Build the RBS parser as a WebAssembly module (JRuby support, step 1)

[soutaro]

Why

This is the first step toward running RBS on Ruby implementations that cannot load the MRI C extension — notably JRuby — using WebAssembly instead of an FFI/dylib.

The parser under src/ is plain, self-contained C with no dependency on the Ruby C API, so it can be compiled to WebAssembly as-is. The plan is to compile the parser to a .wasm module, run it on a JVM WebAssembly runtime under JRuby, and reconstruct RBS::AST objects on the Ruby side.

Plan (split into PRs)

1. This PR — WASM build of the parser. Prove the toolchain end to end: compile src/*.c to wasm32-wasip1, expose a minimal stable ABI, build it from Rake, and smoke-test it in CI.
2. AST serialization. A compact binary serializer (generated from config.yml, alongside the existing ast_translation.c) plus a pure-Ruby deserializer. Validated on CRuby first via a round-trip against the existing C → Ruby translation, so the hardest part is de-risked without any WASM involved.
3. JRuby integration. Load rbs_parser.wasm with Chicory (a pure-JVM WASM runtime, zero native deps), run the serializer entry point, feed the result to the step-2 deserializer, and branch lib/rbs.rb on RUBY_ENGINE. Adds a JRuby CI job.

(Runtime = Chicory, format = compact binary — confirmed up front.)

What this PR does

• wasm/rbs_wasm.c — a reactor-model entry shim exposing a small, stable ABI:

  Export	Signature	Description
  rbs_wasm_alloc	(i32) -> i32	Allocate N bytes in linear memory, return the offset
  rbs_wasm_free	(i32) -> ()	Free a region from rbs_wasm_alloc
  rbs_wasm_parse_signature	(i32 ptr, i32 len) -> i32	Parse UTF-8 source; 0 = ok, 1 = parse error
  rbs_wasm_selftest	() -> i32	Parse a fixed sample; 0 = ok

• rake wasm:build / rake wasm:check — driven by the WASI SDK (WASI_SDK_PATH) and wasmtime (WASMTIME).
• .github/workflows/wasm.yml — installs the WASI SDK + wasmtime and runs rake wasm:check.
• src/util/rbs_allocator.c — drop unused <sys/mman.h> / <fcntl.h> / <sys/types.h> includes (only sysconf() is used) so the arena allocator compiles cleanly against wasi-libc. WASI has no mmap, and the code never called it; this is the only change needed to the shared C source.

No AST is serialized back to the host yet — that's step 2.

Testing

Verified locally with wasi-sdk-33 (clang 22) and wasmtime 45:

$ export WASI_SDK_PATH=/path/to/wasi-sdk
$ rake wasm:check
Built .../wasm/rbs_parser.wasm
WebAssembly selftest passed.

Also confirmed:

• valid RBS → rbs_wasm_selftest returns 0; malformed RBS → 1;
• the native C extension still builds and test/rbs/parser_test.rb passes (42 tests, 395 assertions, 0 failures) after the rbs_allocator.c change.

The compiled rbs_parser.wasm is a build artifact and is gitignored.

https://claude.ai/code/session_01LTveMt3NLbYHEboXuzAKpA

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Generated by Claude Code