> By now I am consciously trying to limit any async parts of the program to a simple and stupid "Rust-light" style without any "fancy" features such as traits or closures, which does not feel like a proper solution.

I disagree that this is bad, and I think you've made a good decision.

I find that too many Rust programmers reach for a closure WAY too often and, even when they should grab a closure, they make it far too complicated. Closures should be short. Inline closures are nice when they're a single line part of "collect()" (although, I've seen some that make me want to hang the author ...).

However, if your closure is 15 lines long invoking a builder chain (this is a common issue in EventLoop type closures), that should be a function. This is before I get started about how builder chains are a gigantic misfeature to paper over the fact that the language doesn't have default/named function arguments.

Anyway ... I have found that "make it a named function and call it" is often a far better way to communicate exactly what your intent was.