Work around negative_impls that allows unsound overlapping Unpin impls #2958

Job	Run time
test (1.56)	1m 24s
test (stable)	49s
test (beta)	43s
test (nightly)	2m 4s
docs / docs	20s
miri / miri	1m 5s
check-external-types / check-external-types	21s
deny / deny	15s
msrv / msrv	6m 34s
tidy / tidy	1m 29s
	15m 4s

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