`<[T]>::sort_by_index` and `<[T]>::sort_by_key_and_index`

I did think of that, but I suspect the T-libs folks won't want the stdlib doing that.

But: this is an expensive check no matter what -- intrinsically has to scan the whole array -- and you might want to use the same permutation multiple times, so maybe a wrapper class something like

struct Permutation<N: usize>([usize; N]);

impl Permutation {
    /// Checks that `indices` define a permutation
    fn from_indices(indices: [usize; N])
        -> Result<Self, NotAPermutationError>;

    /// Uses self to permute the first N elements of `s`
    /// it is an error if s.len() < N
    fn permute<T: Sized>(&self, s: &mut [T]) -> Result<(), TooSmallError>;
}

(API sketch only, type annotations for illustration only and may be bogus)