Change https://golang.org/cl/262018 mentions this issue: x/tools/gopls: support fillstruct for partially-filled structs

https://golang.org/cl/262018 now adds support for partially filled structs - wooo! But, all comments get destroyed. =)

Coming to a crossroads, now that I'm tackling comments. The ast package is proving to be a huge hindrance, since fillstruct doesn't use *ast.File in its implementation. Instead, it prints with token.FileSet. I'm not seeing a way to print subsets of the file (which seems to require token.FileSet) with comments (which appear to only exist on ast.File and printer.CommentedNode).

I had hoped that printer.CommentedNode would do the trick, but printing printer.CommentedNode with an ast.Expr and some comments seems to not work. (Does that meet expectations?)

So, the crossroads I'm at:

Option A: github.com/dave/dst

It looks like github.com/dave/dst has what I'm looking for: comments tied directly to nodes. But, introducing a v0 dependency seems sketchy.

Option B: hacky printing

I vaguely bet there's a way to figure out the fields that are already set (expr.Elts), then just go to the end of the struct and stick zero values of all the not-set fields at the end with printing hackery. That would leave all the comments and set fields up at the top, and all the zero'd values at the bottom. Not pretty but I thiiiink doable?

@joshbaum @stamblerre are there any directions that I'm missing? If not, any preference on the above?

I think Option B would be best, as we'd like to avoid adding dependencies to x/tools.
I would honestly say that overwriting the comments is also fine--but I'm guessing this is incompatible with our test frameworks?

Ah! Ok, that helps a lot. I actually haven't tried all the other tests, but not sure. I'll give option B a shot, and then if that doesn't work will either rewrite comments or just not work when comments exist. :) Will report back soon!