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: early vertical slice (v0.0.1). It already compiles real-world SCSS (variables, nesting, &, interpolation, unit math, a focused color function set, and @import partials) byte-identically to dart-sass 1.100. The north-star target is 100% of the official sass-spec suite, tracked as a ratchet (see Conformance).

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, …)

Not yet implemented: @mixin/@function, control flow (@if/@each/ @for/@while), @extend/placeholders, the @use/@forward module system, and the indented .sass syntax. These are the next ratchet steps.

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.

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. See bench/ for the methodology. In-process startup is effectively free (vs ~140 ms for the dart-sass binary and ~1 s for npx sass), which matters when a build compiles many files.

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

License

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