scovich commented on code in PR #8208:
URL: https://github.com/apache/arrow-rs/pull/8208#discussion_r2296027615


##########
parquet-variant/src/builder.rs:
##########
@@ -589,14 +675,14 @@ enum ParentState<'a> {
     Variant {
         value_builder: &'a mut ValueBuilder,
         saved_value_builder_offset: usize,
-        metadata_builder: &'a mut MetadataBuilder,
+        metadata_builder: &'a mut dyn MetadataBuilder,

Review Comment:
   A related issue we'll hit shortly: `VariantArrayBuilder` is currently 
hard-wired to use a `BasicMetadataBuilder` that produces a new metadata column. 
   
   Once we need a variant array builder that reuses an existing metadata column 
(e.g. for unshredding), we'll be forced to decide whether we want to make 
`VariantArrayBuilder` generic over `M: MetadataBuilder` or just define a second 
builder class. 
   
   I suspect that choice will ultimately decide the generic-vs-not question.



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