TupleMirror inflates bundle size for Scala.JS

[strelec]

Compiler version

3.2.0, regression

Description

Because TupleMirror uses runtime.Tuples.fromProduct, Scala.JS brings Tuple1 ... Tupe21 into the source code, and that ends up consuming around 50KB of minified (fullOpt) JavaScript code.

So, at the very least, the Scaladoc is not accurate, as the effect is exactly opposite for Scala.JS.

But as we have seen with Vector (scala-js/scala-js#4699), it appears that the team is adding minor benefits to JVM at the significant expense to Scala.JS.

I think this should be discussed.

[bishabosha]

so either go back to anonymous classes in Scala.js mode or create some specialised tuple mirrors in the scala.js library?

[strelec]

Inspecting the commit that caused the regression in 3.2.0 and assessing its merit is the best first step.

[nicolasstucki]

Could you provide an example of code that generates the problematic fromProduct call?

We already optimize some of the tuple operations. I wonder if optimizing fromProduct when the product has a known arity might be a possible solution for these cases.