Enable use of wasm-pack with panic_immediate_abort

[sffc]

I'm trying to enable panic_immediate_abort in order to get smaller wasm size (see rust-lang/rust#54981). The workflow is to create a Xargo.toml file:

[dependencies]
std = {default-features=false, features=["panic_immediate_abort"]}

However, I can't figure out how to invoke xargo when using wasm-pack build. I can run xargo build --target wasm32-unknown-unknown --release, which I assume puts the output in the target directory without compiling it to the pkg directory, but then wasm-pack build just overwrites the target directory again with the non-xargo build.

[Ngalstyan4]

I think being able to use xargo instead of cargo has more general uses as well and has to be supported. For example, using wasm32 simd extensions is currently unstable and requires recompiling libcore in order to enable them so in order to take advantage of these in WebAssembly, one has to recompile libcore, typically done by xargo.

For reference, I currently use

xargo build --target wasm32-unknown-unknown --release
wasm-bindgen target/wasm32-unknown-unknown/release/hello_wasm.wasm --out-dir pkg --target web

for the desired effect.

[Pauan]

Note that it's no longer needed to use xargo, since build-std is now a part of Rust itself (nightly only):

wasm-pack build --release -- -Z build-std=std,panic_abort

This works for enabling simd as well. You simply create a .cargo/config file in your project which contains this:

[build]
rustflags = ["-C", "target-feature=+simd128"]