[Rust] Clippy CI is failing

[asfimport]

CI uses "stable" rust
1.50 stable was updated today: https://blog.rust-lang.org/2021/02/11/Rust-1.50.0.html

The new clippy is pickier resulting in many clippy warnings such as https://github.com/apache/arrow/pull/9469/checks?check_run_id=1881854256

We need to get CI back green

Reporter: Andrew Lamb / @alamb
Assignee: Andrew Lamb / @alamb

Related issues:

• [Rust] Fix clippy error (is duplicated by)

PRs and other links:

• GitHub Pull Request #9475

_{Note: This issue was originally created as ARROW-11602. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Dupe of ARROW-11603

[asfimport]

Todd Farmer / @toddfarmer:
Transitioning issue from Resolved to Closed to based on resolution field value.