Faster (and smaller) WASM runtime builds

[nazar-pc]

This tackles underlying issue from #12671 (partially) from a different angle by simply not compiling any dependencies in substrate-wasm-builder when compiling WASM version of the crate in the first place.

It is a bit awkward that WasmBuilder is copy-pasted and not implementing any shared trait with regular one, but I decided it might be acceptable for now.

What this means is that runtime is build faster and smaller.

target/debug/wbuild/node-template-runtime/target size:

• before ~1.5G
• after ~0.6G

I didn't bother measuring compile times though.

[bkchr]

Great idea! But can be improved.

[bkchr]

Good idea, but way too complicated. Just adding target dependencies with not(target_arch = "wasm32") should do it.
Then we also don't need to enable or disable any features.

https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#platform-specific-dependencies

[nazar-pc]

I tried that initially, the problem is that substrate-wasm-builder itself is never compiled for wasm32 architecture, it is always compiled to native machine code. The cleanest way to work around that that I found was to add an std feature.

I was hoping there is a way to distinguish host from target, but apparently not in this context.

[bkchr]

Good point, but you should be able to import wasm-builder conditionally in your downstream project based on the target_arch? Achieving the same without requiring any upstream changes?

[nazar-pc]

Almost, provide_dummy_wasm_binary_if_not_exist is still necessary for WASM, so I thought having it upstream in a backwards compatible way would be more convenient than reimplementing that function in every runtime.

[bkchr]

Why is that required? The wasm binary is only imported on std builds?

[nazar-pc]

Indeed. I recall there was something when I was testing, will check again later.

[nazar-pc]

Replaced with #12790