Treat unknown prototype props as unknown

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
Resolves #4425

Description

Following the argument provided in #4425, this will deoptimize unknown array (and object) properties in the following way:

• Accessing an unknown property is treated as an "unknown" value instead of undefined.

  const object = {};
  if (object.missing) console.log('will be retained because Object.prototype.missing might be defined');

  There are two exceptions:
  • When we know there is no prototype by using __proto__: null in an object literal

    const object = { __proto__: null };
    if (object.missing) console.log('will be removed');

  • When the property is a number because we do not expect builtins to be numbers. This also helps for array literals:

    const object = {};
    if (object[0]) console.log('will be removed');
    const array = [];
    if (array[0]) console.log('will be removed');

• Calling an unknown function if the name is not numeric will deoptimize all properties (i.e. we assume it may mutate the object/array):

  const object = { x: false };
  object.unknown();
  if (object.x) console.log('will be retained because calling unknown could have mutated object');

• Accessing/assigning unknown properties is not expected to have side effects because we do not expect new builtins to be getters/setters with side effects (other than the obvious side effect of assignments to change a value).

This also means that the previous feature of treating unknown object properties as undefined is disabled with this PR.

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#gh-4425_unknown_prototype_props

or load it into the REPL:
https://rollupjs.org/repl/?pr=4428

[codecov]

Codecov Report

Merging #4428 (9c9c6d3) into master (9c8894e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4428   +/-   ##
=======================================
  Coverage   98.76%   98.76%           
=======================================
  Files         204      204           
  Lines        7309     7316    +7     
  Branches     2077     2079    +2     
=======================================
+ Hits         7219     7226    +7     
  Misses         33       33           
  Partials       57       57           

Impacted Files	Coverage Δ
src/ast/nodes/shared/ObjectPrototype.ts	100.00% <100.00%> (ø)

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