Bug 2897 – RandomCover docs wrong: RandomCover needs a random access range.

From RandomCover docs: "Covers a given range r in a random manner, i.e. goes through each element of r once and only once, just in a random order. r must be a forward access range with length." Reading the source code, it's clear that RandomCover really needs random access: ref ElementType!(Range) front() { return _input[_current]; }