rusvelte

An experimental Rust port of the Svelte 5 compiler and tooling, including a faster svelte-check. Not affiliated with the Svelte team; see THIRD_PARTY_NOTICES.md for the projects it ports. The project page lives in site/ and deploys to GitHub Pages. JavaScript inside components is parsed with oxc. Every layer is checked against the JS original by diffing outputs over large corpora.

Most of the code was written with Claude Code, Anthropic's coding agent, under human direction. Nothing is taken on trust: every result in the tables below comes from comparing against the official JavaScript tools, and the scripts to rerun them are in oracle/.

• Parser: parse(source, { modern: true }) and the legacy AST from svelte/compiler 5.57.2.
• svelte2tsx: the component-to-TypeScript transform from svelte2tsx 0.7.61, which svelte-check type-checks.
• svelte-check-rs: a native svelte-check --tsgo. It converts every component in parallel, writes svelte-check's overlay, runs TypeScript 7 (tsgo) and maps the diagnostics back.
• rusvelte (npm): the compiler as a drop-in for svelte/compiler in Vite builds, through a Node binding (bindings/node, packages/rusvelte); see its README.

Status

"The private app" is a 171-component SvelteKit app that isn't public; Windmill, Immich and bits-ui are open source.

Part	Parity with the JS original
Parser	4,567/4,567 Svelte test files (3 differ only in a JS error's position), the private app 171/171
Legacy AST	4,567/4,567, 171/171
Analysis (compiler warnings and errors, compile with generate: false)	Svelte tests 4,564/4,567 (the rest word a JS syntax error differently), the private app 171/171, language-tools 787/803
svelte2tsx	Svelte tests 4,566/4,567, svelte2tsx's samples 247/250 (dts mode skipped), the private app 171/171, Windmill 1,982/1,982
SvelteKit route/hook/param files (upsertKitFile)	all real-world files; 118/121 synthetic samples
svelte-check-rs, TypeScript diagnostics	identical to svelte-check --tsgo 4.7.6 on the private app (2,477 diagnostics under strict options) and Windmill (7,922)
svelte-check-rs, compiler warnings	identical to svelte-check on the private app and Windmill (89 warnings)
svelte-check-rs, CSS diagnostics (css/scss/less)	identical to svelte-check on Windmill (96 warnings in all); the CSS linter matches svelte-check's on 4,567/4,567 Svelte tests, Windmill, the private app and 47,072 synthetic files
CSS output (css.code, css.hasGlobal, injected styles)	identical on every component of the code generation oracle (Svelte test suites 6,929 compilations, the private app 342, Windmill 3,964), plus dev/css: 'injected' variants and the CSS linter's 31,418 synthetic files (1 differs: oxc rejects a mangled TypeScript expression acorn accepts)
Client code generation (compile, generate: 'client', js.code)	identical on every client compilation of the code generation oracle: Svelte test suites (runtime-runes 1,408, runtime-legacy 1,657, hydration 98, css 187/188: a cssHash function the harness can't call, snapshot 33), the private app 171, Windmill 1,982, bits-ui 226, immich 411, language-tools 668; also with dev, hmr, fragments: 'tree', experimental.async, customElement, css: 'injected', compatibility.componentApi: 4 variants, and the runes/legacy/TypeScript/custom-element variants of the test suites and Windmill

The remaining svelte2tsx mismatches are 2 scripts that oxc can't parse but TypeScript recovers from, and 2 samples where npm 0.7.61 throws but the current language-tools source (followed here) doesn't.

Speed

	JS	Rust
Parse the Svelte test corpus	157 ms	19 ms
svelte2tsx, Windmill (1,982 files, warm)	3,425 ms	539 ms, 91 ms on 12 threads
Full check, a private app (svelte-check vs svelte-check-rs)	10.6 s	1.7 s
Full check, Windmill	73.5 s (needs an 8 GB heap)	9.3 s
Incremental re-run, Windmill, no changes	2.4 s (svelte-fast-check)	2.0 s
Print Windmill's compiled JS (3,964 files, 34 MB) with esrap / estree::print	4,268 ms	600 ms (360 ms without source maps)

On Windmill the type-check itself (tsgo, about 8 s) is now most of the time; the project's PostCSS/Melt UI preprocessors run alongside it in Node.

Differences from svelte-check 4.7 --tsgo

These are deliberate fixes:

• Excludes covering the cache directory are dropped. A tsconfig that excludes .svelte-kit made svelte-check exclude its own output, so no component got type-checked.
• package.json subpath imports (#lib/*) get overlay paths that try the generated files first. svelte-check only redirects relative imports and paths, so #lib/X.svelte became an untyped module.
• The overlay keeps the project's rootDir default. With outDir and no rootDir, TypeScript 6+ would make the overlay's own directory the root, report only TS6059 errors and skip type-checking.
• Compiler errors are reported for components svelte2tsx can't convert. svelte-check drops such files silently.

svelte-check-rs

cargo build --release --bin svelte-check-rs
cd my-app && /path/to/svelte-check-rs --tsconfig ./tsconfig.json

The project needs TypeScript 7 (@typescript/native, an alias of typescript@7, or @typescript/native-preview), as with svelte-check --tsgo. Compiler warnings come from the Rust analysis. Node is only started when the project's config has preprocessors (which then run in Node, and the analysis maps positions back through their source maps) or compiler options the analysis doesn't support (then the project's svelte/compiler does the whole job).

