Mark snippets and the bundler target's main file as having side effects

[Liamolucko]

This is a less extreme version of #1208, which only marks snippets and the main file on the bundler target as having side effects instead of all files.

This means that the JS shim file which contains the vast majority of the JS code is still properly marked as having no side effects, allowing bundlers to get rid of things like unused new TextEncoder calls which could theoretically have side effects but don't.

Fixes #972
Fixes rustwasm/wasm-bindgen#3276

[cryptoquick]

I tried testing out your branch, but unfortunately I ran into this issue here: #650 (comment)

It's unrelated to what's solved here, but if I can solve that, I can test if this resolves rustwasm/wasm-bindgen#3306 and #1230.

[Menci]

I think only bundler target's sideEffects should be set to the entry file. web target should keep false.

[Liamolucko]

I think only bundler target's sideEffects should be set to the entry file. web target should keep false.

It's already effectively false for the entry file. On the web target, sideEffects is set to ["./snippets/*"], which means that only the files inside of snippets are treated as though they have side effects. Everything else, including the entry file, is treated as though sideEffects is set to false.

The reason why snippets are marked as having side effects is because they're arbitrary user code; we've got no idea what they do, and we don't want to assume that they have no side effects and cause them to break.

[martinxyz]

I've tested this branch (after a merge with master) and it fixed the issue for me.

[Sytten]

What's missing to move this forward? It's an annoying bug right now

[Liamolucko]

What's missing to move this forward? It's an annoying bug right now

It's just waiting on a review from @drager.

[drager]

Thanks a lot!

[stephanemagnenat]

Thank you for doing that!

[cryptoquick]

Thanks! 🙏