tustvold commented on a change in pull request #6980:
URL: https://github.com/apache/arrow/pull/6980#discussion_r412736972



##########
File path: rust/arrow/src/array/builder.rs
##########
@@ -236,6 +251,14 @@ impl<T: ArrowPrimitiveType> BufferBuilderTrait<T> for 
BufferBuilder<T> {
         self.write_bytes(v.to_byte_slice(), 1)
     }
 
+    default fn append_n(&mut self, n: usize, v: T::Native) -> Result<()> {
+        self.reserve(n)?;
+        for _ in 0..n {
+            self.write_bytes(v.to_byte_slice(), 1)?;
+        }

Review comment:
       That would be incorrect though? The method should write the value `v` 
`n` times. Calling `write_bytes(v.to_byte_slice(), n)` would write `v` once and 
then increase the recorded element count by n, which would leave things in an 
odd state. 
   
   For the general case there is little reason for this method to exist, aside 
from reserving memory once. It mainly exists because the bitmap specialization 
can more efficiently append the same value multiple times than a naive loop 
such as this.




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