Options follow svelte-check:

• --workspace, --tsconfig
• --output human|human-verbose|machine|machine-verbose
• --threshold, --ignore, --fail-on-warnings
• --diagnostic-sources js,svelte,css, --compiler-warnings code:ignore|error,...
• --incremental, --watch, --preserveWatchOutput

--timings prints where the time went.

SVELTE_CHECK_TSGO=/path/to/tsgo uses another TypeScript 7 binary, such as tsc-rs, and SVELTE_CHECK_TSGO_ARGS passes it extra flags (--singleThreaded makes results independent of how files are split between checkers).

CSS output

rusvelte::compile_css(source, filename, &CssOptions) returns result.css of compile (code and has_global), and transform::compile_styles also the stylesheet injected into the JS with css: 'injected' or custom elements. It is a port of render_stylesheet (src/transform/css.rs), with the cssHash, css, dev, customElement and rootDir options. Source maps aren't produced yet.

Analysis

rusvelte::analyze::compile_diagnostics(source, filename) reproduces the warnings of compile(source, { dev: true, generate: false, filename }), or the error it throws. That covers scopes, runes and legacy analysis, a11y, CSS pruning (css_unused_selector) and svelte-ignore. compile_diagnostics_with takes the runes, customElement and experimental.async options. It takes about 51 ms for Svelte's test corpus against 361 ms for compile on Node.

oracle/gen_variants.mjs makes runes, legacy, TypeScript and custom-element variants of a corpus, to push the analysis down other paths.

Setup for the parity checks

git clone https://github.com/sveltejs/svelte.git svelte-upstream
git -C svelte-upstream checkout 707c28146b0f0a6d5404a1bd4769874c3c24851a
git clone https://github.com/sveltejs/language-tools.git language-tools-upstream
cd oracle && npm install && cd ..

Each oracle script runs the JS original over a corpus and writes one JSON file per component. The matching binary compares and groups failures, and VERBOSE=1 prints the first difference.

Oracle	Compare binary
oracle/gen.mjs	compare (parser, legacy AST)
oracle/gen_compile.mjs	compare_compile (analysis)
oracle/gen_htmlx2jsx.mjs	compare_s2t (template half)
oracle/gen_svelte2tsx.mjs <corpus> <out> [check|samples|svelte-check]	compare_s2t (full svelte2tsx)
oracle/gen_kit.mjs	compare_kit (SvelteKit files)
oracle/css_oracle.cjs (and gen_css_samples.mjs for synthetic corpora)	compare_css (CSS diagnostics)
oracle/gen_css_output.mjs <corpus> <out> [base options], or gen_codegen.mjs	compare_css_output (CSS output)
oracle/gen_esrap.mjs [--keep-minified] <dir|@list> <out.json> [base]	compare_esrap (JS printer; BENCH=n times it)
oracle/gen_builders.mjs	expected output for the builder tests in src/estree/tests.rs

tools/check_sanity.py <language-tools> <node_modules> runs svelte-check's own sanity fixtures against svelte-check-rs.

Layout

• Parser
  • src/parser/: port of phases/1-parse.
  • src/js.rs: oxc integration and acorn-compatible JSON (ToJson).
  • src/ast.rs, src/css.rs: the typed template and CSS ASTs.
  • src/legacy.rs: the Svelte 4-shaped AST that svelte2tsx consumes.
  • src/errors.rs, src/warning_codes.rs: generated from Svelte's messages.
• Analysis
  • src/analyze/: port of phases/2-analyze and phases/scope.js. nodes.rs views the template AST and oxc's AST as the ESTree/Svelte tree the JS walks.
  • warnings.rs and src/errors.rs are generated by tools/gen_messages.mjs, and a11y_data.rs by tools/gen_a11y.mjs.
  • acorn.rs adds the parse errors acorn raises that oxc's parser doesn't.
• svelte2tsx
  • src/magic_string.rs: port of magic-string.
  • src/svelte2tsx/template.rs, elements.rs, slots.rs, periscope.rs: the template half (htmlxtojsx_v2).
  • src/svelte2tsx/script/: the script half, which walks oxc's AST the way svelte2tsx walks TypeScript's.
  • src/svelte2tsx/kit.rs, rewrite_imports.rs: SvelteKit files and external imports.
• Code generation
  • src/estree/: the owned ESTree AST the transform builds (Node/NodeKind), with convert.rs (oxc → acorn-shaped ESTree, TypeScript removed like remove_typescript_nodes), builders.rs (port of utils/builders.js) and print.rs (port of esrap 2.4.0 with its ts language, including comments and source map mappings).
  • src/transform/: port of phases/3-transform: css/index.js (render_stylesheet), the client transform (client/: transform-client.js, transform-template/*, visitors/*).
• CSS diagnostics

License

MIT, see LICENSE. The ported code keeps its original notices in THIRD_PARTY_NOTICES.md.