Fix false-positive unused_mut lints comming from macro #688

Job	Run time
cargo-fmt	15s
cargo-lock	6s
cargo-machete	3s
cargo-doc (ubuntu)	23s
cargo-miri	1m 32s
cargo-doc (windows)	1m 30s
compilation-benchmarks	2m 7s
cargo-doc (macos)	32s
prettier	11s
runtime-benchmarks (args_3)	51s
taplo-fmt	3s
runtime-benchmarks (args_5)	52s
test-msrv (ubuntu)	1m 1s
test-stable (ubuntu)	1m 13s
runtime-benchmarks (args_10_alloc)	56s
test-msrv (windows)	1m 51s
test-stable (windows)	3m 55s
runtime-benchmarks (args_10_structs)	53s
test-msrv (macos)	1m 22s
test-stable (macos)	1m 8s
test-unstable (beta)	25s
runtime-benchmarks (args_10)	1m 4s
test-unstable (nightly)	30s
runtime-benchmarks (args_20)	59s
typos	4s
website-build	24s
	24m 10s

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