Does `&[!]` have a restriction on its length metadata?

[scottmcm]

I think by the "size_of_val needs to not overflow"-motivated rule it arguably doesn't have any range restriction, same as &[()].

Should it require that the length is zero?

[RalfJung]

So what would the general rule be? For &T, if T has unsized tail [U], and if U is uninhabited, then the length must be 0?

That does seem consistent with the fact that we'd consider &[!; 0] to be valid but &[!; 1] is insta-UB to create.