add some explanation and todos

parquet/src/file/column_crypto_metadata.rs

@@ -26,6 +26,9 @@ use crate::format::{
 use crate::parquet_thrift::{FieldType, ThriftCompactInputProtocol};
 use crate::{thrift_struct, thrift_union};
 
+// define this and ColumnCryptoMetadata here so they're only defined when
+// the encryption feature is enabled
+
 thrift_struct!(
 /// Encryption metadata for a column chunk encrypted with a column-specific key
 pub struct EncryptionWithColumnKey {

parquet/src/file/metadata/mod.rs

@@ -430,6 +430,7 @@ impl From<ParquetMetaData> for ParquetMetaDataBuilder {
     }
 }
 
+// TODO: should this move to thrift_gen?
 thrift_struct!(
 /// A key-value pair for [`FileMetaData`].
 pub struct KeyValue {
@@ -552,6 +553,7 @@ impl FileMetaData {
     }
 }
 
+// TODO: should this move to thrift_gen?
 thrift_struct!(
 /// Sort order within a RowGroup of a leaf column
 pub struct SortingColumn {

parquet/src/file/page_encoding_stats.rs

@@ -22,6 +22,7 @@ use crate::errors::{ParquetError, Result};
 use crate::parquet_thrift::{FieldType, ThriftCompactInputProtocol};
 use crate::thrift_struct;
 
+// TODO: This should probably all be moved to thrift_gen
 thrift_struct!(
 /// PageEncodingStats for a column chunk and data page.
 pub struct PageEncodingStats {