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:

Description

This will add a new option treeshake.correctVarValueBeforeDeclaration. This option will deoptimize the value of any var declarations to make sure that Rollup does not make wrong assumptions when variables are accessed before their declaration. This will even work in pathological examples like

checkFoo();
var foo = true;

function checkFoo() {
  if (foo) console.log('should not be shown');
}

without this option, this will be simplified to

checkFoo();

function checkFoo() {
  console.log('should not be shown');
}

which is obviously wrong. As this option can lead to heavy deoptimizations depending on the code, it is not switched on by default. It is still better than turning of tree-shaking altogether as basic tree-shaking will still work well: If a variable is not used, it will be removed.

In the meantime, we can work on finding a proper fix for this situation, at which point this option will be deprecated again.

Currently, it will be part of the "safest" tree-shaking preset introduced in #4131.