scovich commented on code in PR #8324:
URL: https://github.com/apache/arrow-rs/pull/8324#discussion_r2344616766


##########
parquet-variant/src/builder.rs:
##########
@@ -1683,26 +1712,31 @@ pub trait VariantBuilderExt {
 
     /// Creates a nested list builder. See e.g. [`VariantBuilder::new_list`]. 
Panics if the nested
     /// builder cannot be created, see e.g. [`ObjectBuilder::new_list`].
-    fn new_list(&mut self) -> ListBuilder<'_> {
+    fn new_list(&mut self) -> ListBuilder<'_, Self::State<'_>> {

Review Comment:
   NOTE: If associated type defaults were stable, we could simplify the code 
even further:
   ```rust
   pub trait VariantBuilderExt {
       /// The builder specific state used by nested builders
       type State<'a>: BuilderSpecificState + 'a
       where
           Self: 'a;
       
       type ListBuilder<'a> = ListBuilder<'a, Self::State<'a>> 
       where 
           Self: 'a;
       
       type ObjectBuilder<'a> = ObjectBuilder<'a, Self::State<'a>> 
       where 
           Self: 'a;
   
       ...
   
       fn try_new_list(&mut self) -> Result<Self::ListBuilder<'_>, ArrowError>;
       fn try_new_object(&mut self) -> Result<Self::ObjectBuilder<'_>, 
ArrowError>;
   }
   ```



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