WASM Support

[elliotsayes]

Trying to build for web gives:
don't know how to build Lua for wasm32-unknown-unknown
..originating from the lua-src-rs package. That package does support wasm, but only the wasm32-unknown-emscripten target which doesn't work with bevy

[makspll]

As you see this is likely not possible until both mlua and bevy support a common wasm target

[thomas992]

What about WASMtime?

[Shatur]

Looks like we'll get a WASM-compatible backend soon: https://mastodon.world/@danielsreichenbach/116108388959479041

[makspll]

What about WASMtime?

can WASMtime run in browser? What would be the use case for that target ?

[makspll]

Looks like we'll get a WASM-compatible backend soon: mastodon.world/@danielsreichenbach/116108388959479041

Yup! Daniel has popped in to the bevy discord too, exciting!

[julien-blanchon]

Hey @makspll any news about that. I need this feature in my projects and I'm currently using the danielsreichenbach fork (https://github.com/danielsreichenbach/bevy_mod_scripting/tree/feature/rilua-backend). Is this expected to come here anytime soon ?

[makspll]

@julien-blanchon I am not currently planning on adding new language backends (i.e. rilua), I believe mlua should be able to support WASM as a target soon, which would mostly work, although I imagine we'd need to fix up some things in the codebase. In theory though if rilua supports WASM, an external crate could implement its scripting plugin without my help (would be curious to see what needs to change in the codebase if anything to compile fine on WASM)

[ianm199]

Working on unblocking this. First step is getting the core to actually run on wasm — opened #543 which fixes a std::time::Instant call in the pipeline that panics at runtime on wasm32-unknown-unknown (swaps to bevy_platform::time::Instant). The remaining blocker is the Lua backend's C dependency; I'm looking at a pure-Rust Lua VM for that next.

[ianm199]

@makspll follow-up to my earlier comment. The external backend works now.

A pure-Rust Lua backend running in a Bevy wasm build: https://ianm199.github.io/bms-lua-rs/

It's Conway's Game of Life scripted in Lua, running on wasm32-unknown-unknown with no C and no emscripten. The script is bms's own game_of_life.lua, used as-is: it reads and writes the LifeState component through the reflection bridge (world.get_type_by_name, world.query, the component's cells). The page also lets you edit the Lua live and shows sliders the script declares for itself.

Some details:

• It's a separate backend crate (bevy_mod_scripting_lua_rs) that sits alongside mlua/rhai/rune and doesn't touch the core, which is the external-backend approach you mentioned for this.
• It's backed by lua-rs (https://github.com/ianm199/lua-rs), a pure-Rust Lua 5.4 that passes the upstream test suite, now on crates.io. Being pure Rust is what lets it link into the wasm-bindgen/wgpu module Bevy emits.
• The only core change it needs is fix: Use bevy_platform::time::Instant in the script pipeline #543, the std::time::Instant fix that panics on wasm. That one would help any wasm backend, including mlua once it builds for wasm.

Repo with the full write-up, including the one annoying bug (a wasm stack overflow inside the pipeline): https://github.com/ianm199/bms-lua-rs

Caveats: it needs a bigger wasm stack via a RUSTFLAGS link-arg (16 MB), it depends on the patched core until #543 lands, and the reflection coverage is enough to run the example rather than a complete binding. Posting it in case it's useful to anyone here.