WASM is really the greatest thing, but it's lacking proper support from compilers. Binaryen seems like a beast to use. I don't really understand why WASM is not just supported natively directly by clang or gcc.
Binaryen is an optional optimizer that you can run on the emitted wasm, to make it smaller (either manually, or a toolchain may integrate it for you, like emscripten or wasm-pack).
I mean it is still under development but I think it is usable. Here's a quick intro I found some time ago: https://dassur.ma/things/c-to-webassembly/
We wouldn't have been able to put wasm in Firefox like this if there wasn't decent compiler/runtime support via the clang ecosystem.
The Rust code, by contrast, slurps up the entire file contents into a buffer before writing it out. If someone is going to write code that way why even bother with a low-level language? (This "memory is infinite" mentality is something I've noticed in other Rust projects, even major ones like mio.)
Beyond that, that file is just a simple example for showing how to work with the toolchain and the sandbox.