Rewrite most of the crate with updated dependencies

[LukasKalbertodt]

Closes #8

So I'm basically done with rewriting everything. Of course I didn't just rewrite it (just) for fun, but to update syn, quote and proc_macro. We now use the following versions (the newest as of time of writing):

• proc-macro2 0.4.6
• quote 0.6.3
• syn 0.14.4

The new implementation can now compile the compile_test examples (and more). I guess I reached feature parity. I am not aware that I changed anything about how this crate works. The attribute syntax is the same as before, and I generate the same impl blocks as before (again, as far as I am aware).

I think the old and new implementations are somewhat different:

• The new implementation uses much less own data structures and instead uses syn structures to pass information around.
• Most functionality in the new version is implemented in free functions which usually receive syn data structures and return a token stream.
• The new implementation uses proc_macro::TokenStream instead of the proc_macro2 version where possible in a useful way.

I managed to reduce total line count despite my code having more comments and adding two examples. I guess this is simply due to more ergonomic APIs by syn and proc_macro.

---

As far as I can tell, I improved diagnostics in the new version. Example:

---

Notes for the Review

I know this is a pretty big PR and reviewing it thoroughly might not be possible, because it would take too long. But I'd appreciate it if you could take a quick look and maybe tell me if anything is still unclear. I can always add more explanations in comments if necessary.

---

Things I plan on doing after this PR has been merged

(I'd create an issue for each of those points)

• Make this work with in-band lifetimes (there is a TODO comment about this in the code)
• Improve the test suite, in particular:
  • More unit tests
  • More compile_test tests
  • Add compile-fail tests
  • Make cargo test execute the compile-test and compile-fail tests
• Create better documentation and update the README, if necessary
• Improve names for the lifetime and type parameter we introduce (just take T or something simple if it's not taken yet)
• A subset of the proc-macro API will be stabilized for Rust 2018 (see rustc: Stabilize the proc_macro feature rust-lang/rust#52081). Unfortunately, the API in proc_macro that allows the nice kind of diagnostics shown above won't be stabilized by that PR and will thus probably remain unstable for a while. We probably should talk about whether we want this to work on stable Rust 2018 or not. Of course, the best solution would be to use awesome diagnostics on nightly and fall back to a less nice solution on stable. But I'm not yet sure how difficult it would be to use such a fallback mechanism.
• Possibly more, but I can't think of anything I missed...

[KodrAus]

Hi @LukasKalbertodt!

Thanks so much for taking this on! I'll set some time aside this week to go through it all. I'm really keen to see how it all comes out using the newer APIs, I think auto_impl is a pretty nice use-case for showing off proc macros.

[LukasKalbertodt]

@KodrAus I updated the code in response to rust-lang/rust#52081.

After this PR is merged, it's time to talk about what nightly features we really want to use. As far as I understand it, the status from the linked PR is what will "land" in Rust 2018. This excludes nice diagnostics (which I use in this PR) and some Span methods, most importantly def_site() for def site hygiene (which I currently don't use, but I think should be used by this crate).

As I probably already said somewhere, I think it's possible to add a nightly feature to this crate which enables better diagnostics and let's this crate avoid the ugly __AutoImplProxyT hack (which I intend to make less ugly, but ... using proper hygiene is better IMO). Without the nightly feature, this crate should be able to compile on stable. That would be pretty neat, as I agree with you that this crate is a nice example crate to show off proc macros!

[KodrAus]

@LukasKalbertodt Thanks for your patience on this. I just moved house over the weekend and tried to get to this before then, but missed the boat. This is looking nice and clean! You've done an awesome job. I'm happy to get this merged.

As for the nightly feature, I really like the idea of getting this working on stable. I've fallen a bit behind on the state of proc macros recently, where do you expect the seams to be? Would the nightly feature mostly just be adding in support for missing features without deeply affecting the way things are structured?

[LukasKalbertodt]

Awesome, thanks for taking a look and merging!

Would the nightly feature mostly just be adding in support for missing features without deeply affecting the way things are structured?

I think it should be possible to add nightly features on top without restructuring everything, yes. Right now it's mainly Diagnostics anyway. For the stable version we could just define our own Diagnostics type that simply panics to show the error. So yeah, I think it would work fine.

---

So I'll probably make a plan of things I want to do in the next couple of weeks. I'll create issues for everything that is worth discussing (like the two issues I already opened). I hope you don't mind me creating issues and maybe a milestone on your repo?

[KodrAus]

I'll create issues for everything that is worth discussing (like the two issues I already opened). I hope you don't mind me creating issues and maybe a milestone on your repo?

Please go right ahead! At this stage I think it's fair to say you've got a much clearer idea of where this crate should go than I do.