hybrid-array: make ArraySize an unsafe trait
#914
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It's used when checking the lengths of slices match an array size prior to using a pointer cast to convert types. If someone were to make their own `typenum::Unsigned` type and impl `ArraySize` for it with an `ArrayType` whose size does not match `Unsigned::USIZE`, that would be UB. Really `ArraySize` is not intended for downstream crates to impl anyway, but making it an `unsafe trait` at least captures the UB potential. Additionally this adds more debug checks to `check_slice_length` to ensure that if there is a length mismatch, it's at least caught in debug builds.
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It's used when checking the lengths of slices match an array size prior to using a pointer cast to convert types.
If someone were to make their own
typenum::Unsignedtype and implArraySizefor it with anArrayTypewhose size does not matchUnsigned::USIZE, that would be UB.Really
ArraySizeis not intended for downstream crates to impl anyway, but making it anunsafe traitat least captures the UB potential.Additionally this adds more debug checks to
check_slice_lengthto ensure that if there is a length mismatch, it's at least caught in debug builds.