That is a very unusual Rust experience. I find "application code" very pleasant to write in Rust. Of course there are things that aren't as ergonomic in Rust as in other languages (e.g. callbacks) but that's true of pretty much any language.

I have heard this reaction from others before. One of the Rust expert friends I consulted with told me "I'm not convinced you're not trying to write Haskell-style code in Rust;" I told him the patterns I was struggling with were both trivial and common in Java.

The things I found quite difficult or impossible in Rust were to me pretty basic patterns for modularity and removing duplication that it's really shocking that these complaints are not more common.

I currently have but two hypotheses for why.

First, the second problem I mentioned only comes from using tokio, which causes your top-level program to secretly be using a defunctionalized continuation data type, derived from where exactly in other files you put your await's, that might not be Send. If you're not using tokio, you won't experience that issue.

Second...I was kinda told to just give up on deduplication and have lots of copy+pasted code. This raises the very uncomfortable hypothesis that Rust afficionados are some combination of people who came to Rust early and never learned traditional software design and don't know what they're missing, and people who were raised on traditional good software engineering but then got hit with Rust's metaphorical baseball bat of lack-of-modularity over and over until they got used to being hit with a baseball bat as a normal pain of life.

I don't like either of these explanations (esp. with tokio seeming quite dominant), so I'm awaiting an explanation that makes more sense. https://xkcd.com/3210/