Fix vscalar feature to depend on vtab-arrow (#542)

The vscalar feature uses WritableVector from the vtab::arrow module.

Fixes #441

Author: mlafeldt

crates/duckdb/Cargo.toml

@@ -20,7 +20,7 @@ default = []
 bundled = ["libduckdb-sys/bundled"]
 json = ["libduckdb-sys/json", "bundled"]
 parquet = ["libduckdb-sys/parquet", "bundled"]
-vscalar = []
+vscalar = ["vtab-arrow"]
 vscalar-arrow = []
 vtab = []
 vtab-loadable = ["vtab", "duckdb-loadable-macros"]

crates/duckdb/src/vtab/arrow.rs

@@ -1218,6 +1218,7 @@ mod test {
     }
 
     #[test]
+    #[cfg(feature = "appender-arrow")]
     fn test_append_struct() -> Result<(), Box<dyn Error>> {
         let db = Connection::open_in_memory()?;
         db.execute_batch("CREATE TABLE t1 (s STRUCT(v VARCHAR, i INTEGER))")?;
@@ -1254,6 +1255,7 @@ mod test {
     }
 
     #[test]
+    #[cfg(feature = "appender-arrow")]
     fn test_append_struct_contains_null() -> Result<(), Box<dyn Error>> {
         let db = Connection::open_in_memory()?;
         db.execute_batch("CREATE TABLE t1 (s STRUCT(v VARCHAR, i INTEGER))")?;