Thats much smaller than I expected, especially given how large rust native binaries usually are. What allocator does that use? Does that 65k include its own malloc implementation like we needed for asmjs, or does WASM expose a system malloc library or something?

wasm exposes "memory sections", which the module states up-front how much it wants. It can also generally call a "grow memory" function that makes this space bigger. That's it, from the direct wasm perspective.

That said, this target uses https://github.com/alexcrichton/dlmalloc-rs

It should be possible to reduce that a bunch more, 65kB sounds high.

Running the binaryen optimizer on that wasm shrinks it by 6%. Probably more can be done on the Rust side.

Absolutely, yeah. Having a real linker will help with that as well. Until then, you can also compile with

    [profile.release]
    opt-level = "s"

(i.e., optimizing for size), to get it down to 23kB gzipped.