x/tools/gopls: fillstruct heuristic fails to look beneath current point, or in other files

[adonovan]

The fillstruct code action looks for declarations in the current file, before the current point, as candidates for the initializers of each missing field. That seems doubly arbitrary: package-level declarations in other files should be candidates too, as should declarations beneath the current point.

[findleyr]

Skimming the logic, it appears that fillstruct does a syntactic analysis to find candidate identifiers, but then only uses their name.

I don't know why all of this logic couldn't just be replaced by a simple scope traversal.

We could also traverse the package scopes of direct imports, though that is less likely to be what the user wants, IMO.

[findleyr]

CC @madelinekalil, who has been looking at the fillstruct code action recently.