Improve comments heuristic

[Nadrieril]

Fixes #388.

[msprotz]

FYI, just gave this a quick try, and I don't seem to be receiving the comments at all anymore (or at least, the one I noticed had a problem)

[Nadrieril]

Yeah I marked it as draft because it doesn't work for complex cases such as destructuring or assert! that you test in Eurydice

[sonmarcho]

This seems to be rotting. @Nadrieril do you have any plan for this PR?

[Nadrieril]

I was hoping to get back to it when higher-priority items are done, but tbf I don't know how to go about this. The few heuristics I tried don't work and the last idea I had required an important change to the visitors library we're using (so we can hold on to borrows while visiting)

[msprotz]

Is no one else in the Rust ecosystem doing this? (Retaining inline comments.)

Should this be a discussion with upstream Rustc to add more support in the compiler itself?

[Nadrieril]

I don't know of anyone else doing that no