Compile error when `vscalar` feature is used without `vtab-arrow` feature

[Hiroaki Yutani]

I see this compile error when I enable vscalar feature. I'm not sure if it's supposed to work without arrow interface to use the vscalar.

error[E0432]: unresolved import `crate::vtab::arrow`
  --> C:\...\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\duckdb-1.2.0\src\vscalar\mod.rs:12:11
   |
12 |     vtab::arrow::WritableVector,
   |           ^^^^^ could not find `arrow` in `vtab`
   |

[era127]

Try the vscalar-arrow feature as well.

[Hiroaki Yutani]

Thanks. What I was wondering is, the API design allows us to implement a scalar function without arrow. But, I found scalar function support was merged only very recently, so this problem should be a minor oversight in the pull request.

#429