Some small refactorings

[jplatte]

I first thought I couldn't remove use diesel; because some crates didn't have edition = "2018" and started working on that. Then I realized the code the macros generate have to work in the 2015 edition so I probably can't remove the redundant import. (I still don't understand why things break inside diesel when I remove it, but that's not that important)

If you want me to split this PR or remove the first commit altogether, I'm fine with that.

[weiznich]

I would prefer to have the 2018 edition change in a different PR.

Then I realized the code the macros generate have to work in the 2015 edition so I probably can't remove the redundant import. (I still don't understand why things break inside diesel when I remove it, but that's not that important)

So the problem here is the following: We use the macros also in diesel itself, as they are useful for us too. For example sql_function! is quite widely used internally to define various sql functions. In the end all of those macros generate some code that relies on types from diesel being scope. That needs to be wrapped somehow by the macro, as we cannot assume that the user will import all of those types (Some of them are even not considered part of the public API). That implies we need something to import from. For cases where someone else uses diesel we just assume that there exists a crate named diesel that we can import. That use diesel; brings it into local scope. For our internal use things are a bit more complicated. We decided to workaround that by just having a mod diesel { pub use super::*;'} in our crate root. That allows proc macros to refer to diesel as like it was an external crate even inside diesel itself. In the end it's just a hack to workaround that there is nothing like $crate for proc macros.

[jplatte]

Interesting! Should I add a comment to the use diesel; import that explains this?

[weiznich]

That's a good idea, otherwise I've forgotten that the next time I look at that code 🙈

[jplatte]

So apparently CI does cargo fmt on 1.36.0 while you have a rust-toolchain file with 1.37.0? Not that it makes a difference here (they produce the same output, some later version changes things up in one place), I had a local rustup override to see newer clippy lints, but it does confuse me a bit.

[jplatte]

CI failure seems unrelated.