Clean up Rust 2021 linting warning

[asfimport]

Reporter: QP Hou / @houqp
Assignee: QP Hou / @houqp

PRs and other links:

• GitHub Pull Request #9535

_{Note: This issue was originally created as ARROW-11708. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Issue resolved by pull request 9535
#9535