UPDATED PROPOSAL

UglifyJS2, as of 2.8.0, now supports marking pure with this comment:

/*#__PURE__*/

I propose that Rollup adopt similar behavior to help with tree-shaking.

PREVIOUS PROPOSAL (before UglifyJS release)

I'm proposing adding eslint-style comment annotations to give users more control over tree-shaking:

tree-shaking: pure "Affected expressions will not mark their nodes for inclusion"
tree-shaking: auto "Current behavior"
tree-shaking: side-effects "Affected expressions mark their nodes for inclusion"

Overriding expression marks on a single line:

Greeting.propTypes = {  // rollup-tree-shaking pure
...
}

Overriding expression marks in a block, reverting to auto at the end of the block:

/* rollup-tree-shaking pure */
Greeting.propTypes = {
...
}

Greeting.defaultProps = {
...
};
/* rollup-tree-shaking auto */

Overriding marks for all expressions in a file:

/* rollup-tree-shaking pure */

Marking a line as having side-effects. This expression is marked for inclusion:

documentElement.style.color = 'red';  // rollup-tree-shaking side-effects

Pros

• The mutable nature of JS makes it impossible to statically analyze common statements, so a lot of potentially pure statements are left on the table when tree-shaking. These annotations allow users to say "I'm not writing maniacal JS" and get drastically smaller bundle sizes.

Cons

• Build tools that alter code before passing to rollup could shift comments to the wrong line, and apply annotations to the wrong statement(s).
• Adding program changing logic in comments. There's different camps of thought on this, but generally, comments aren't meant to alter code.
• Build tools that strip comments could remove annotations if applied in the wrong order.
• Rollup specific logic. Might make it harder to interop with other build tools.
• Could become a crutch that prevents rollup tree-shaking bugs from being fixed. If rollup is doing the wrong thing, adding the annotation would allow users to patch it and move on.
• If users don't understand the risk of overriding rollup's side-effect detection, it could cause unexpected side-effects. Maybe the pure annotation should be unsafe-pure to make the risk clear.