split async function handle_button_press() { … }

This would cause the bundler to inject a split point & know how to hide that + know what needs bundling and what doesn’t. GWT pioneered almost 20 years ago although not a fan of the syntax they invented to keep everything running within stock Java syntax: https://www.gwtproject.org/doc/latest/DevGuideCodeSplitting....

Why do you want them explicit in the code?

The tooling producing such annotaion bwsed on historical visits whould conctantly change the annotated set of functions, I suspect, whith app versions evolving.

Note, the inactive code parts are very often in 3rd party libraries.

Some are also in your own libraries shared between your different application. So the same shared library whould need one set of split-annotated functions for one app, and another set for another app. So these conflicting sets of annotations can not live in the library source code simultaneously.

For example the automated system could take your non-split codebase & inject splits at any async function & self-optimize as needed. The manual annotation support would be to get the ecosystem going as an initial step because I suspect targeting auto-optimizing right from the get go may be too big a pill to swallow.