[RFC] Rework embind_gen.js to be able to improve typings based on actual Emscripten settings

[Dabolus]

As of today, TypeScript typings generation is a bit limited by the fact that-to run the JS wrapper script that generates the typings-the settings provided to Emscripten need to be tweaked (see https://github.com/emscripten-core/emscripten/blob/main/tools/link.py#L1907-L1955).

TS generation is also limited by the current approach to accessing settings in the scripts, that doesn't allow e.g. to iterate over the exported runtime methods to decide what to expose on the typings side.

I'm not sure what the best approach would be, but it would be great to make the script receive the original settings as native JS object somehow.

To demonstrate what I mean, I created a PoC branch on my fork (see main...Dabolus:feat/better-output-types), where I successfully managed to create more reliable types by providing the original settings as a base64 encoded arg to the JS script (I know this isn't ideal, but again, it's just to better explain my point).

[sbc100]

Is this a duplicate of #20296?

[Dabolus]

Not exactly, but it's related to it. Finding a proper way to provide Emscripten settings to the JS tool would solve that issue, plus many other use cases. A good example is what I did in my branch by adding the const declaration and the default export when the corresponding flags are provided, which is something I think isn't tracked anywhere but that would definitely make the typings more accurate.

[sbc100]

I think that is what #20296 is all about. The status quo is that --embind-emit-tsd specifically relates only toe embind stuff (i.e. stuff defined in EMSCRIPTEN_BINDINGS).

Extending our typescript generation to apply to other things like EXPORTED_RUNTIME_METHODS is what #20296 is about. It will probably involve renaming the setting to something less embind-specific and running the code somewhere that is not in src/embind/ I think.

[sbc100]

BTW, the JS library code such as embind_gen.js should already have full access to all settings as global variables. You can use the {{{ }}} macro system to embed them at build time.

For example var foo = {{{ EXPORTED_RUNTIME_METHODS }}}; will generate code that statically contains the EXPORTED_RUNTIME_METHODS list.

[sbc100]

I think the direction you are taking here looks good though! Would you be interesting in help work on this feature? I think @brendandahl has some thought about the way forward.

One thing I think we need to more testing of the typescript bindings in our CI so we can verify that working and correct over time.

[Dabolus]

Cool, didn't know about the {{{ }}} macro! However, those are still the currently defined Emscripten settings, right?
Cause one issue I've found while trying working on this is that some settings are overridden when running embind_gen.js, so for example you have no way to understand whether EXPORT_ES6 was initially enabled or not due to https://github.com/emscripten-core/emscripten/blob/main/tools/link.py#L1927, which is the reason why I provided to the script as arg original_settings instead of just settings.

Any help would definitely be appreciated! I started using Emscripten only recently, so my knowledge is still very limited. 🙏🏻

[Dabolus]

Bumping this issue since I'm still very interested in helping with this 🙏🏻

In particular, the main blocking point is having a proper way to access the original_settings variable from the embind_gen.js tool

[brendandahl]

I added support for generating ts definitions from runtime exports in #21517. Was there more that is needed?