chore: correctly cfg unused type #3385

Job	Run time
cfg-check	28s
clippy	1m 5s
docs	54s
doctest	2m 17s
feature-checks	5m 40s
fmt	14s
no-std	28s
wasm-unknown	1m 14s
test ubuntu-latest stable --no-default-features	2m 26s
wasm-wasi	2m 10s
test ubuntu-latest stable	2m 21s
test ubuntu-latest nightly --no-default-features	1m 8s
test ubuntu-latest nightly	1m 8s
test ubuntu-latest 1.76 --no-default-features	1m 34s
test ubuntu-latest 1.76	1m 38s
test windows-latest stable --no-default-features	5m 14s
test windows-latest stable	5m 11s
test windows-latest nightly --no-default-features	4m 11s
test windows-latest nightly	4m 16s
test windows-latest 1.76 --no-default-features	3m 53s
test windows-latest 1.76	3m 16s
	50m 46s

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