Improve Rust compile times

[horenso]

Adding a newline to main.rs the backend and recompiling it takes ~8 seconds. This is a drag and there might be oportunities to improve this.

We could split up the backend into seperate crates, in Rust a crate is one compilation unit. As an experiment, I seperated the model (database and dto) into its own layer on this branch and it improved it already to ~5 seconds, but it was done very quicky and I believe it could be done much better.

How would the backend be split up?

There are different ways to structure projects in Rust. I suggest:

The backend directory:

Cargo.toml
lib-db/
   Cargo.toml
   ...
src/

What does this improve?

• Compilation speed
• Seperation of concerns, the database layer can have its own dependencies.

Disadvantages

• It moves files around, so it should be done quickly (in the sense that if or when we split up into crates it should be reviewed quickly to not stall development).

It's not an urgent issue, but at some point when compile times increase we should spilt up the backend at some point.

[markus2330]

Thank you for creating this issue!

The gain of compilation speed 8->5s is quite minimal and I also don't think there is much more potential (as afaik Diesel causes most of the compilation time).

Splitting up in 3 modules make sense, so that the module structure represents our architecture. But only if it does not involve too much time effort, as the gain is not huge.