tustvold opened a new issue, #1850:
URL: https://github.com/apache/arrow-rs/issues/1850
**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**
* #1842 is necessary to provide these APIs safely
* BufferBuilder should be moved to its own module to avoid others mutating
its inner MutableArray (#1843)
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