SWC browser WASM transpiler

[merryman]

Summary

• Replace Babel with SWC WASM for in-browser module transpilation on fastLoad=false world loads
• Compiles the existing Rust SWC transforms into a standalone browser WASM module (~5MB), loaded lazily at startup
• 176 modules transpiled via SWC, 0 Babel fallbacks, 0 runtime errors
• Automatic fallback to Babel if WASM loading fails — fully backwards-compatible

Performance

Metric	SWC	Babel	Improvement
Total page load	34s	46s	1.3x faster
Transpile time (176 modules)	3.1s	~15s+	~5x faster
Babel fallbacks	0	N/A	—

Architecture

Cargo workspace (3 crates sharing transform code):

• lively-swc-transforms/ — shared library (7 AST visitors, ~4000 lines)
• lively-swc-plugin/ — existing server-side SWC plugin (wasm32-wasip1)
• lively-swc-browser/ — new browser WASM entry point (wasm32-unknown-unknown)

JS integration (lively.source-transform/swc/):

• browser-transform.js — WASM loader + transform wrapper
• transpiler-setup.js — SystemJS transpiler registration, config mapping, Babel fallback

Key post-processing passes (Phase 3 in lib.rs):

• fix_async_execute — strips spurious async from execute()
• hoist_recorder_init — moves recorder init from execute() to factory body
• rewrite_setters — adds defVar wrapper + default params to SystemJS setters
• fix_nested_fn_export_calls — strips _export() inside nested functions where the export name is shadowed by a local let/const (SWC system_js scoping bug workaround)
• strip/restore_export_aliases — export { Y as a } → __export_a__ desugaring to prevent system_js hoisting

Test plan

• cargo build --target wasm32-unknown-unknown -p lively-swc-browser compiles
• cargo build --target wasm32-wasip1 -p lively-swc-plugin still works
• World loads at http://localhost:9011/projects/load?name=__newProject__&fastLoad=false with 0 errors
• All 176 modules transpiled by SWC (0 Babel fallbacks)
• ?babelOnly flag falls back to Babel correctly
• Loading screen build succeeds
• Compared SWC output against Babel gold standard for structural parity

🤖 Generated with Claude Code