Is your feature request related to a problem or challenge? Please describe what you are trying to do.

BufferBuilder is effectively a Vec<T: ArrowNativeType> that can be converted to Buffer without needing to copy. This is an incredibly useful abstraction for incrementally building and mutating data in place, prior to freezing it for hand-off to other systems.

Unfortunately it is currently lacking some relatively minor functionality to enable this use-case. In particular access to the data that has been already written. The parquet crate currently rolls its own ScalarBuffer to work around this, but as described in #1849 it would be nice to avoid this duplication.

Describe the solution you'd like

Add the following APIs to BufferBuilder

fn as_slice(&self) -> &[T] {
    unsafe { std::slice::from_raw_parts(self.buffer.as_ptr(), self.len) }
}

fn as_slice_mut(&mut self) -> &mut [T] {
    unsafe { std::slice::from_raw_parts_mut(self.buffer.as_ptr_mut(), self.len) }
}

fn truncate(&mut self, len: usize) {}

fn extend_zeroed(&mut self, len: usize) {}

Describe alternatives you've considered

We could not do this

Additional context

• Implement UnionArray FieldData using Type Erasure #1842 is necessary to provide these APIs safely
• BufferBuilder should be moved to its own module to avoid others mutating its inner MutableArray (Split up arrow::array::builder module #1843)