cleanup(rust): Simplify code using closure capture in Rust 2021 edition

[teor2345]

Motivation

Rust's new closure capture rules in the 2021 edition allow us to simplify some code.

There are some new clippy errors in Rust nightly.

Reference

In Rust 2021 edition, closures only capture the fields they need to, not the entire self or argument:
https://doc.rust-lang.org/edition-guide/rust-2021/disjoint-capture-in-closures.html

Solution

Remove a temporary variable

Fix clippy lints

Review

This is a low priority cleanup.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[codecov]

Codecov Report

Merging #6737 (c7464b1) into main (a972144) will increase coverage by 0.14%.
The diff coverage is 60.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6737      +/-   ##
==========================================
+ Coverage   77.92%   78.07%   +0.14%     
==========================================
  Files         308      308              
  Lines       40874    40869       -5     
==========================================
+ Hits        31851    31908      +57     
+ Misses       9023     8961      -62