Fix clippy warnings for //tools/rustfmt

[mypmc]

Close #744

//tools/rustfmt has been built successfully using following .bazelrc.

build --aspects=@rules_rust//rust:defs.bzl%rust_clippy_aspect
build --output_groups=+clippy_checks

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[mypmc]

@googlebot I signed it!

[illicitonion]

Thanks!