helper generates the file ../server_gen.go (in package internal/lsp) which contains stub declarations of server methods.
To invoke it, run go generate in the gopls/internal/lsp directory.
It is derived from gopls/internal/lsp/protocol/tsserver.go, which itself is generated from the protocol downloaded from VSCode, so be sure to run go generate in the protocol first. Or run go generate ./... twice in the gopls directory.
It decides what stubs are needed and their signatures by looking at the Server interface (-t flag). These all look somewhat like Resolve(context.Context, *CompletionItem) (*CompletionItem, error).
It then parses the lsp directory (-u flag) to see if there is a corresponding implementation function (which in this case would be named resolve). If so it discovers the parameter names needed, and generates (in server_gen.go) code like
func (s *Server) resolve(ctx context.Context, params *protocol.CompletionItem) (*protocol.CompletionItem, error) {
return s.resolve(ctx, params)
}
If resolve is not defined (and it is not), then the body of the generated function is
return nil, notImplemented("resolve")
So to add a capability currently not implemented, just define it somewhere in lsp. In this case, just define func (s *Server) resolve(...) and re-generate server_gen.go.