Squashing .wasm files

[jzabinski-dolios]

If I use these settings in Cargo.toml:

[profile.release]
lto = true
opt-level = "z"

And I compile this code:

fn main() {
}

#[no_mangle]
fn flip(a: u32, b: u32) -> (u32, u32) {
    (b, a)
}

Using RUSTFLAGS="-C target-feature=+multivalue" cargo build --release --target=wasm32-unknown-unknown

I end up with a file that is about 65KB. The only thing being used in the code will be flip, called directly from Javascript in a browser. The vast majority of that 65KB will be unused. (wasm-opt optimizes away about 2KB.)

How do I minimize the code size further?

[bjorn3]

• Try adding the following to Cargo.toml:

  [lib]
  crate-type = ["cdylib"]
  

  This also avoids the need to add fn main() {}.
• Add strip = "debuginfo" to the release profile to ensure the debuginfo for the standard library is removed. This doesn't happen by default even when not emitting debuginfo for the crate itself.
• I think -C target-feature=+multivalue requires recompiling the standard library with it to prevent ABI incompatibilities. This requires the unstable cargo feature -Zbuild-std.