Golang x/tools/gopls: regression in semantic tokens (Data field is JSON "null")

@neild reports that neovim with gopls at master produces an error similar to https://github.com/neovim/neovim/issues/21387. I suspect https://go.dev/cl/577655 introduced a shortcut path with a nil Data slice. This patch appears to be an effective workaround:

diff --git a/gopls/internal/server/semantic.go b/gopls/internal/server/semantic.go
index ca3df78e10..7f7a3474d3 100644
--- a/gopls/internal/server/semantic.go
+++ b/gopls/internal/server/semantic.go
@@ -38,7 +38,7 @@ func (s *server) semanticTokens(ctx context.Context, td protocol.TextDocumentIde
                // Previously, an error was returned here to achieve the same effect, but
                // that had the side effect of very noisy "semantictokens are disabled"
                // logs on every keystroke.
-               return new(protocol.SemanticTokens), nil
+               return &protocol.SemanticTokens{Data: []uint32{}}, nil

Comment From: findleyr

Thanks all for catching this (right before we were about to cut the prerelease, too!).

I suspect adding "omitempty" would also fix this, and may be more principled.

Comment From: adonovan

Yes, the patch was just evidence. I think the cleanest solution would be to make sure that every slice in our JSON schema is marked omitempty.

Comment From: gopherbot

Change https://go.dev/cl/591415 mentions this issue: gopls/internal/server: return a non-nil slice for empty token result

Comment From: findleyr

It looks like omitempty may not be appropriate here, because the fields are not marked as optional in the schema.

For now, went with the fix suggested in Alan's patch: just return a non-nil empty slice.