That isn't really inferring the size of the array though. [T; N]::map(impl FnMut(T) -> U) defines its return type as [U; N]. N is fixed at the original array literal.
It's inferring it from the array literal in the first place, but it is a different kind of inference from the original one. But I think that's what the question was asking for:
> What's a sane/legit use case of such inferring in general?
> What's a sane/legit use case of such inferring in general?
Depends how you interpret "such inferring"