feat!: replace the Python CSS implementation with a Rust NIF

[shahryarjb]

Replaces the Python/tinycss2 implementation with a precompiled Rust NIF built on Biome's lossless CSS CST, and deletes priv/python, plibs/, rebuild_wheel.sh and the pythonx dependency — no Python, no Node, no external process. The load-bearing decision is that the tree is never reprinted: operations parse losslessly, locate byte ranges, and splice text into the original source, so comments, indentation and property order outside an edit are preserved by construction rather than by effort. That yields four properties, each asserted for every operation against every fixture rather than spot-checked — comments are never lost, diffs contain only the lines the codemod meant to change, every operation is idempotent and reports changed: false on a re-run, and a file that cannot be patched safely is refused rather than half-edited. Selector matching is deliberately strict (top-level only, normalised comparison, never substring or fuzzy) and an ambiguous selector is an error rather than a guess, which is how the previous implementation produced surprising diffs.

Related: ash-project/ash#2834

Examples

Inserting an at-rule: it lands after the at-rule prologue rather than at the top of the file, and the inline comment on .btn survives the value change.

css = """
@import "tailwindcss";
@source "../js";

/* ===== Layout ===== */

.btn {
  color: red; /* brand */
  margin: 0;
}
"""

{:ok, a} = IgniterCss.ensure_at_rule(css, ~s|@plugin "daisyui";|)
{:ok, b} = IgniterCss.set_declaration(a.source, ".btn", "color", "var(--brand)")

@import "tailwindcss";
@source "../js";
@plugin "daisyui";

/* ===== Layout ===== */

.btn {
  color: var(--brand); /* brand */
  margin: 0;
}

Re-running is a no-op, which is what makes it safe inside an installer:

{:ok, again} = IgniterCss.ensure_at_rule(b.source, ~s|@plugin "daisyui";|)
again.changed
#=> false

Removing a declaration takes the comments it owns and leaves the section header:

{:ok, c} = IgniterCss.remove_declaration(b.source, ".btn", "color")

@plugin "daisyui";

/* ===== Layout ===== */

.btn {
  margin: 0;
}

It refuses rather than guessing:

IgniterCss.set_declaration(".a{}\n.a{}", ".a", "color", "red")
#=> {:error, "selector \".a\" matches 2 top-level rules; refusing to guess which one to patch"}

IgniterCss.ensure_rule(".broken {\n  color: red;\n", ".x")
#=> {:error, "cannot safely patch this file: braces are unbalanced; refusing to patch"}

Inside an installer:

igniter
|> IgniterCss.Codemods.ensure_at_rule(path, ~s|@plugin "daisyui";|)
|> IgniterCss.Codemods.ensure_rule(path, ".hide-scrollbar")
|> IgniterCss.Codemods.set_declaration(path, ".hide-scrollbar", "scrollbar-width", "none")

Verified

rust tests:        339   (unit, corpus-invariant, property/fuzz)
elixir tests:      268 + 39 doctests
clippy warnings:   0
cargo fmt:         ok
mix format:        ok
credo --strict:    no issues
dialyzer:          Total errors: 0, Skipped: 0

Phase 0 gate, which everything else rests on — parse.syntax().to_string() == source byte-for-byte across the whole fixture corpus, and it stays in CI:

round_trip_is_byte_identical_with_default_options ... ok
round_trip_is_byte_identical_in_strict_mode ... ok
round_trip_is_byte_identical_with_css_modules_enabled ... ok
round_trip_holds_even_when_the_parse_has_errors ... ok
tailwind_v4_at_rules_parse_without_diagnostics ... ok

The corpus is real-world shaped on purpose: a Phoenix app.css, Tailwind v4 (@theme, @plugin with and without a block, @source, @custom-variant, @variant, @utility, @apply, @reference — all parse with zero diagnostics), comments in awkward places, CRLF, a BOM, no trailing newline, minified vendor CSS, non-ASCII content, and files that are simply broken.

Patching a fresh Phoenix app.css end to end — two at-rules plus a new rule — touches 6 lines and no others, with every original line still present verbatim.

cargo test, cargo fmt and cargo clippy pass on the shared ash-ci workflow on both matrix legs:

success  ash-ci (1.17.0-otp-26, 26.0, false) / cargo test, fmt and clippy
success  ash-ci (default, default, true)     / cargo test, fmt and clippy

Notes

Two behaviour changes worth flagging. IgniterCss.CSS.CssProcessor is removed — its pipeline mixed codemods with whole-file rewriting, which this design keeps separate: IgniterCss patches, IgniterCss.Transform (minify/beautify/merge) rewrites and must not be pointed at a file a user maintains. And IgniterCss.Parsers.Parser keeps its function names and {:ok, :function_name, result} shape, but its mutating functions are now diff-minimal instead of reprinting the stylesheet.

Releasing depends on ash-project/ash#PENDING — ash-ci's build-release job currently hardcodes igniter_js as the crate to build, so tagged releases here would build a crate this repository does not contain. Branch and PR checks are unaffected.

[zachdaniel]

🚀 Thank you for your contribution! 🚀