[Rust] Add capacity field in Buffer

[asfimport]

Currently Buffer only has len, but no capacity. We should add both.

Reporter: Chao Sun / @sunchao
Assignee: Chao Sun / @sunchao

PRs and other links:

• GitHub Pull Request #4331

_{Note: This issue was originally created as ARROW-5357. Please see the migration documentation for further details.}

[asfimport]

Paddy Horan / @paddyhoran:
What do you think of introducing capacity in addition to len, we can then use capacity in flatbuffers?

[asfimport]

Chao Sun / @sunchao:
Yes we can perhaps do that - this is how C++ does anyway.

Originally the problem was: when I compare a buffer I created via Buffer::from(&[1, 2, 3]) versus a buffer created from flatbuffer with the same data, it will fail because on the left hand side the len is less than capacity, but on the right hand side len = capacity.

However thinking on it more, maybe we should not compare buffer directly without other contexts, such as how many valid elements are stored in the buffer. Also, unless we always zero-out the padded bytes in a buffer, comparison on buffer will likely to fail.

[asfimport]

Paddy Horan / @paddyhoran:

"Also, unless we always zero-out the padded bytes in a buffer, comparison on buffer will likely to fail."

Yea and I don't think we want to guarantee that the padded bytes will be 0 as certain algorithms will be easier to implement such as SIMD operations.

[asfimport]

Chao Sun / @sunchao:
Re-purposing this Jira to add capacity info in Buffer struct.

[asfimport]

Paddy Horan / @paddyhoran:
Issue resolved by pull request 4331
#4331