go/ast: trailing func comments mishandled in CommentMap

[mvdan]

Working example: https://play.golang.org/p/TlsDxVbGow

Prints: g: "trailing comment in f\n"

Seems like the correct output should either be nothing (if it is associated with _ = x), or f: "trailing comment in f\n".

What's currently happening is that the "_ = x" in func f is the most recent "important" node when we encounter the comment. Since the comment isn't on the line immediately following "_ = x", it doesn't
get associated with it. We don't look further up the "important nodes" stack to notice that it's within the bounds of f. Instead, we just give up and associate it with g.

/cc @griesemer

[griesemer]

Feel free to take a shot at it.

[alindeman]

Is there a way to have it associated with _ = x or f while still following the three current rules? Or would a new rule need to be added?

- g starts on the same line as n ends

Not applicable in this circumstance. This is the foo := bar // baz case, as I understand it.

- g starts on the line immediately following n, and there is
  at least one empty line after g and before the next node

The line break between _ = x and the comment prevents this one from applying.

- g starts before n and is not associated to the node before n
  via the previous rules

At this point, g can only be attached to a node after its appearance.