DO NOT MERGE Simulation: What if we dropped support for ES 5.1. #5129
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That support is the biggest source of alternative code paths and polyfills in our codebase. The polyfills and other alternative implementations we need to support ES 5.1 include: * `Math.fround` to support `Float` operations (in `CoreJSLib`). * Other bit manipulation magic for floating-point numbers (in `FloatingPointBits`). * Using dynamic JS `Array`s instead of typed arrays for Scala `Array`s of primitive types. * Using custom Java `Map`s instead of `js.Map`. * Various implementations of math methods in `jl.Math` (although these may come back for a Wasm-only target, along with more of them). * Using run-time generated random property names instead of `Symbol`s. * Very complex code to handle surrogate pairs in `ju.regex.*`. * (Imperfect) desugaring of `...rest` parameters and `...spread` operators. Moreover, the ES 5.1 output does not even have exactly the same semantics as later versions: * JS classes are not true `class`es. Notably, that means they cannot extend native ES classes, and they do not inherit static members. * Top-level exports are declared as `var`s instead of `let`s. 10 years after the instruction of ECMAScript 2015, I believe it is time to deprecate the support for Es 5.1, so that we can eventually remove it.
In this commit, we do the minimum changes to remove `ESVersion.ES5_1`. All the supporting code is still present elsewhere. We will add separate commits for the various areas that can be removed.
And remove support for `!useECMAScript2015Semantics` in the linker.
Remove all the `assumeTrue(esVersion >= ES2015)` tests, as it is now always true by construction.
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Except in `ju.regex`, for which we will have a separate commit.
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#5128 was merged. This PR did its job. I guess we'll see these commits again in a few years. 😅 |
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This is a simulation of all the things we could remove if we dropped support for ES 5.1 (in the future). This is meant as "supporting evidence" for #5128.