How to handle the differing syntax of each databases would be handled at a higher level, by the query builder or orm. The orm could decide to let the user decide for themselves whether to write portable sql or not and fail at runtime, or like diesel in rust, enforce the useable features through its types.
In my experience that is something that is currently missing in the rust ecosystem. Each library seem to have its own connection logic and if I want to write a program that could run on different databases in rust right now, I have to write different code to connect to each database I would like to support.
Diesel has been serving me really well but I currently need to choose a specific backend when I am building a query, and implementing the above dynamic style of connection would be quite cumbersome to implement. It would be nice if we could choose another a standard sql backend where I would only be able to use a subset of features supported by most sql databases and build queries against that. Hopefully the RDBC project could help with something like that.