — dart-sass has no in-process loop mode. So sasso is ~29× faster than dart-sass on pure compute, ~30× on a cold single file, and ~77× on startup; vs grass it is ~2.3× cold / ~2.8× batch / ~2.9× pure.

² Startup compiles a 1-rule file, so it sits at the OS process-spawn floor — sasso and grass measure identically (1.8 ms) here (the mean is dominated by scheduler jitter at this sub-2 ms scale). The native library and wasm builds remove process startup entirely. A scoped bump-arena allocator (one audited unsafe module, Miri- and AddressSanitizer-verified; the rest of the library stays unsafe-free) gives a further ~1.5× by turning each compile's allocations into a pointer bump freed wholesale at the end. Composite values (strings, lists, maps) are reference-counted, so reading a $variable is an O(1) refcount bump, not a deep copy — on a large list passed through a call chain without mutation this cuts both instructions (~7×) and peak memory (~13×). Full methodology, per-file numbers and the correctness diff are in bench/three_way.md; run it yourself with cd bench && RUNS=12 WARMUP=3 LOOP_N=200 bash scripts/run_bench.sh.

WebAssembly

Because the library is zero-dependency and pure std, it compiles to wasm32-unknown-unknown and wasm32-wasip1 out of the box (built in CI). The deployable .wasm cdylib ships in two variants, published to npm as @momiji-rs/sasso:

³ in-process compile of the same large file, Node 22 (best-of-N). The wasm tax over native sasso (7.7 ms) is ~1.5× for the speed build and ~3.5× for the size build; the wasm build runs without the bump arena. Even so the speed build (~12 ms) beats native grass (21 ms) and every dart-sass form a Node toolchain can run.

A whole modern Sass compiler — @use/@forward, @extend, the calc engine, CSS Color 4 — in a few hundred KB gzipped, far smaller than shipping the dart-sass compiler as JavaScript. A browser playground is tracked in the issues.

Language bindings

The compiler is usable beyond Rust:

The Ruby gem lives in its own repo (the norm for Rust-backed gems) and pins a published sasso crate version; file gem-specific issues there.

Testing & coverage

$ cargo test                                  # unit + integration + doctests (offline)
$ SASSO_PARITY=1 cargo test --test parity # live diff vs dart-sass (needs `npx sass`)
$ cargo llvm-cov --workspace                  # coverage report
$ cargo clippy --all-targets -- -D warnings
$ cargo fmt --check

Changelog

Notable changes are recorded in CHANGELOG.md.

Code of Conduct

As a Sass implementation, sasso adopts the Sass Community Guidelines — see CODE_OF_CONDUCT.md.

License

Licensed under either of Apache-2.0 or MIT at your option.