There should be a way to disable `sideEffects: false`

[RReverser]

🐛 Bug description

Custom JS snippets can contain top-level side-effects. Right now, wasm-pack generated package.json includes sideEffects: false which marks every file as side-effect free, leading to potential bugs after bundling.

🤔 Expected Behavior

When JS snippet files are generated, they should be put into the sideEffects array, and only the JS file generated by wasm-bindgen should be assumed to be side-effect free since it's the only one wasm-bindgen has control over.

package.json allows to list potentially side-effect-y files as

{
  "sideEffects": [
    "path/to/snippet.js"
  ]
}

This will avoid bugs for snippets with top-level side-effects, while it won't decrease optimization level for common scenarios where the module is used via its main entry point.

[stephanemagnenat]

The problem is indeed significant. Because of side effects are ignored by default, the "main" Rust function annotated with #[wasm_bindgen(start)] is sometimes not called, because the call to wasm.__wbindgen_start(); in index.js is dropped during tree shaking. See #989.