Ability to mark a code block as non-removable.

[icywolfy]

Feature Use Case

If rollup incorrectly removes a chunk of code, the ability to force an expression to not be tree-shaken.
Provides a workaround for:

• users of rollup in non-standard environment, where rollup assumes a method is pure.
• uncommon scenarios, where rollup assumes no side-effects.
• bug in rollup, pending a fix

Admittedly, this is not for the main-stream user of rollup, but would be valuable for users who run into an issue.

Feature Proposal

/* @rollup-keep */
if (1 > 2) {
  doSomething();
}

In the specific case I am working with, the embedded js engine, in rare occasions, breaks the behaviour of >.
To work around this, we have to add the above if (1 > 2) check, which of course, is tree-shaken.
To work around rollup, we must add

  if (Math.random()*99.99 > 2) {
     doSomething();
  }

Then, on the rollup/minified bundle, replace the conditional back to (1 > 2).
Admittedly, seeing doSomething() report metrics once every few weeks is baffling.

Re: #2963
Re: #1771

[lukastaegert]

I assume putting such code into a separate file for which you explicitly deactivate tree-shaking is not an option?

[cefn]

This would also be valuable for my use-case. I am bundling Typescript to an ES5 environment (the CouchDB Spidermonkey engine), where a script is evaluated to define a map or reduce function.

So for example the following script evaluates to the value of myMap and defines a map function...

function myMap(doc){
  emit(doc.id)
}
myMap;

Since myMap is never actually referenced, when this runs with treeshake: true all code is removed as it is not exported. It would be valuable to be able to explicitly flag a symbol as being in use. Currently I simply have to disable treeshaking and have every imported function in its own file to prevent unused items making it into the bundle.

function myMap(doc){
  emit(doc.id)
}
/* @rollup-keep */
myMap;