Add `wasm32-wasip1` target support for `ruby-rbs-sys` (cross-platform bindgen issues)

Summary

ruby-rbs-sys 0.3.0's build.rs has no wasm32-wasi* target awareness — there's no way to point cc at a wasi-targeting compiler or propagate the same flags to bindgen. I drafted a build refactor mirroring ruby-prism-sys's pattern (split build.rs into build/main.rs + build/vendored.rs, detect the wasm target, route everything through wasi-sdk). The C compile of vendored librbs to wasm32-wasip1 then succeeds cleanly. However, the bindgen step fails differently across three different libclang implementations, and the failure modes suggest a deeper bindgen-on-wasm issue that may also affect ruby-prism-sys — but isn't surfacing there because its wasm CI doesn't actually run.

This issue documents what I found in case maintainers (or anyone with deeper bindgen-on-wasm experience) can spot the underlying cause. I'm happy to send a PR for the build-refactor portion regardless — that part is mechanical and parallels prism cleanly. The bindgen question is what I'd like guidance on before proceeding.

Goal

Build ruby-rbs (and consumer crates) for wasm32-wasip1 so it can run in browser environments via wasi-libc shims. Used in a downstream project that ingests RBS signatures alongside Ruby source for a transpile-time type-resolution pipeline.

What works after the build refactor

• Vendored librbs C compiles to wasm32-wasip1 cleanly with WASI_SDK_PATH set, using -D_WASI_EMULATED_MMAN=1 (defensive <sys/mman.h> includes in rbs_allocator.c; no actual mmap calls, so no link against libwasi-emulated-mman needed).
• Native build path is unaffected; the wasm code is gated on target.contains("wasm32") && target.contains("wasi").
• Bindgen is invoked with the same -I, -D, and --sysroot flags as cc via BINDGEN_EXTRA_CLANG_ARGS_<target> propagation — same convention ruby-prism-sys uses.

Three libclangs, three failure modes

The pattern looks like bindgen handles forward-decl-then-typedef structs differently across libclang versions when running with --target=wasm32-wasip1 --sysroot=<wasi-sysroot>. The native build (no wasm target, no sysroot override) handles them correctly across all three libclangs.

Why prism's pattern alone may not be a sufficient precedent

I initially assumed ruby-prism-sys had wasm support figured out and copying its build pattern would suffice. The build-script pattern transferred cleanly, but the bindgen issues surfaced anyway. On checking: ruby-prism's wasm CI doesn't appear to actually run. The repo has rust/ruby-prism/examples/wasm/{main.rs,run.sh} and the rake cargo:examples task that invokes them, but I don't see a GHA workflow job that exercises the wasm path end-to-end — rust-bindings.yml adds targets: wasm32-wasip1 to the rustup-action but I couldn't find a job that actually runs cargo build --target=wasm32-wasip1 on prism. Maintainers can correct me if I'm reading this wrong.

If that's right, prism may share a similar issue; it just hasn't been forced to surface. If it's wrong, the prism-specific differences (e.g., PRISM_EXPORT_SYMBOLS macro, .size_t_is_usize(true), .layout_tests(true) configuration) are clues for what RBS may need.

What might help

I'm not sure what the right fix is. Plausible directions:

1. Disable layout_tests for the affected struct names. Workaround for the brew-libclang failure mode; doesn't address the Apple/Ubuntu modes.
2. Investigate the typedef-after-forward-decl handling — possibly an ordering or __attribute__((visibility)) interaction with wasm targets that bindgen's libclang sees differently across versions. May require a minimal repro filed against rust-bindgen.
3. Generate bindings on the host platform, vendor them into the crate, skip bindgen entirely for wasm builds. Ugly but unblocks downstream.

If maintainers have a preferred direction, I'd be glad to send a PR. The build-script refactor (the wasm-detection block alone, mirroring prism) is independently useful and could land first.

Start with build.rs and the proposed build/main.rs and build/vendored.rs split, comparing the referenced ruby-prism-sys pattern. Reproduce the wasm32-wasip1 build with WASI_SDK_PATH and the reported libclang variants, then determine whether bindgen flags or layout handling resolve the generated bindings; done means native builds remain unaffected and wasm consumer crates compile.