Fix unused field warnings, 0.12 backport #3120

Job	Run time
build-binaries (1.71)	1m 3s
docs	1m 11s
build-binaries (stable)	59s
build-crate (--no-default-features, 1.71)	1m 15s
build-crate (--no-default-features, stable)	1m 1s
build-crate (--all-features, 1.71)	1m 31s
build-crate (--all-features, stable)	1m 24s
build-crate (--features=default, 1.71)	1m 14s
build-crate (--features=default, stable)	1m 3s
build-crate (--features=test-util, 1.71)	1m 21s
build-crate (--features=test-util, stable)	1m 5s
build-crate (--features=multithreaded, 1.71)	1m 19s
build-crate (--features=multithreaded, stable)	1m 6s
build-crate (--features=test-util,multithreaded, 1.71)	1m 15s
build-crate (--features=test-util,multithreaded, stable)	1m 10s
build-crate (--features=prio2, 1.71)	1m 17s
build-crate (--features=prio2, stable)	1m 10s
build-crate (--features=prio2,test-util, 1.71)	1m 15s
build-crate (--features=prio2,test-util, stable)	1m 16s
build-crate (--features=prio2,multithreaded, 1.71)	1m 17s
build-crate (--features=prio2,multithreaded, stable)	1m 16s
build-crate (--features=prio2,test-util,multithreaded, 1.71)	1m 33s
build-crate (--features=prio2,test-util,multithreaded, stable)	1m 14s
build-crate (--features=experimental, 1.71)	1m 22s
build-crate (--features=experimental, stable)	1m 13s
build-crate (--features=experimental,multithreaded, 1.71)	1m 27s
build-crate (--features=experimental,multithreaded, stable)	1m 19s
	33m 36s

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