Building with build.rs

[max-sixty]

Is there any facility for building wasm binaries within build.rs, rather than calling into wasm-bindgen's CLI?

Over at PRQL/prql#2881 we're hoping to use a build.rs file to build our wasm binaries (edit: and JS shims), mostly to take advantage of cargo's caching — wasm-pack seems to require a full rebuild even if no files have changed.

We were planning to use https://crates.io/crates/substrate-wasm-builder. That seems to allow building from build.rs. It's less focused on JS projects though, so probably need additional CLI calls into wasm-bindgen.

I also found https://github.com/rustminded/xtask-wasm/blob/main/src/dist.rs which also looks interesting, but doesn't seem to support building from build.rs.

(I also posted this at rustwasm/wasm-pack#916, so feel free to close if that's a better place for it)

Thank you!

[daxpedda]

You probably mean how to run wasm-bindgen from build.rs. Because wasm-bindgen itself doesn't build Wasm binaries, it only modifies them and produces a JS shim.

This is indeed possible with wasm-bindgen-cli-support. Which is whats used by wasm-bindgen-cli internally and includes all that's necessary to do the same.

[max-sixty]

Thanks @daxpedda .

Do you have any thoughts on whether the concept of building from build.rs is reasonable? I had a look at the dependents of wasm-bindgen-cli-support, and they all look to be build tools.

I'm surprised I'm the first person to think that cargo build should build the full project, including the JS shims — am I missing something?

[daxpedda]

I'm surprised I'm the first person to think that cargo build should build the full project, including the JS shims — am I missing something?

That is the desired goal ofc, unfortunately this requires a dedicated target in Rust, which doesn't currently exist. Something like wasm32-web. I'm not entirely sure what you intend to do btw, but you can't build the JS shim from build.rs for that same project.

The current flow looks like this:
Rust runs your build.rs -> then it compiles your crate -> produces a .wasm file -> you run it through wasm-bindgen-cli.

You can't let your build.rs generate the JS shim and fix you .wasm file because it runs before compiling the crate in the first place.
What you can do is do it from another crate.

[max-sixty]

am I missing something?

You can't let your build.rs generate the JS shim and fix you .wasm file because it runs before compiling the crate in the first place.

Ah, this is what I was missing! Thank you. So it's more difficult than I thought.

Thanks!

[max-sixty]

FYI — re the "can we do it all from build.rs" question — I think that https://crates.io/crates/substrate-wasm-builder is doing this by creating a new crate, compiling it, and then using the artifact as the .wasm files.

So it is indeed possible, but not easy...

[daxpedda]

FYI — re the "can we do it all from build.rs" question — I think that https://crates.io/crates/substrate-wasm-builder is doing this by creating a new crate, compiling it, and then using the artifact as the .wasm files.

Wow, that's pretty hacky. As far as I can read though they didn't find a nice way to prevent the native build, so it builds twice, once during build.rs and then the crate itself.

I think whats missing here is some post-build options for Cargo. Configuring custom runners works pretty nicely, I've been using wasm-server-runner for a while now and it's incredibly convenient.