Unify wasm-bindgen output under -sWASM_BINDGEN, add -sWASM_BINDGEN=auto

[guybedford]

This depends on wasm-bindgen's __export/__force attribute support from wasm-bindgen/wasm-bindgen#5253 (landed on wasm-bindgen main, building on the attribute support that landed here in #27436). Until that ships in a wasm-bindgen release, the test crates and CI pin the wasm-bindgen library and CLI to the same wasm-bindgen main rev; they can move to a version pin once released.

This unifies wasm-bindgen output generation under a single -sWASM_BINDGEN path and adds -sWASM_BINDGEN=auto, building on the wasm-bindgen side change in wasm-bindgen/wasm-bindgen#5210. It supersedes #27179.

Previously -sWASM_BINDGEN was effectively a staticlib-only flow where emcc had to discover and own the export set itself. With #5210, wasm-bindgen hoists its clean exported API (functions, classes, enums) into top-level library symbols that self-register as public exports via the __export/__force symbol attributes (#27436, wasm-bindgen/wasm-bindgen#5253) in its --js-library. The user-facing API now comes from wasm-bindgen's own library in every flow, so emscripten no longer needs to guess or own exports — it just surfaces what wasm-bindgen registered. That lets one -sWASM_BINDGEN path serve both whether cargo/rustc drives the link (bin crate, emcc as the linker, rustc supplies -sEXPORTED_FUNCTIONS) or emcc drives it (staticlib, exports discovered locally).

The model for a public export is the union across both systems: wasm-bindgen's self-registered API, plus emscripten's own EMSCRIPTEN_KEEPALIVE exports — minus wasm-bindgen's internal expansion glue, which must not spill into the public surface.

What's implemented:

• The exports the wasm-bindgen expansion reaches by name — the supplied EXPORTED_FUNCTIONS (method shims, the __wbindgen_* runtime, the marker, main) plus anything its expansion adds — are captured as internal glue and kept off every export layer: the ESM wrapper (user_requested_exports), the factory Module attachment (EXPORTED_FUNCTIONS, via should_export), and the keepalive pass in finalize_wasm. main still runs automatically on init; _main isn't surfaced.
• A genuine EMSCRIPTEN_KEEPALIVE C/C++ export is not in that internal set and remains surfaced, so a hand-written native export composes with wasm-bindgen's API in the same module.
• Placeholder symbols wasm-bindgen consumes (__wbindgen_describe*, __externref_*, ...) are stripped so they aren't reported as undefined exports.
• nm-based export discovery only runs for explicit -sWASM_BINDGEN when no driver supplied EXPORTED_FUNCTIONS; the rustc-driven link already lists them exactly.
• Imported JS is wired through: library_bindgen.extern-pre.js is fed as extern-pre-js and the snippets/ dir is copied next to the output so relative imports resolve.
• Under WASM_ESM_INTEGRATION, wasmExports is provided via a namespace import of the wasm so wasm-bindgen's by-name export access works. This is gated behind WASM_BINDGEN so plain ESM integration keeps per-symbol named imports and their tree-shakability. (The wrapper re-export of self-registered JS library symbols itself landed in Support JS libraries self-registering their exports with visibility attributes #27436.)

-sWASM_BINDGEN=auto (replacing #27179's implicit marker detection, per review feedback there): runs wasm-bindgen only when the linked wasm carries wasm-bindgen's __wasm_bindgen_emscripten_marker custom section — how cargo/rustc opts in when driving emcc as the linker. Otherwise it's a no-op and wasm-bindgen need not be installed, so rustc can always pass it via -Clink-arg without affecting ordinary builds.

if settings.WASM_BINDGEN == 'auto':
    settings.WASM_BINDGEN = 1 if building.is_wasm_bindgen_module(in_wasm) else 0

Both output modes then expose only the clean API (e.g. a Greeter class).

Testing:

• End-to-end test parameterized over the ESM (-sWASM_ESM_INTEGRATION) and factory (-sMODULARIZE -sEXPORT_ES6) output modes, built via cargo build with -sWASM_BINDGEN=auto (marker auto-detected), asserting the clean Greeter API works, main runs on init, and none of the raw wasm exports leak. (The test crate passes -sDEFAULT_TO_CXX as a link arg since rust's precompiled std requires emscripten's C++ runtime libraries but rustc links via emcc, needed since Disable C++ linking (DEFAULT_TO_CXX) by default #27469.)
• A no-marker test asserting -sWASM_BINDGEN=auto is a no-op for an ordinary hello_world.c build (doesn't require wasm-bindgen installed).
• The staticlib integration test is extended to assert an EMSCRIPTEN_KEEPALIVE native export survives alongside the wasm-bindgen API (fails under a blanket export wipe).
• CI installs a pinned wasm-bindgen-cli alongside rust so the flow is always exercised.

Not in scope (follow-up): preserving human-supplied EXPORTED_FUNCTIONS through wasm-bindgen linkage (the rustc-supplied set is currently indistinguishable from glue, so explicit exports aren't surfaced).

[guybedford]

@walkingeyerobot has now also confirmed that this unification approach works in his testing too.

[walkingeyerobot]

@sbc100 Sam, this looks good to me, but I'd appreciate it if you could give it a quick look before we hit the merge button. Thanks!

[guybedford]

This is now ready for final review.