alamb commented on code in PR #7922:
URL: https://github.com/apache/arrow-rs/pull/7922#discussion_r2207866251
##########
parquet-variant/src/builder.rs:
##########
@@ -402,6 +402,11 @@ impl<S: AsRef<str>> FromIterator<S> for MetadataBuilder {
impl<S: AsRef<str>> Extend<S> for MetadataBuilder {
fn extend<T: IntoIterator<Item = S>>(&mut self, iter: T) {
+ let iter = iter.into_iter();
+ let (min, max) = iter.size_hint();
+
+ self.field_names.reserve(max.unwrap_or(min));
+
for field_name in iter {
self.upsert_field_name(field_name.as_ref());
Review Comment:
I agree in theory it is not a great idea
However in practice I think it is common to reserve capacity (allocate)
based on the size hint and it is done several other places in this crate
https://github.com/apache/arrow-rs/blob/474f1924fff30d3150f7c737205bb9f903686d53/parquet/src/arrow/arrow_reader/selection.rs#L129-L128
https://github.com/apache/arrow-rs/blob/be338f9bdaab719dd569130ea882e10a58ef9d17/arrow-array/src/array/primitive_array.rs#L1408-L1407
https://github.com/apache/arrow-rs/blob/a2cc42639b4ad5579d052e6f2317a413e2407e0f/arrow-array/src/array/boolean_array.rs#L435-L434
I also think it is done in the standard library for `Vec` , for example in
https://doc.rust-lang.org/src/alloc/vec/mod.rs.html#3550
<img width="1138" height="732" alt="Screenshot 2025-07-15 at 11 36 06 AM"
src="https://github.com/user-attachments/assets/83e7311f-1de2-4b21-a983-94a348574992";
/>
Thus I think it is acceptable to do here too
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