Fix `pad_using` bug producing items when it shouldn't #1199

Job	Run time
doc	17s
miri	3m 0s
check format	14s
msrv	45s
test	56s
check	22s
check (--no-default-features --features use_alloc)	16s
semver-checks	19s
check (--no-default-features)	14s
check (--all-targets --all-features)	27s
All checks succeeded	2s
	6m 52s

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